synchronized snapshots

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 0b6a109..ce8653e 100644
*** a/doc/src/sgml/func.sgml
--- b/doc/src/sgml/func.sgml
*************** FOR EACH ROW EXECUTE PROCEDURE suppress_
*** 15023,15026 ****
--- 15023,15100 ----
          <xref linkend="SQL-CREATETRIGGER">.
      </para>
    </sect1>
+ 
+   <sect1 id="functions-snapshotsync">
+    <title>Snapshot Synchronization Functions</title>
+ 
+    <indexterm>
+      <primary>pg_export_snapshot</primary>
+    </indexterm>
+ 
+    <para>
+      <productname>PostgreSQL</> allows different sessions to synchronize their
+      snapshots. A database snapshot determines which data is visible to
+      the client that is using this snapshot. Synchronized snapshots are necessary when
+      two clients need to see the same content in the database. If these two clients
+      just connected to the database and opened their transactions, then they could
+      never be sure that there was no data modification right between both
+      connections.
+    </para>
+    <para>
+      As a solution, <productname>PostgreSQL</> offers the function
+      <function>pg_export_snapshot</> which saves the snapshot internally and
+      from then on until the end of the saving transaction, the snapshot can be
+      copied into a a new transaction with the <xref linkend="sql-set-transaction">
+      command to open a second transaction with the
+      exact same snapshot. Now both transactions are guaranteed to see the exact same
+      data even though they might have connected at different times.
+    </para>
+    <para>
+      Note that a snapshot can only be used to start a new transaction as long
+      as the transaction that originally saved it is held open. Also note that even
+      after the synchronization both clients still run their own independent
+      transactions. As a consequence, even though synchronized with respect to
+      reading pre-existing data, both transactions won't be able to see each other's
+      uncommitted data.
+    </para>
+    <table id="functions-snapshot-synchronization">
+     <title>Snapshot Synchronization Functions</title>
+     <tgroup cols="3">
+      <thead>
+       <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry>
+       </row>
+      </thead>
+ 
+      <tbody>
+       <row>
+        <entry>
+         <literal><function>pg_export_snapshot()</function></literal>
+        </entry>
+        <entry><type>text</type></entry>
+        <entry>Save the snapshot and return its identifier</entry>
+       </row>
+      </tbody>
+     </tgroup>
+    </table>
+ 
+    <para>
+       The function <function>pg_export_snapshot</> does not take an argument
+       and returns the snapshot's identifier as <type>text</type> data. Internally the
+       function will save the snapshot data to a file so that it can be retrieved
+       from a different backend process later on. Note that as soon as the
+       transaction ends, any saved snapshots become invalid and their
+       identifiers cannot be used in other transactions anymore. If the function
+       has been executed, the transaction cannot be prepared anymore with <xref
+       linkend="sql-prepare-transaction">.
+    </para>
+ <programlisting>
+ SELECT pg_export_snapshot();
+ 
+  pg_export_snapshot
+ --------------------
+  000003A1-1
+ (1 row)
+ </programlisting>
+   </sect1>
  </chapter>
+ 
diff --git a/doc/src/sgml/ref/set_transaction.sgml b/doc/src/sgml/ref/set_transaction.sgml
index e28a7e1..059b94b 100644
*** a/doc/src/sgml/ref/set_transaction.sgml
--- b/doc/src/sgml/ref/set_transaction.sgml
*************** SET SESSION CHARACTERISTICS AS TRANSACTI
*** 40,45 ****
--- 40,46 ----
      ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
      READ WRITE | READ ONLY
      [ NOT ] DEFERRABLE
+     SNAPSHOT <replaceable class="parameter">snapshot-id</replaceable>
  </synopsis>
   </refsynopsisdiv>
  
*************** SET SESSION CHARACTERISTICS AS TRANSACTI
*** 146,151 ****
--- 147,167 ----
     contributing to or being canceled by a serialization failure.  This mode
     is well suited for long-running reports or backups.
    </para>
+ 
+   <para>
+    The <literal>SNAPSHOT</literal> property allows a new transaction to run
+    with the same snapshot as an already running transaction.  A call to
+    <literal>pg_export_snapshot</literal> (see <xref
+    linkend="functions-snapshotsync">) must have been executed in the other
+    transaction. This function returns a snapshot id which must be passed as a
+    parameter to this command to create a second transaction running with the same
+    snapshot. You also need to make the transaction <literal>ISOLATION LEVEL
+    SERIALIZABLE</literal> or <literal>ISOLATION LEVEL REPEATABLE READ</literal>.
+    If the transaction has already executed a query, started a subtransaction or
+    assigned a snapshot, no further snapshot assignment is possible in this
+    transaction.
+   </para>
+ 
   </refsect1>
  
   <refsect1>
*************** SET SESSION CHARACTERISTICS AS TRANSACTI
*** 178,183 ****
--- 194,225 ----
    </para>
   </refsect1>
  
+  <refsect1>
+   <title>Examples</title>
+ 
+   <para>
+    To begin a new transaction block with the same snapshot as an already
+    existing transaction, first export the snapshot from the existing
+    transaction. This will return the snapshot id:
+ 
+ <programlisting>
+ # SELECT pg_export_snapshot();
+  pg_export_snapshot
+ --------------------
+  000003A1-1
+ (1 row)
+ </programlisting>
+ 
+    Then reference this snapshot id as the first command in the newly opened
+    transaction:
+ 
+ <programlisting>
+ BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
+ SET TRANSACTION SNAPSHOT '000003A1-1';
+ </programlisting>
+   </para>
+  </refsect1>
+ 
   <refsect1 id="R1-SQL-SET-TRANSACTION-3">
    <title>Compatibility</title>
  
*************** SET SESSION CHARACTERISTICS AS TRANSACTI
*** 198,206 ****
    </para>
  
    <para>
!    The <literal>DEFERRABLE</literal>
     <replaceable class="parameter">transaction_mode</replaceable>
!    is a <productname>PostgreSQL</productname> language extension.
    </para>
  
    <para>
--- 240,250 ----
    </para>
  
    <para>
!    <literal>DEFERRABLE</literal>
     <replaceable class="parameter">transaction_mode</replaceable>
!    and <literal>SNAPSHOT</literal> <replaceable
!    class="parameter">snapshot-id</replaceable> are
!    <productname>PostgreSQL</productname> language extensions.
    </para>
  
    <para>
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index de3f965..8b58cb5 100644
*** a/src/backend/access/transam/xact.c
--- b/src/backend/access/transam/xact.c
*************** CommitTransaction(void)
*** 1852,1857 ****
--- 1852,1863 ----
  	 */
  	PreCommit_Notify();
  
+ 	/*
+ 	 * Cleans up exported snapshots (this needs to happen before we update
+ 	 * our MyProc entry).
+ 	 */
+ 	PreCommit_Snapshot();
+ 
  	/* Prevent cancel/die interrupt while cleaning up */
  	HOLD_INTERRUPTS();
  
*************** PrepareTransaction(void)
*** 2070,2075 ****
--- 2076,2086 ----
  				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
  				 errmsg("cannot PREPARE a transaction that has operated on temporary tables")));
  
+ 	if (exportedSnapshots)
+ 		ereport(ERROR,
+ 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+ 				 errmsg("cannot PREPARE a transaction that has exported snapshots")));
+ 
  	/* Prevent cancel/die interrupt while cleaning up */
  	HOLD_INTERRUPTS();
  
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 8f65ddc..331a564 100644
*** a/src/backend/access/transam/xlog.c
--- b/src/backend/access/transam/xlog.c
***************
*** 58,63 ****
--- 58,64 ----
  #include "utils/guc.h"
  #include "utils/ps_status.h"
  #include "utils/relmapper.h"
+ #include "utils/snapmgr.h"
  #include "utils/timestamp.h"
  #include "pg_trace.h"
  
*************** StartupXLOG(void)
*** 6373,6378 ****
--- 6374,6384 ----
  		CheckRequiredParameterValues();
  
  		/*
+ 		 * We can delete any saved transaction snapshots that still exist
+ 		 */
+ 		DeleteAllExportedSnapshotFiles();
+ 
+ 		/*
  		 * We're in recovery, so unlogged relations relations may be trashed
  		 * and must be reset.  This should be done BEFORE allowing Hot Standby
  		 * connections, so that read-only backends don't try to read whatever
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index e9f3896..e1ab161 100644
*** a/src/backend/parser/gram.y
--- b/src/backend/parser/gram.y
*************** static void processCASbits(int cas_bits,
*** 553,560 ****
  
  	SAVEPOINT SCHEMA SCROLL SEARCH SECOND_P SECURITY SELECT SEQUENCE SEQUENCES
  	SERIALIZABLE SERVER SESSION SESSION_USER SET SETOF SHARE
! 	SHOW SIMILAR SIMPLE SMALLINT SOME STABLE STANDALONE_P START STATEMENT
! 	STATISTICS STDIN STDOUT STORAGE STRICT_P STRIP_P SUBSTRING
  	SYMMETRIC SYSID SYSTEM_P
  
  	TABLE TABLES TABLESPACE TEMP TEMPLATE TEMPORARY TEXT_P THEN TIME TIMESTAMP
--- 553,560 ----
  
  	SAVEPOINT SCHEMA SCROLL SEARCH SECOND_P SECURITY SELECT SEQUENCE SEQUENCES
  	SERIALIZABLE SERVER SESSION SESSION_USER SET SETOF SHARE
! 	SHOW SIMILAR SIMPLE SMALLINT SNAPSHOT SOME STABLE STANDALONE_P START
! 	STATEMENT STATISTICS STDIN STDOUT STORAGE STRICT_P STRIP_P SUBSTRING
  	SYMMETRIC SYSID SYSTEM_P
  
  	TABLE TABLES TABLESPACE TEMP TEMPLATE TEMPORARY TEXT_P THEN TIME TIMESTAMP
*************** set_rest:	/* Generic SET syntaxes: */
*** 1286,1291 ****
--- 1286,1299 ----
  					n->args = $2;
  					$$ = n;
  				}
+ 			| TRANSACTION SNAPSHOT Sconst
+ 				{
+ 					VariableSetStmt *n = makeNode(VariableSetStmt);
+ 					n->kind = VAR_SET_VALUE;
+ 					n->name = "TRANSACTION SNAPSHOT";
+ 					n->args = list_make1(makeStringConst($3, @3));
+ 					$$ = n;
+ 				}
  			| SESSION CHARACTERISTICS AS TRANSACTION transaction_mode_list
  				{
  					VariableSetStmt *n = makeNode(VariableSetStmt);
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 9489012..b665d75 100644
*** a/src/backend/storage/ipc/procarray.c
--- b/src/backend/storage/ipc/procarray.c
*************** ProcArrayShmemSize(void)
*** 166,174 ****
  {
  	Size		size;
  
- 	/* Size of the ProcArray structure itself */
- #define PROCARRAY_MAXPROCS	(MaxBackends + max_prepared_xacts)
- 
  	size = offsetof(ProcArrayStruct, procs);
  	size = add_size(size, mul_size(sizeof(PGPROC *), PROCARRAY_MAXPROCS));
  
--- 166,171 ----
*************** ProcArrayShmemSize(void)
*** 179,192 ****
  	 * TransactionIdIsInProgress() and GetRunningTransactionData(). All of the
  	 * main structures created in those functions must be identically sized,
  	 * since we may at times copy the whole of the data structures around. We
! 	 * refer to this size as TOTAL_MAX_CACHED_SUBXIDS.
  	 *
  	 * Ideally we'd only create this structure if we were actually doing hot
  	 * standby in the current run, but we don't know that yet at the time
  	 * shared memory is being set up.
  	 */
- #define TOTAL_MAX_CACHED_SUBXIDS \
- 	((PGPROC_MAX_CACHED_SUBXIDS + 1) * PROCARRAY_MAXPROCS)
  
  	if (EnableHotStandby)
  	{
--- 176,187 ----
  	 * TransactionIdIsInProgress() and GetRunningTransactionData(). All of the
  	 * main structures created in those functions must be identically sized,
  	 * since we may at times copy the whole of the data structures around. We
! 	 * refer to this size as TOTAL_MAX_CACHED_SUBXIDS, defined in procarray.h.
  	 *
  	 * Ideally we'd only create this structure if we were actually doing hot
  	 * standby in the current run, but we don't know that yet at the time
  	 * shared memory is being set up.
  	 */
  
  	if (EnableHotStandby)
  	{
*************** GetOldestXmin(bool allDbs, bool ignoreVa
*** 1144,1150 ****
   * not statically allocated (see xip allocation below).
   */
  Snapshot
! GetSnapshotData(Snapshot snapshot)
  {
  	ProcArrayStruct *arrayP = procArray;
  	TransactionId xmin;
--- 1139,1145 ----
   * not statically allocated (see xip allocation below).
   */
  Snapshot
! GetSnapshotData(Snapshot snapshot, Snapshot stemplate)
  {
  	ProcArrayStruct *arrayP = procArray;
  	TransactionId xmin;
*************** GetSnapshotData(Snapshot snapshot)
*** 1158,1163 ****
--- 1153,1181 ----
  	Assert(snapshot != NULL);
  
  	/*
+ 	 * We only get a valid snapshot in stemplate if the snapshot
+ 	 * synchronization feature used. In that case we just need to copy the
+ 	 * values that we get onto the snapshot we return.
+ 	 * Note that in this case we always duplicate an existing snapshot, that is
+ 	 * currently held by another active transaction. That's why we do not need
+ 	 * to update any { RecentGlobalXmin, RecentXmin, globalxmin }.
+ 	 */
+ 	if (stemplate != InvalidSnapshot)
+ 	{
+ 		/*
+ 		 * 'stemplate' is only read and its values are copied onto 'snapshot'.
+ 		 */
+ 		CopySnapshotOnto(stemplate, snapshot);
+ 
+ 		/*
+ 		 * We can use the result of the copy except for that this snapshot
+ 		 * should look like new and not copied.
+ 		 */
+ 		snapshot->copied = false;
+ 		return snapshot;
+ 	}
+ 
+ 	/*
  	 * Allocating space for maxProcs xids is usually overkill; numProcs would
  	 * be sufficient.  But it seems better to do the malloc while not holding
  	 * the lock, so we can't look at numProcs.  Likewise, we allocate much
diff --git a/src/backend/storage/lmgr/predicate.c b/src/backend/storage/lmgr/predicate.c
index 1754180..6df30d8 100644
*** a/src/backend/storage/lmgr/predicate.c
--- b/src/backend/storage/lmgr/predicate.c
*************** static void OldSerXidSetActiveSerXmin(Tr
*** 416,423 ****
  
  static uint32 predicatelock_hash(const void *key, Size keysize);
  static void SummarizeOldestCommittedSxact(void);
! static Snapshot GetSafeSnapshot(Snapshot snapshot);
! static Snapshot RegisterSerializableTransactionInt(Snapshot snapshot);
  static bool PredicateLockExists(const PREDICATELOCKTARGETTAG *targettag);
  static bool GetParentPredicateLockTag(const PREDICATELOCKTARGETTAG *tag,
  						  PREDICATELOCKTARGETTAG *parent);
--- 416,423 ----
  
  static uint32 predicatelock_hash(const void *key, Size keysize);
  static void SummarizeOldestCommittedSxact(void);
! static Snapshot GetSafeSnapshot(Snapshot snapshot, Snapshot stemplate);
! static Snapshot RegisterSerializableTransactionInt(Snapshot snapshot, Snapshot stemplate);
  static bool PredicateLockExists(const PREDICATELOCKTARGETTAG *targettag);
  static bool GetParentPredicateLockTag(const PREDICATELOCKTARGETTAG *tag,
  						  PREDICATELOCKTARGETTAG *parent);
*************** SummarizeOldestCommittedSxact(void)
*** 1487,1493 ****
   *		one of them could possibly create a conflict.
   */
  static Snapshot
! GetSafeSnapshot(Snapshot origSnapshot)
  {
  	Snapshot	snapshot;
  
--- 1487,1493 ----
   *		one of them could possibly create a conflict.
   */
  static Snapshot
! GetSafeSnapshot(Snapshot origSnapshot, Snapshot stemplate)
  {
  	Snapshot	snapshot;
  
*************** GetSafeSnapshot(Snapshot origSnapshot)
*** 1501,1507 ****
  		 * caller passed to us. It returns a copy of that snapshot and
  		 * registers it on TopTransactionResourceOwner.
  		 */
! 		snapshot = RegisterSerializableTransactionInt(origSnapshot);
  
  		if (MySerializableXact == InvalidSerializableXact)
  			return snapshot;	/* no concurrent r/w xacts; it's safe */
--- 1501,1507 ----
  		 * caller passed to us. It returns a copy of that snapshot and
  		 * registers it on TopTransactionResourceOwner.
  		 */
! 		snapshot = RegisterSerializableTransactionInt(origSnapshot, stemplate);
  
  		if (MySerializableXact == InvalidSerializableXact)
  			return snapshot;	/* no concurrent r/w xacts; it's safe */
*************** GetSafeSnapshot(Snapshot origSnapshot)
*** 1554,1560 ****
   * It should be current for this process and be contained in PredXact.
   */
  Snapshot
! RegisterSerializableTransaction(Snapshot snapshot)
  {
  	Assert(IsolationIsSerializable());
  
--- 1554,1560 ----
   * It should be current for this process and be contained in PredXact.
   */
  Snapshot
! RegisterSerializableTransaction(Snapshot snapshot, Snapshot stemplate)
  {
  	Assert(IsolationIsSerializable());
  
*************** RegisterSerializableTransaction(Snapshot
*** 1564,1576 ****
  	 * thereby avoid all SSI overhead once it's running..
  	 */
  	if (XactReadOnly && XactDeferrable)
! 		return GetSafeSnapshot(snapshot);
  
! 	return RegisterSerializableTransactionInt(snapshot);
  }
  
  static Snapshot
! RegisterSerializableTransactionInt(Snapshot snapshot)
  {
  	PGPROC	   *proc;
  	VirtualTransactionId vxid;
--- 1564,1576 ----
  	 * thereby avoid all SSI overhead once it's running..
  	 */
  	if (XactReadOnly && XactDeferrable)
! 		return GetSafeSnapshot(snapshot, stemplate);
  
! 	return RegisterSerializableTransactionInt(snapshot, stemplate);
  }
  
  static Snapshot
! RegisterSerializableTransactionInt(Snapshot snapshot, Snapshot stemplate)
  {
  	PGPROC	   *proc;
  	VirtualTransactionId vxid;
*************** RegisterSerializableTransactionInt(Snaps
*** 1608,1614 ****
  	} while (!sxact);
  
  	/* Get and register a snapshot */
! 	snapshot = GetSnapshotData(snapshot);
  	snapshot = RegisterSnapshotOnOwner(snapshot, TopTransactionResourceOwner);
  
  	/*
--- 1608,1614 ----
  	} while (!sxact);
  
  	/* Get and register a snapshot */
! 	snapshot = GetSnapshotData(snapshot, stemplate);
  	snapshot = RegisterSnapshotOnOwner(snapshot, TopTransactionResourceOwner);
  
  	/*
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index a71729c..9ffe3a1 100644
*** a/src/backend/utils/misc/guc.c
--- b/src/backend/utils/misc/guc.c
***************
*** 72,77 ****
--- 72,78 ----
  #include "utils/plancache.h"
  #include "utils/portal.h"
  #include "utils/ps_status.h"
+ #include "utils/snapmgr.h"
  #include "utils/tzparser.h"
  #include "utils/xml.h"
  
*************** ExecSetVariableStmt(VariableSetStmt *stm
*** 6094,6099 ****
--- 6095,6109 ----
  	switch (stmt->kind)
  	{
  		case VAR_SET_VALUE:
+ 			if (strcmp(stmt->name, "TRANSACTION SNAPSHOT") == 0)
+ 			{
+ 				if (!ImportSnapshot(ExtractSetVariableArgs(stmt)))
+ 					ereport(ERROR,
+ 							(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ 							 errmsg("could not import the requested snapshot")));
+ 				break;
+ 			}
+ 			/* fallthrough */
  		case VAR_SET_CURRENT:
  			set_config_option(stmt->name,
  							  ExtractSetVariableArgs(stmt),
diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index bb25ac6..5d3f492 100644
*** a/src/backend/utils/time/snapmgr.c
--- b/src/backend/utils/time/snapmgr.c
***************
*** 25,36 ****
   */
  #include "postgres.h"
  
  #include "access/transam.h"
  #include "access/xact.h"
  #include "storage/predicate.h"
  #include "storage/proc.h"
  #include "storage/procarray.h"
! #include "utils/memutils.h"
  #include "utils/memutils.h"
  #include "utils/snapmgr.h"
  #include "utils/tqual.h"
--- 25,42 ----
   */
  #include "postgres.h"
  
+ #include <sys/types.h>
+ #include <sys/stat.h>
+ #include <unistd.h>
+ 
  #include "access/transam.h"
  #include "access/xact.h"
+ #include "miscadmin.h"
+ #include "storage/fd.h"
  #include "storage/predicate.h"
  #include "storage/proc.h"
  #include "storage/procarray.h"
! #include "utils/builtins.h"
  #include "utils/memutils.h"
  #include "utils/snapmgr.h"
  #include "utils/tqual.h"
*************** static Snapshot CopySnapshot(Snapshot sn
*** 108,125 ****
  static void FreeSnapshot(Snapshot snapshot);
  static void SnapshotResetXmin(void);
  
  
  /*
!  * GetTransactionSnapshot
   *		Get the appropriate snapshot for a new query in a transaction.
   *
!  * Note that the return value may point at static storage that will be modified
!  * by future calls and by CommandCounterIncrement().  Callers should call
!  * RegisterSnapshot or PushActiveSnapshot on the returned snap if it is to be
!  * used very long.
   */
! Snapshot
! GetTransactionSnapshot(void)
  {
  	/* First call in transaction? */
  	if (!FirstSnapshotSet)
--- 114,137 ----
  static void FreeSnapshot(Snapshot snapshot);
  static void SnapshotResetXmin(void);
  
+ /* What we need for exporting snapshots */
+ #define SNAPSHOT_EXPORT_DIR "pg_snapshots"
+ #define XactExportFilePath(path, xid, num, suffix) \
+     snprintf(path, MAXPGPATH, SNAPSHOT_EXPORT_DIR "/%08X-%d%s", xid, num, suffix)
+ 
+ List *exportedSnapshots = NIL;
  
  /*
!  * GetTransactionSnapshotFromTemplate
   *		Get the appropriate snapshot for a new query in a transaction.
   *
!  * A template snapshot is passed for the synchronized snapshots feature.
!  * In that case we want to have a snapshot back that has the template's
!  * values. We just pass it along and the lower level functions take care
!  * of it.
   */
! static Snapshot
! GetTransactionSnapshotFromTemplate(Snapshot stemplate)
  {
  	/* First call in transaction? */
  	if (!FirstSnapshotSet)
*************** GetTransactionSnapshot(void)
*** 134,150 ****
  		if (IsolationUsesXactSnapshot())
  		{
  			if (IsolationIsSerializable())
! 				CurrentSnapshot = RegisterSerializableTransaction(&CurrentSnapshotData);
  			else
  			{
! 				CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);
  				CurrentSnapshot = RegisterSnapshotOnOwner(CurrentSnapshot,
  												TopTransactionResourceOwner);
  			}
  			registered_xact_snapshot = true;
  		}
  		else
! 			CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);
  
  		FirstSnapshotSet = true;
  		return CurrentSnapshot;
--- 146,170 ----
  		if (IsolationUsesXactSnapshot())
  		{
  			if (IsolationIsSerializable())
! 				CurrentSnapshot = RegisterSerializableTransaction(&CurrentSnapshotData,
! 																  stemplate);
  			else
  			{
! 				CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData, stemplate);
  				CurrentSnapshot = RegisterSnapshotOnOwner(CurrentSnapshot,
  												TopTransactionResourceOwner);
  			}
  			registered_xact_snapshot = true;
  		}
  		else
! 		{
! 			/*
! 			 * template is only used for the synchronized snapshot feature. Which in
! 			 * turn is only allowed for IsolationUsesXactSnapshot() == true transactions
! 			 */
! 			Assert(stemplate == InvalidSnapshot);
! 			CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData, InvalidSnapshot);
! 		}
  
  		FirstSnapshotSet = true;
  		return CurrentSnapshot;
*************** GetTransactionSnapshot(void)
*** 153,164 ****
  	if (IsolationUsesXactSnapshot())
  		return CurrentSnapshot;
  
! 	CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);
  
  	return CurrentSnapshot;
  }
  
  /*
   * GetLatestSnapshot
   *		Get a snapshot that is up-to-date as of the current instant,
   *		even if we are executing in transaction-snapshot mode.
--- 173,205 ----
  	if (IsolationUsesXactSnapshot())
  		return CurrentSnapshot;
  
! 	/* see comment above */
! 	Assert(stemplate == InvalidSnapshot);
! 	CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData, InvalidSnapshot);
  
  	return CurrentSnapshot;
  }
  
  /*
+  * GetTransactionSnapshot
+  *		Get the appropriate snapshot for a new query in a transaction.
+  *
+  * This is the public interface for anything different than the snapshot
+  * synchronization feature.
+  *
+  * Note that the return value may point at static storage that will be modified
+  * by future calls and by CommandCounterIncrement().  Callers should call
+  * RegisterSnapshot or PushActiveSnapshot on the returned snap if it is to be
+  * used very long.
+  */
+ Snapshot
+ GetTransactionSnapshot(void)
+ {
+ 	return GetTransactionSnapshotFromTemplate(InvalidSnapshot);
+ }
+ 
+ 
+ /*
   * GetLatestSnapshot
   *		Get a snapshot that is up-to-date as of the current instant,
   *		even if we are executing in transaction-snapshot mode.
*************** GetLatestSnapshot(void)
*** 170,176 ****
  	if (!FirstSnapshotSet)
  		elog(ERROR, "no snapshot has been set");
  
! 	SecondarySnapshot = GetSnapshotData(&SecondarySnapshotData);
  
  	return SecondarySnapshot;
  }
--- 211,217 ----
  	if (!FirstSnapshotSet)
  		elog(ERROR, "no snapshot has been set");
  
! 	SecondarySnapshot = GetSnapshotData(&SecondarySnapshotData, InvalidSnapshot);
  
  	return SecondarySnapshot;
  }
*************** SnapshotSetCommandId(CommandId curcid)
*** 192,234 ****
  }
  
  /*
!  * CopySnapshot
!  *		Copy the given snapshot.
   *
!  * The copy is palloc'd in TopTransactionContext and has initial refcounts set
!  * to 0.  The returned snapshot has the copied flag set.
   */
! static Snapshot
! CopySnapshot(Snapshot snapshot)
  {
- 	Snapshot	newsnap;
  	Size		subxipoff;
- 	Size		size;
- 
- 	Assert(snapshot != InvalidSnapshot);
  
! 	/* We allocate any XID arrays needed in the same palloc block. */
! 	size = subxipoff = sizeof(SnapshotData) +
! 		snapshot->xcnt * sizeof(TransactionId);
! 	if (snapshot->subxcnt > 0)
! 		size += snapshot->subxcnt * sizeof(TransactionId);
! 
! 	newsnap = (Snapshot) MemoryContextAlloc(TopTransactionContext, size);
! 	memcpy(newsnap, snapshot, sizeof(SnapshotData));
  
! 	newsnap->regd_count = 0;
! 	newsnap->active_count = 0;
! 	newsnap->copied = true;
  
  	/* setup XID array */
  	if (snapshot->xcnt > 0)
  	{
! 		newsnap->xip = (TransactionId *) (newsnap + 1);
! 		memcpy(newsnap->xip, snapshot->xip,
  			   snapshot->xcnt * sizeof(TransactionId));
  	}
  	else
! 		newsnap->xip = NULL;
  
  	/*
  	 * Setup subXID array. Don't bother to copy it if it had overflowed,
--- 233,265 ----
  }
  
  /*
!  * CopySnapshotOnto
!  *      Copy the given snapshot onto an already sufficiently allocated other
!  *      snapshot.
   *
!  * Return the modified snapshot (onto).
   */
! Snapshot
! CopySnapshotOnto(Snapshot snapshot, Snapshot onto)
  {
  	Size		subxipoff;
  
! 	subxipoff = sizeof(SnapshotData) + snapshot->xcnt * sizeof(TransactionId);
! 	memcpy(onto, snapshot, sizeof(SnapshotData));
  
! 	onto->regd_count = 0;
! 	onto->active_count = 0;
! 	onto->copied = true;
  
  	/* setup XID array */
  	if (snapshot->xcnt > 0)
  	{
! 		onto->xip = (TransactionId *) (onto + 1);
! 		memcpy(onto->xip, snapshot->xip,
  			   snapshot->xcnt * sizeof(TransactionId));
  	}
  	else
! 		onto->xip = NULL;
  
  	/*
  	 * Setup subXID array. Don't bother to copy it if it had overflowed,
*************** CopySnapshot(Snapshot snapshot)
*** 239,252 ****
  	if (snapshot->subxcnt > 0 &&
  		(!snapshot->suboverflowed || snapshot->takenDuringRecovery))
  	{
! 		newsnap->subxip = (TransactionId *) ((char *) newsnap + subxipoff);
! 		memcpy(newsnap->subxip, snapshot->subxip,
  			   snapshot->subxcnt * sizeof(TransactionId));
  	}
  	else
! 		newsnap->subxip = NULL;
  
! 	return newsnap;
  }
  
  /*
--- 270,308 ----
  	if (snapshot->subxcnt > 0 &&
  		(!snapshot->suboverflowed || snapshot->takenDuringRecovery))
  	{
! 		onto->subxip = (TransactionId *) ((char *) onto + subxipoff);
! 		memcpy(onto->subxip, snapshot->subxip,
  			   snapshot->subxcnt * sizeof(TransactionId));
  	}
  	else
! 		onto->subxip = NULL;
  
! 	return onto;
! }
! 
! /*
!  * CopySnapshot
!  *		Copy the given snapshot.
!  *
!  * The copy is palloc'd in TopTransactionContext and has initial refcounts set
!  * to 0.  The returned snapshot has the copied flag set.
!  */
! static Snapshot
! CopySnapshot(Snapshot snapshot)
! {
! 	Snapshot	newsnap;
! 	Size		size;
! 
! 	Assert(snapshot != InvalidSnapshot);
! 
! 	/* We allocate any XID arrays needed in the same palloc block. */
! 	size = sizeof(SnapshotData) + snapshot->xcnt * sizeof(TransactionId);
! 	if (snapshot->subxcnt > 0)
! 		size += snapshot->subxcnt * sizeof(TransactionId);
! 
! 	newsnap = (Snapshot) MemoryContextAlloc(TopTransactionContext, size);
! 
! 	return CopySnapshotOnto(snapshot, newsnap);
  }
  
  /*
*************** AtEOXact_Snapshot(bool isCommit)
*** 576,578 ****
--- 632,1026 ----
  	FirstSnapshotSet = false;
  	registered_xact_snapshot = false;
  }
+ 
+ /*
+  * PreCommit_Snapshot
+  *		Cleans up exported snapshots (this needs to happen before we update
+  *		our MyProc entry, hence it is in PreCommit).
+  */
+ void
+ PreCommit_Snapshot(void)
+ {
+ 	ListCell   *snapshot;
+ 	int			i;
+ 	char		buf[MAXPGPATH];
+ 
+ 	if (exportedSnapshots == NIL)
+ 		return;
+ 
+ 	Assert(list_length(exportedSnapshots) > 0);
+ 	Assert(TransactionIdIsValid(GetTopTransactionIdIfAny()));
+ 
+ 	for(i = 1; i <= list_length(exportedSnapshots); i++)
+ 	{
+ 		XactExportFilePath(buf, GetTopTransactionId(), i, "");
+ 		unlink(buf);
+ 	}
+ 
+ 	foreach(snapshot, exportedSnapshots)
+ 		UnregisterSnapshotFromOwner(lfirst(snapshot), TopTransactionResourceOwner);
+ 
+ 	exportedSnapshots = NIL;
+ }
+ 
+ /*
+  * DeleteAllExportedSnapshotFiles
+  *		Cleans up any files that have been left behind by a crashed backend
+  *		that had exported snapshots before it died.
+  */
+ void
+ DeleteAllExportedSnapshotFiles(void)
+ {
+ 	char		buf[MAXPGPATH];
+ 	DIR		   *s_dir;
+ 	struct dirent *s_de;
+ 
+ 	if (!(s_dir = AllocateDir(SNAPSHOT_EXPORT_DIR)))
+ 	{
+ 		/*
+ 		 * We really should have that directory in a sane cluster setup. But
+ 		 * then again if we don't it's not fatal enough to make it FATAL.
+ 		 */
+ 		elog(WARNING,
+ 			"could not open directory \"%s\": %m",
+ 			SNAPSHOT_EXPORT_DIR);
+ 		return;
+ 	}
+ 
+ 	while ((s_de = ReadDir(s_dir, SNAPSHOT_EXPORT_DIR)) != NULL)
+ 	{
+ 		if (strcmp(s_de->d_name, ".") == 0 ||
+ 			strcmp(s_de->d_name, "..") == 0)
+ 			continue;
+ 
+ 		snprintf(buf, MAXPGPATH, SNAPSHOT_EXPORT_DIR "/%s", s_de->d_name);
+ 		unlink(buf);
+ 	}
+ 	FreeDir(s_dir);
+ }
+ 
+ /*
+  * ExportSnapshot
+  *		Export the snapshot to a file so that other backends can import the same
+  *		snapshot.
+  *		Returns the token (the file name) that can be used to import this
+  *		snapshot.
+  */
+ static char *
+ ExportSnapshot(Snapshot snapshot)
+ {
+ #define SNAPSHOT_APPEND(x, y) (appendStringInfo(&buf, (x), (y)))
+ 	TransactionId *children, topXid;
+ 	FILE	   *f;
+ 	int			i;
+ 	int			nchildren;
+ 	MemoryContext oldcxt;
+ 	char		path[MAXPGPATH];
+ 	char		pathtmp[MAXPGPATH];
+ 	StringInfoData buf;
+ 
+ 	Assert(IsTransactionState());
+ 
+ 	/*
+ 	 * This will also assign a transaction id if we do not yet have one.
+ 	 */
+ 	topXid = GetTopTransactionId();
+ 
+ 	Assert(TransactionIdIsValid(GetTopTransactionIdIfAny()));
+ 
+ 	/*
+ 	 * We cannot export a snapshot from a subtransaction because in a
+ 	 * subtransaction we don't see our open subxip values in the snapshot so
+ 	 * they would be missing in the backend applying it.
+ 	 */
+ 	if (GetCurrentTransactionNestLevel() != 1)
+ 		ereport(ERROR,
+ 				(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
+ 				 errmsg("cannot export a snapshot from a subtransaction")));
+ 
+ 	/*
+ 	 * We do however see our already committed subxip values and add them to
+ 	 * the subxip array.
+ 	 */
+ 	nchildren = xactGetCommittedChildren(&children);
+ 
+ 	initStringInfo(&buf);
+ 
+ 	/* Write up all the data that we return */
+ 	SNAPSHOT_APPEND("xid:%d ", topXid);
+ 	SNAPSHOT_APPEND("xmi:%d ", snapshot->xmin);
+ 	SNAPSHOT_APPEND("xma:%d ", snapshot->xmax);
+ 	/* Include our own transaction ID into the count. */
+ 	SNAPSHOT_APPEND("xcnt:%d ", snapshot->xcnt + 1);
+ 	for (i = 0; i < snapshot->xcnt; i++)
+ 		SNAPSHOT_APPEND("xip:%d ", snapshot->xip[i]);
+ 	/*
+ 	 * Finally add our own XID, since by definition we will still be running
+ 	 * when the other transaction takes over the snapshot.
+ 	 */
+ 	SNAPSHOT_APPEND("xip:%d ", topXid);
+ 	if (snapshot->suboverflowed || snapshot->subxcnt + nchildren > TOTAL_MAX_CACHED_SUBXIDS)
+ 		SNAPSHOT_APPEND("sof:%d ", 1);
+ 	else
+ 	{
+ 		SNAPSHOT_APPEND("sxcnt:%d ", snapshot->subxcnt + nchildren);
+ 		for (i = 0; i < snapshot->subxcnt; i++)
+ 			SNAPSHOT_APPEND("sxp:%d ", snapshot->subxip[i]);
+ 		/* Add already committed subtransactions. */
+ 		for (i = 0; i < nchildren; i++)
+ 			SNAPSHOT_APPEND("sxp:%d ", children[i]);
+ 	}
+ 
+ 	/*
+ 	 * buf ends with a trailing space but we leave it in for simplicity. The
+ 	 * parsing routines also depend on it.
+ 	 */
+ 
+ 	/* Register the snapshot and add it to the list of exported snapshots */
+ 	snapshot = RegisterSnapshotOnOwner(snapshot, TopTransactionResourceOwner);
+ 
+ 	oldcxt = MemoryContextSwitchTo(TopTransactionContext);
+ 	exportedSnapshots = lappend(exportedSnapshots, snapshot);
+ 	MemoryContextSwitchTo(oldcxt);
+ 
+ 	XactExportFilePath(pathtmp, topXid, list_length(exportedSnapshots), ".tmp");
+ 	if (!(f = AllocateFile(pathtmp, PG_BINARY_W)))
+ 		ereport(ERROR,
+ 				(errcode_for_file_access(),
+ 				 errmsg("could not create file \"%s\": %m", pathtmp)));
+ 
+ 	if (fwrite(buf.data, buf.len, 1, f) != 1)
+ 		/* Aborting the transaction will also call FreeFile() */
+ 		ereport(ERROR,
+ 				(errcode_for_file_access(),
+ 				 errmsg("could not write to file \"%s\": %m", pathtmp)));
+ 
+ 	if (FreeFile(f))
+ 		ereport(ERROR,
+ 				(errcode_for_file_access(),
+ 				 errmsg("could not write to file \"%s\": %m", pathtmp)));
+ 
+ 	/*
+ 	 * Now that we have written everything into a .tmp file we rename the file
+ 	 * and remove the .tmp suffix. Our filename is predictable and we're
+ 	 * paranoid enough to not let us read a partially written file (we can't
+ 	 * read a .tmp file because this would fail the valid characters check in
+ 	 * ImportSnapshot).
+ 	 */
+ 	XactExportFilePath(path, topXid, list_length(exportedSnapshots), "");
+ 
+ 	if (rename(pathtmp, path) < 0)
+ 		ereport(WARNING,
+ 				(errcode_for_file_access(),
+ 				 errmsg("could not rename file \"%s\" to \"%s\": %m",
+ 						pathtmp, path)));
+ 
+ 	/*
+ 	 * The basename of the file is what we return from pg_export_snapshot().
+ 	 * It's already in path in a textual format and we know that the path
+ 	 * starts with SNAPSHOT_EXPORT_DIR. Skip over the prefix and over the
+ 	 * slash and pstrdup it to not return a local variable.
+ 	 */
+ 	return pstrdup(path + strlen(SNAPSHOT_EXPORT_DIR) + 1);
+ #undef SNAPSHOT_APPEND
+ }
+ 
+ /*
+  * Poor man's type independent parser. We only use it in the three functions
+  * below so there's no need to get ambitious about putting extra (x) around the
+  * arguments.
+  */
+ #define SNAPSHOT_PARSE(valFunc, inFunc, type, strpp, prfx, notfound)		\
+ 	do {																	\
+ 		char	   *n, *p = strstr(*strpp, prfx);							\
+ 		type		v;														\
+ 																			\
+ 		if (!p)																\
+ 			return notfound;												\
+ 		p += strlen(prfx);													\
+ 		n = strchr(p, ' ');													\
+ 		if (!n)																\
+ 			return notfound;												\
+ 		*n = '\0';															\
+ 		v = valFunc(DirectFunctionCall1(inFunc, CStringGetDatum(p)));		\
+ 		*strpp = n + 1;														\
+ 		return v;															\
+ 	} while (0);
+ 
+ static int
+ parseIntFromText(char **s, const char *prefix)
+ {
+ 	SNAPSHOT_PARSE(DatumGetInt32, int4in, int, s, prefix, 0);
+ }
+ 
+ static bool
+ parseBoolFromText(char **s, const char *prefix)
+ {
+ 	SNAPSHOT_PARSE(DatumGetInt32, int4in, bool, s, prefix, false);
+ }
+ 
+ static TransactionId
+ parseXactFromText(char **s, const char *prefix)
+ {
+ 	SNAPSHOT_PARSE(DatumGetTransactionId, xidin, TransactionId,
+ 				   s, prefix, InvalidTransactionId);
+ }
+ 
+ #undef SNAPSHOT_PARSE
+ 
+ /*
+  * ImportSnapshot
+  *      Import a previously exported snapshot. We expect that whatever we get
+  *      is a filename in SNAPSHOT_EXPORT_DIR. Load the snapshot from that file.
+  *      This is called from "START TRANSACTION (SNAPSHOT = 'foo')" so we always
+  *      start fresh from zero with respect to the transaction state that we
+  *      work on.  Returns true on success and false on failure.
+  */
+ bool
+ ImportSnapshot(char *idstr)
+ {
+ 	char		path[MAXPGPATH];
+ 	FILE	   *f;
+ 	int			i;
+ 	char	   *s;
+ 	struct stat	stat_buf;
+ 	int			sxcnt, xcnt;
+ 	TransactionId xid, origXid, myXid;
+ 	SnapshotData snapshot = {HeapTupleSatisfiesMVCC};
+ 
+ 	if (FirstSnapshotSet || GetCurrentTransactionNestLevel() != 1)
+ 		ereport(ERROR,
+ 			(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
+ 			 errmsg("SET TRANSACTION SNAPSHOT must be called before any query")));
+ 
+ 	/*
+ 	 * If we were in read committed mode then the next query would execute with a
+ 	 * new snapshot thus making this function call quite useless.
+ 	 */
+ 	if (!IsolationUsesXactSnapshot())
+ 		ereport(ERROR,
+ 			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+ 			 errmsg("A snapshot importing transaction must have ISOLATION "
+ 					"LEVEL SERIALIZABLE or ISOLATION LEVEL REPEATABLE READ")));
+ 
+ 	/* We're lucky to always start off from a pretty clean state */
+ 	Assert(IsTransactionState());
+ 	Assert(GetCurrentTransactionNestLevel() == 1);
+ 	Assert(GetTopTransactionIdIfAny() == InvalidTransactionId);
+ 	Assert(CurrentSnapshot == NULL);
+ 	Assert(SecondarySnapshot == NULL);
+ 	Assert(registered_xact_snapshot == false);
+ 
+ 	/* verify the identifier, only 0-9,A-F and a hyphen are allowed... */
+ 	s = idstr;
+ 	while (*s)
+ 	{
+ 		if (!isdigit(*s) && !(*s >= 'A' && *s <= 'F') && *s != '-')
+ 			return false;
+ 		s++;
+ 	}
+ 
+ 	/*
+ 	 * Assign a transaction id. We only do this to detect a possible
+ 	 * transaction id wraparound which is somewhere between unlikely
+ 	 * and impossible...
+ 	 */
+ 	myXid = GetTopTransactionId();
+ 
+ 	snprintf(path, MAXPGPATH, SNAPSHOT_EXPORT_DIR "/%s", idstr);
+ 
+ 	/* get the size of the file so that we know how much memory we need */
+ 	if (stat(path, &stat_buf) != 0)
+ 		return false;
+ 
+ 	if (!(f = AllocateFile(path, PG_BINARY_R)))
+ 		return false;
+ 
+ 	s = palloc(stat_buf.st_size + 1);
+ 	if (fread(s, stat_buf.st_size, 1, f) != 1)
+ 		return false;
+ 
+ 	s[stat_buf.st_size] = '\0';
+ 
+ 	FreeFile(f);
+ 
+ 	origXid = parseXactFromText(&s, "xid:");
+ 
+ 	snapshot.xmin = parseXactFromText(&s, "xmi:");
+ 	Assert(snapshot.xmin != InvalidTransactionId);
+ 	snapshot.xmax = parseXactFromText(&s, "xma:");
+ 	Assert(snapshot.xmax != InvalidTransactionId);
+ 
+ 	xcnt = parseIntFromText(&s, "xcnt:");
+ 	/*
+ 	 * This snapshot only serves as a template, there is no need for it to have
+ 	 * maxProcs entries, so let's make it just as large as we need it.
+ 	 */
+ 	snapshot.xip = palloc(xcnt * sizeof(TransactionId));
+ 
+ 	i = 0;
+ 	while ((xid = parseXactFromText(&s, "xip:")) != InvalidTransactionId)
+ 		snapshot.xip[i++] = xid;
+ 	snapshot.xcnt = i;
+ 	Assert(snapshot.xcnt == xcnt);
+ 
+ 	/*
+ 	 * We only write "sof:1" if the snapshot overflowed. If not, then there is
+ 	 * no "sof:x" entry at all and parseBoolFromText() will return false.
+ 	 */
+ 	snapshot.suboverflowed = parseBoolFromText(&s, "sof:");
+ 
+ 	if (!snapshot.suboverflowed)
+ 	{
+ 		sxcnt = parseIntFromText(&s, "sxcnt:");
+ 		snapshot.subxip = palloc(sxcnt * sizeof(TransactionId));
+ 
+ 		i = 0;
+ 		while ((xid = parseXactFromText(&s, "sxp:")) != InvalidTransactionId)
+ 			snapshot.subxip[i++] = xid;
+ 		snapshot.subxcnt = i;
+ 		Assert(snapshot.subxcnt == sxcnt);
+ 	} else {
+ 		snapshot.subxip = NULL;
+ 		snapshot.subxcnt = 0;
+ 	}
+ 
+ 	/* complete the snapshot data structure */
+ 	snapshot.curcid = 0;
+ 	snapshot.takenDuringRecovery = RecoveryInProgress();
+ 
+ 	/*
+ 	 * Note that MyProc->xmin can go backwards here. However this is safe
+ 	 * because the xmin we set here is the same as in the backend's proc->xmin
+ 	 * whose snapshot we are copying. At this very moment, anybody computing a
+ 	 * minimum will calculate at least this xmin as the overall xmin with or
+ 	 * without us setting MyProc->xmin to this value.
+ 	 */
+ 	LWLockAcquire(ProcArrayLock, LW_SHARED);
+ 	MyProc->xmin = snapshot.xmin;
+ 	LWLockRelease(ProcArrayLock);
+ 
+ 	/* bail out if the original transaction is not running anymore... */
+ 	if (!TransactionIdIsInProgress(origXid) || TransactionIdPrecedes(myXid, origXid))
+ 		return false;
+ 
+ 	/*
+ 	 * Install the snapshot as if we got it through GetTransactionSnapshot().
+ 	 * This will set up CurrentSnapshot and also set up the predicate locks for a
+ 	 * serializable transaction.
+ 	 */
+ 	GetTransactionSnapshotFromTemplate(&snapshot);
+ 	return true;
+ }
+ 
+ Datum
+ pg_export_snapshot(PG_FUNCTION_ARGS)
+ {
+ 	char	   *snapshotData;
+ 
+ 	RequireTransactionChain(true, "pg_export_snapshot()");
+ 
+ 	snapshotData = ExportSnapshot(GetTransactionSnapshot());
+ 	PG_RETURN_TEXT_P(cstring_to_text(snapshotData));
+ }
+ 
diff --git a/src/bin/initdb/initdb.c b/src/bin/initdb/initdb.c
index e535fda..0e981cd 100644
*** a/src/bin/initdb/initdb.c
--- b/src/bin/initdb/initdb.c
*************** main(int argc, char *argv[])
*** 2557,2562 ****
--- 2557,2563 ----
  		"pg_serial",
  		"pg_subtrans",
  		"pg_twophase",
+ 		"pg_snapshots",
  		"pg_multixact/members",
  		"pg_multixact/offsets",
  		"base",
diff --git a/src/include/access/twophase.h b/src/include/access/twophase.h
index 0019df5..4706f10 100644
*** a/src/include/access/twophase.h
--- b/src/include/access/twophase.h
***************
*** 22,30 ****
   */
  typedef struct GlobalTransactionData *GlobalTransaction;
  
- /* GUC variable */
- extern int	max_prepared_xacts;
- 
  extern Size TwoPhaseShmemSize(void);
  extern void TwoPhaseShmemInit(void);
  
--- 22,27 ----
diff --git a/src/include/catalog/pg_proc.h b/src/include/catalog/pg_proc.h
index 96f43fe..a4e0387 100644
*** a/src/include/catalog/pg_proc.h
--- b/src/include/catalog/pg_proc.h
*************** DATA(insert OID = 2171 ( pg_cancel_backe
*** 2853,2858 ****
--- 2853,2860 ----
  DESCR("cancel a server process' current query");
  DATA(insert OID = 2096 ( pg_terminate_backend		PGNSP PGUID 12 1 0 0 0 f f f t f v 1 0 16 "23" _null_ _null_ _null_ _null_ pg_terminate_backend _null_ _null_ _null_ ));
  DESCR("terminate a server process");
+ DATA(insert OID = 3122 ( pg_export_snapshot		PGNSP PGUID 12 1 0 0 0 f f f t f v 0 0 25 "" _null_ _null_ _null_ _null_ pg_export_snapshot _null_ _null_ _null_ ));
+ DESCR("export a snapshot");
  DATA(insert OID = 2172 ( pg_start_backup		PGNSP PGUID 12 1 0 0 0 f f f t f v 2 0 25 "25 16" _null_ _null_ _null_ _null_ pg_start_backup _null_ _null_ _null_ ));
  DESCR("prepare for taking an online backup");
  DATA(insert OID = 2173 ( pg_stop_backup			PGNSP PGUID 12 1 0 0 0 f f f t f v 0 0 25 "" _null_ _null_ _null_ _null_ pg_stop_backup _null_ _null_ _null_ ));
diff --git a/src/include/miscadmin.h b/src/include/miscadmin.h
index 9d19417..15326cf 100644
*** a/src/include/miscadmin.h
--- b/src/include/miscadmin.h
*************** extern PGDLLIMPORT int NBuffers;
*** 134,139 ****
--- 134,142 ----
  extern int	MaxBackends;
  extern int	MaxConnections;
  
+ /* GUC variable */
+ extern int	max_prepared_xacts;
+ 
  extern PGDLLIMPORT int MyProcPid;
  extern PGDLLIMPORT pg_time_t MyStartTime;
  extern PGDLLIMPORT struct Port *MyProcPort;
diff --git a/src/include/parser/kwlist.h b/src/include/parser/kwlist.h
index 12c2faf..3d170bc 100644
*** a/src/include/parser/kwlist.h
--- b/src/include/parser/kwlist.h
*************** PG_KEYWORD("show", SHOW, UNRESERVED_KEYW
*** 337,342 ****
--- 337,343 ----
  PG_KEYWORD("similar", SIMILAR, TYPE_FUNC_NAME_KEYWORD)
  PG_KEYWORD("simple", SIMPLE, UNRESERVED_KEYWORD)
  PG_KEYWORD("smallint", SMALLINT, COL_NAME_KEYWORD)
+ PG_KEYWORD("snapshot", SNAPSHOT, UNRESERVED_KEYWORD)
  PG_KEYWORD("some", SOME, RESERVED_KEYWORD)
  PG_KEYWORD("stable", STABLE, UNRESERVED_KEYWORD)
  PG_KEYWORD("standalone", STANDALONE_P, UNRESERVED_KEYWORD)
diff --git a/src/include/storage/predicate.h b/src/include/storage/predicate.h
index 5ddbc1d..f4f0303 100644
*** a/src/include/storage/predicate.h
--- b/src/include/storage/predicate.h
*************** extern void CheckPointPredicate(void);
*** 42,48 ****
  extern bool PageIsPredicateLocked(Relation relation, BlockNumber blkno);
  
  /* predicate lock maintenance */
! extern Snapshot RegisterSerializableTransaction(Snapshot snapshot);
  extern void RegisterPredicateLockingXid(TransactionId xid);
  extern void PredicateLockRelation(Relation relation, Snapshot snapshot);
  extern void PredicateLockPage(Relation relation, BlockNumber blkno, Snapshot snapshot);
--- 42,48 ----
  extern bool PageIsPredicateLocked(Relation relation, BlockNumber blkno);
  
  /* predicate lock maintenance */
! extern Snapshot RegisterSerializableTransaction(Snapshot snapshot, Snapshot stemplate);
  extern void RegisterPredicateLockingXid(TransactionId xid);
  extern void PredicateLockRelation(Relation relation, Snapshot snapshot);
  extern void PredicateLockPage(Relation relation, BlockNumber blkno, Snapshot snapshot);
diff --git a/src/include/storage/procarray.h b/src/include/storage/procarray.h
index a11d438..dfe57aa 100644
*** a/src/include/storage/procarray.h
--- b/src/include/storage/procarray.h
*************** extern void ExpireOldKnownAssignedTransa
*** 39,45 ****
  
  extern RunningTransactions GetRunningTransactionData(void);
  
! extern Snapshot GetSnapshotData(Snapshot snapshot);
  
  extern bool TransactionIdIsInProgress(TransactionId xid);
  extern bool TransactionIdIsActive(TransactionId xid);
--- 39,45 ----
  
  extern RunningTransactions GetRunningTransactionData(void);
  
! extern Snapshot GetSnapshotData(Snapshot snapshot, Snapshot stemplate);
  
  extern bool TransactionIdIsInProgress(TransactionId xid);
  extern bool TransactionIdIsActive(TransactionId xid);
*************** extern void XidCacheRemoveRunningXids(Tr
*** 69,72 ****
--- 69,78 ----
  						  int nxids, const TransactionId *xids,
  						  TransactionId latestXid);
  
+ /* Size of the ProcArray structure itself */
+ #define PROCARRAY_MAXPROCS	(MaxBackends + max_prepared_xacts)
+ 
+ #define TOTAL_MAX_CACHED_SUBXIDS \
+ 	((PGPROC_MAX_CACHED_SUBXIDS + 1) * PROCARRAY_MAXPROCS)
+ 
  #endif   /* PROCARRAY_H */
diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h
index c969a37..d25b1cd 100644
*** a/src/include/utils/snapmgr.h
--- b/src/include/utils/snapmgr.h
*************** extern TransactionId TransactionXmin;
*** 22,27 ****
--- 22,29 ----
  extern TransactionId RecentXmin;
  extern TransactionId RecentGlobalXmin;
  
+ extern List *exportedSnapshots;
+ 
  extern Snapshot GetTransactionSnapshot(void);
  extern Snapshot GetLatestSnapshot(void);
  extern void SnapshotSetCommandId(CommandId curcid);
*************** extern void UpdateActiveSnapshotCommandI
*** 32,46 ****
--- 34,54 ----
  extern void PopActiveSnapshot(void);
  extern Snapshot GetActiveSnapshot(void);
  extern bool ActiveSnapshotSet(void);
+ extern Snapshot CopySnapshotOnto(Snapshot onto, Snapshot snapshot);
  
  extern Snapshot RegisterSnapshot(Snapshot snapshot);
  extern void UnregisterSnapshot(Snapshot snapshot);
  extern Snapshot RegisterSnapshotOnOwner(Snapshot snapshot, ResourceOwner owner);
  extern void UnregisterSnapshotFromOwner(Snapshot snapshot, ResourceOwner owner);
  
+ extern void PreCommit_Snapshot(void);
  extern void AtSubCommit_Snapshot(int level);
  extern void AtSubAbort_Snapshot(int level);
  extern void AtEarlyCommit_Snapshot(void);
  extern void AtEOXact_Snapshot(bool isCommit);
  
+ extern Datum pg_export_snapshot(PG_FUNCTION_ARGS);
+ extern bool ImportSnapshot(char *idstr);
+ extern void DeleteAllExportedSnapshotFiles(void);
+ 
  #endif   /* SNAPMGR_H */

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 0b6a109..ce8653e 100644
*** a/doc/src/sgml/func.sgml
--- b/doc/src/sgml/func.sgml
*************** FOR EACH ROW EXECUTE PROCEDURE suppress_
*** 15023,15026 ****
--- 15023,15100 ----
          <xref linkend="SQL-CREATETRIGGER">.
      </para>
    </sect1>
+ 
+   <sect1 id="functions-snapshotsync">
+    <title>Snapshot Synchronization Functions</title>
+ 
+    <indexterm>
+      <primary>pg_export_snapshot</primary>
+    </indexterm>
+ 
+    <para>
+      <productname>PostgreSQL</> allows different sessions to synchronize their
+      snapshots. A database snapshot determines which data is visible to
+      the client that is using this snapshot. Synchronized snapshots are necessary when
+      two clients need to see the same content in the database. If these two clients
+      just connected to the database and opened their transactions, then they could
+      never be sure that there was no data modification right between both
+      connections.
+    </para>
+    <para>
+      As a solution, <productname>PostgreSQL</> offers the function
+      <function>pg_export_snapshot</> which saves the snapshot internally and
+      from then on until the end of the saving transaction, the snapshot can be
+      copied into a a new transaction with the <xref linkend="sql-set-transaction">
+      command to open a second transaction with the
+      exact same snapshot. Now both transactions are guaranteed to see the exact same
+      data even though they might have connected at different times.
+    </para>
+    <para>
+      Note that a snapshot can only be used to start a new transaction as long
+      as the transaction that originally saved it is held open. Also note that even
+      after the synchronization both clients still run their own independent
+      transactions. As a consequence, even though synchronized with respect to
+      reading pre-existing data, both transactions won't be able to see each other's
+      uncommitted data.
+    </para>
+    <table id="functions-snapshot-synchronization">
+     <title>Snapshot Synchronization Functions</title>
+     <tgroup cols="3">
+      <thead>
+       <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry>
+       </row>
+      </thead>
+ 
+      <tbody>
+       <row>
+        <entry>
+         <literal><function>pg_export_snapshot()</function></literal>
+        </entry>
+        <entry><type>text</type></entry>
+        <entry>Save the snapshot and return its identifier</entry>
+       </row>
+      </tbody>
+     </tgroup>
+    </table>
+ 
+    <para>
+       The function <function>pg_export_snapshot</> does not take an argument
+       and returns the snapshot's identifier as <type>text</type> data. Internally the
+       function will save the snapshot data to a file so that it can be retrieved
+       from a different backend process later on. Note that as soon as the
+       transaction ends, any saved snapshots become invalid and their
+       identifiers cannot be used in other transactions anymore. If the function
+       has been executed, the transaction cannot be prepared anymore with <xref
+       linkend="sql-prepare-transaction">.
+    </para>
+ <programlisting>
+ SELECT pg_export_snapshot();
+ 
+  pg_export_snapshot
+ --------------------
+  000003A1-1
+ (1 row)
+ </programlisting>
+   </sect1>
  </chapter>
+ 
diff --git a/doc/src/sgml/ref/set_transaction.sgml b/doc/src/sgml/ref/set_transaction.sgml
index e28a7e1..059b94b 100644
*** a/doc/src/sgml/ref/set_transaction.sgml
--- b/doc/src/sgml/ref/set_transaction.sgml
*************** SET SESSION CHARACTERISTICS AS TRANSACTI
*** 40,45 ****
--- 40,46 ----
      ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
      READ WRITE | READ ONLY
      [ NOT ] DEFERRABLE
+     SNAPSHOT <replaceable class="parameter">snapshot-id</replaceable>
  </synopsis>
   </refsynopsisdiv>
  
*************** SET SESSION CHARACTERISTICS AS TRANSACTI
*** 146,151 ****
--- 147,167 ----
     contributing to or being canceled by a serialization failure.  This mode
     is well suited for long-running reports or backups.
    </para>
+ 
+   <para>
+    The <literal>SNAPSHOT</literal> property allows a new transaction to run
+    with the same snapshot as an already running transaction.  A call to
+    <literal>pg_export_snapshot</literal> (see <xref
+    linkend="functions-snapshotsync">) must have been executed in the other
+    transaction. This function returns a snapshot id which must be passed as a
+    parameter to this command to create a second transaction running with the same
+    snapshot. You also need to make the transaction <literal>ISOLATION LEVEL
+    SERIALIZABLE</literal> or <literal>ISOLATION LEVEL REPEATABLE READ</literal>.
+    If the transaction has already executed a query, started a subtransaction or
+    assigned a snapshot, no further snapshot assignment is possible in this
+    transaction.
+   </para>
+ 
   </refsect1>
  
   <refsect1>
*************** SET SESSION CHARACTERISTICS AS TRANSACTI
*** 178,183 ****
--- 194,225 ----
    </para>
   </refsect1>
  
+  <refsect1>
+   <title>Examples</title>
+ 
+   <para>
+    To begin a new transaction block with the same snapshot as an already
+    existing transaction, first export the snapshot from the existing
+    transaction. This will return the snapshot id:
+ 
+ <programlisting>
+ # SELECT pg_export_snapshot();
+  pg_export_snapshot
+ --------------------
+  000003A1-1
+ (1 row)
+ </programlisting>
+ 
+    Then reference this snapshot id as the first command in the newly opened
+    transaction:
+ 
+ <programlisting>
+ BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
+ SET TRANSACTION SNAPSHOT '000003A1-1';
+ </programlisting>
+   </para>
+  </refsect1>
+ 
   <refsect1 id="R1-SQL-SET-TRANSACTION-3">
    <title>Compatibility</title>
  
*************** SET SESSION CHARACTERISTICS AS TRANSACTI
*** 198,206 ****
    </para>
  
    <para>
!    The <literal>DEFERRABLE</literal>
     <replaceable class="parameter">transaction_mode</replaceable>
!    is a <productname>PostgreSQL</productname> language extension.
    </para>
  
    <para>
--- 240,250 ----
    </para>
  
    <para>
!    <literal>DEFERRABLE</literal>
     <replaceable class="parameter">transaction_mode</replaceable>
!    and <literal>SNAPSHOT</literal> <replaceable
!    class="parameter">snapshot-id</replaceable> are
!    <productname>PostgreSQL</productname> language extensions.
    </para>
  
    <para>
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 3dab45c..7ce516e 100644
*** a/src/backend/access/transam/xact.c
--- b/src/backend/access/transam/xact.c
*************** CommitTransaction(void)
*** 1852,1857 ****
--- 1852,1863 ----
  	 */
  	PreCommit_Notify();
  
+ 	/*
+ 	 * Cleans up exported snapshots (this needs to happen before we update
+ 	 * our MyProc entry).
+ 	 */
+ 	InvalidateExportedSnapshots();
+ 
  	/* Prevent cancel/die interrupt while cleaning up */
  	HOLD_INTERRUPTS();
  
*************** PrepareTransaction(void)
*** 2067,2072 ****
--- 2073,2083 ----
  				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
  				 errmsg("cannot PREPARE a transaction that has operated on temporary tables")));
  
+ 	if (exportedSnapshots)
+ 		ereport(ERROR,
+ 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+ 				 errmsg("cannot PREPARE a transaction that has exported snapshots")));
+ 
  	/* Prevent cancel/die interrupt while cleaning up */
  	HOLD_INTERRUPTS();
  
*************** AbortTransaction(void)
*** 2228,2233 ****
--- 2239,2247 ----
  	/* Prevent cancel/die interrupt while cleaning up */
  	HOLD_INTERRUPTS();
  
+ 	/* Invalidate any exported snapshots */
+ 	InvalidateExportedSnapshots();
+ 
  	/* Make sure we have a valid memory context and resource owner */
  	AtAbort_Memory();
  	AtAbort_ResourceOwner();
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 8f65ddc..331a564 100644
*** a/src/backend/access/transam/xlog.c
--- b/src/backend/access/transam/xlog.c
***************
*** 58,63 ****
--- 58,64 ----
  #include "utils/guc.h"
  #include "utils/ps_status.h"
  #include "utils/relmapper.h"
+ #include "utils/snapmgr.h"
  #include "utils/timestamp.h"
  #include "pg_trace.h"
  
*************** StartupXLOG(void)
*** 6373,6378 ****
--- 6374,6384 ----
  		CheckRequiredParameterValues();
  
  		/*
+ 		 * We can delete any saved transaction snapshots that still exist
+ 		 */
+ 		DeleteAllExportedSnapshotFiles();
+ 
+ 		/*
  		 * We're in recovery, so unlogged relations relations may be trashed
  		 * and must be reset.  This should be done BEFORE allowing Hot Standby
  		 * connections, so that read-only backends don't try to read whatever
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index e9f3896..e1ab161 100644
*** a/src/backend/parser/gram.y
--- b/src/backend/parser/gram.y
*************** static void processCASbits(int cas_bits,
*** 553,560 ****
  
  	SAVEPOINT SCHEMA SCROLL SEARCH SECOND_P SECURITY SELECT SEQUENCE SEQUENCES
  	SERIALIZABLE SERVER SESSION SESSION_USER SET SETOF SHARE
! 	SHOW SIMILAR SIMPLE SMALLINT SOME STABLE STANDALONE_P START STATEMENT
! 	STATISTICS STDIN STDOUT STORAGE STRICT_P STRIP_P SUBSTRING
  	SYMMETRIC SYSID SYSTEM_P
  
  	TABLE TABLES TABLESPACE TEMP TEMPLATE TEMPORARY TEXT_P THEN TIME TIMESTAMP
--- 553,560 ----
  
  	SAVEPOINT SCHEMA SCROLL SEARCH SECOND_P SECURITY SELECT SEQUENCE SEQUENCES
  	SERIALIZABLE SERVER SESSION SESSION_USER SET SETOF SHARE
! 	SHOW SIMILAR SIMPLE SMALLINT SNAPSHOT SOME STABLE STANDALONE_P START
! 	STATEMENT STATISTICS STDIN STDOUT STORAGE STRICT_P STRIP_P SUBSTRING
  	SYMMETRIC SYSID SYSTEM_P
  
  	TABLE TABLES TABLESPACE TEMP TEMPLATE TEMPORARY TEXT_P THEN TIME TIMESTAMP
*************** set_rest:	/* Generic SET syntaxes: */
*** 1286,1291 ****
--- 1286,1299 ----
  					n->args = $2;
  					$$ = n;
  				}
+ 			| TRANSACTION SNAPSHOT Sconst
+ 				{
+ 					VariableSetStmt *n = makeNode(VariableSetStmt);
+ 					n->kind = VAR_SET_VALUE;
+ 					n->name = "TRANSACTION SNAPSHOT";
+ 					n->args = list_make1(makeStringConst($3, @3));
+ 					$$ = n;
+ 				}
  			| SESSION CHARACTERISTICS AS TRANSACTION transaction_mode_list
  				{
  					VariableSetStmt *n = makeNode(VariableSetStmt);
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 9489012..b665d75 100644
*** a/src/backend/storage/ipc/procarray.c
--- b/src/backend/storage/ipc/procarray.c
*************** ProcArrayShmemSize(void)
*** 166,174 ****
  {
  	Size		size;
  
- 	/* Size of the ProcArray structure itself */
- #define PROCARRAY_MAXPROCS	(MaxBackends + max_prepared_xacts)
- 
  	size = offsetof(ProcArrayStruct, procs);
  	size = add_size(size, mul_size(sizeof(PGPROC *), PROCARRAY_MAXPROCS));
  
--- 166,171 ----
*************** ProcArrayShmemSize(void)
*** 179,192 ****
  	 * TransactionIdIsInProgress() and GetRunningTransactionData(). All of the
  	 * main structures created in those functions must be identically sized,
  	 * since we may at times copy the whole of the data structures around. We
! 	 * refer to this size as TOTAL_MAX_CACHED_SUBXIDS.
  	 *
  	 * Ideally we'd only create this structure if we were actually doing hot
  	 * standby in the current run, but we don't know that yet at the time
  	 * shared memory is being set up.
  	 */
- #define TOTAL_MAX_CACHED_SUBXIDS \
- 	((PGPROC_MAX_CACHED_SUBXIDS + 1) * PROCARRAY_MAXPROCS)
  
  	if (EnableHotStandby)
  	{
--- 176,187 ----
  	 * TransactionIdIsInProgress() and GetRunningTransactionData(). All of the
  	 * main structures created in those functions must be identically sized,
  	 * since we may at times copy the whole of the data structures around. We
! 	 * refer to this size as TOTAL_MAX_CACHED_SUBXIDS, defined in procarray.h.
  	 *
  	 * Ideally we'd only create this structure if we were actually doing hot
  	 * standby in the current run, but we don't know that yet at the time
  	 * shared memory is being set up.
  	 */
  
  	if (EnableHotStandby)
  	{
*************** GetOldestXmin(bool allDbs, bool ignoreVa
*** 1144,1150 ****
   * not statically allocated (see xip allocation below).
   */
  Snapshot
! GetSnapshotData(Snapshot snapshot)
  {
  	ProcArrayStruct *arrayP = procArray;
  	TransactionId xmin;
--- 1139,1145 ----
   * not statically allocated (see xip allocation below).
   */
  Snapshot
! GetSnapshotData(Snapshot snapshot, Snapshot stemplate)
  {
  	ProcArrayStruct *arrayP = procArray;
  	TransactionId xmin;
*************** GetSnapshotData(Snapshot snapshot)
*** 1158,1163 ****
--- 1153,1181 ----
  	Assert(snapshot != NULL);
  
  	/*
+ 	 * We only get a valid snapshot in stemplate if the snapshot
+ 	 * synchronization feature used. In that case we just need to copy the
+ 	 * values that we get onto the snapshot we return.
+ 	 * Note that in this case we always duplicate an existing snapshot, that is
+ 	 * currently held by another active transaction. That's why we do not need
+ 	 * to update any { RecentGlobalXmin, RecentXmin, globalxmin }.
+ 	 */
+ 	if (stemplate != InvalidSnapshot)
+ 	{
+ 		/*
+ 		 * 'stemplate' is only read and its values are copied onto 'snapshot'.
+ 		 */
+ 		CopySnapshotOnto(stemplate, snapshot);
+ 
+ 		/*
+ 		 * We can use the result of the copy except for that this snapshot
+ 		 * should look like new and not copied.
+ 		 */
+ 		snapshot->copied = false;
+ 		return snapshot;
+ 	}
+ 
+ 	/*
  	 * Allocating space for maxProcs xids is usually overkill; numProcs would
  	 * be sufficient.  But it seems better to do the malloc while not holding
  	 * the lock, so we can't look at numProcs.  Likewise, we allocate much
diff --git a/src/backend/storage/lmgr/predicate.c b/src/backend/storage/lmgr/predicate.c
index d39f897..462cc59 100644
*** a/src/backend/storage/lmgr/predicate.c
--- b/src/backend/storage/lmgr/predicate.c
*************** static void OldSerXidSetActiveSerXmin(Tr
*** 416,423 ****
  
  static uint32 predicatelock_hash(const void *key, Size keysize);
  static void SummarizeOldestCommittedSxact(void);
! static Snapshot GetSafeSnapshot(Snapshot snapshot);
! static Snapshot GetSerializableTransactionSnapshotInt(Snapshot snapshot);
  static bool PredicateLockExists(const PREDICATELOCKTARGETTAG *targettag);
  static bool GetParentPredicateLockTag(const PREDICATELOCKTARGETTAG *tag,
  						  PREDICATELOCKTARGETTAG *parent);
--- 416,423 ----
  
  static uint32 predicatelock_hash(const void *key, Size keysize);
  static void SummarizeOldestCommittedSxact(void);
! static Snapshot GetSafeSnapshot(Snapshot snapshot, Snapshot stemplate);
! static Snapshot GetSerializableTransactionSnapshotInt(Snapshot snapshot, Snapshot stemplate);
  static bool PredicateLockExists(const PREDICATELOCKTARGETTAG *targettag);
  static bool GetParentPredicateLockTag(const PREDICATELOCKTARGETTAG *tag,
  						  PREDICATELOCKTARGETTAG *parent);
*************** SummarizeOldestCommittedSxact(void)
*** 1491,1497 ****
   *		area that can safely be passed to GetSnapshotData.
   */
  static Snapshot
! GetSafeSnapshot(Snapshot origSnapshot)
  {
  	Snapshot	snapshot;
  
--- 1491,1497 ----
   *		area that can safely be passed to GetSnapshotData.
   */
  static Snapshot
! GetSafeSnapshot(Snapshot origSnapshot, Snapshot stemplate)
  {
  	Snapshot	snapshot;
  
*************** GetSafeSnapshot(Snapshot origSnapshot)
*** 1505,1511 ****
  		 * our caller passed to us.  The pointer returned is actually the same
  		 * one passed to it, but we avoid assuming that here.
  		 */
! 		snapshot = GetSerializableTransactionSnapshotInt(origSnapshot);
  
  		if (MySerializableXact == InvalidSerializableXact)
  			return snapshot;	/* no concurrent r/w xacts; it's safe */
--- 1505,1511 ----
  		 * our caller passed to us.  The pointer returned is actually the same
  		 * one passed to it, but we avoid assuming that here.
  		 */
! 		snapshot = GetSerializableTransactionSnapshotInt(origSnapshot, stemplate);
  
  		if (MySerializableXact == InvalidSerializableXact)
  			return snapshot;	/* no concurrent r/w xacts; it's safe */
*************** GetSafeSnapshot(Snapshot origSnapshot)
*** 1562,1568 ****
   * within this function.
   */
  Snapshot
! GetSerializableTransactionSnapshot(Snapshot snapshot)
  {
  	Assert(IsolationIsSerializable());
  
--- 1562,1568 ----
   * within this function.
   */
  Snapshot
! GetSerializableTransactionSnapshot(Snapshot snapshot, Snapshot stemplate)
  {
  	Assert(IsolationIsSerializable());
  
*************** GetSerializableTransactionSnapshot(Snaps
*** 1572,1584 ****
  	 * thereby avoid all SSI overhead once it's running.
  	 */
  	if (XactReadOnly && XactDeferrable)
! 		return GetSafeSnapshot(snapshot);
  
! 	return GetSerializableTransactionSnapshotInt(snapshot);
  }
  
  static Snapshot
! GetSerializableTransactionSnapshotInt(Snapshot snapshot)
  {
  	PGPROC	   *proc;
  	VirtualTransactionId vxid;
--- 1572,1584 ----
  	 * thereby avoid all SSI overhead once it's running.
  	 */
  	if (XactReadOnly && XactDeferrable)
! 		return GetSafeSnapshot(snapshot, stemplate);
  
! 	return GetSerializableTransactionSnapshotInt(snapshot, stemplate);
  }
  
  static Snapshot
! GetSerializableTransactionSnapshotInt(Snapshot snapshot, Snapshot stemplate)
  {
  	PGPROC	   *proc;
  	VirtualTransactionId vxid;
*************** GetSerializableTransactionSnapshotInt(Sn
*** 1616,1622 ****
  	} while (!sxact);
  
  	/* Get the snapshot */
! 	snapshot = GetSnapshotData(snapshot);
  
  	/*
  	 * If there are no serializable transactions which are not read-only, we
--- 1616,1622 ----
  	} while (!sxact);
  
  	/* Get the snapshot */
! 	snapshot = GetSnapshotData(snapshot, stemplate);
  
  	/*
  	 * If there are no serializable transactions which are not read-only, we
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index a71729c..9ffe3a1 100644
*** a/src/backend/utils/misc/guc.c
--- b/src/backend/utils/misc/guc.c
***************
*** 72,77 ****
--- 72,78 ----
  #include "utils/plancache.h"
  #include "utils/portal.h"
  #include "utils/ps_status.h"
+ #include "utils/snapmgr.h"
  #include "utils/tzparser.h"
  #include "utils/xml.h"
  
*************** ExecSetVariableStmt(VariableSetStmt *stm
*** 6094,6099 ****
--- 6095,6109 ----
  	switch (stmt->kind)
  	{
  		case VAR_SET_VALUE:
+ 			if (strcmp(stmt->name, "TRANSACTION SNAPSHOT") == 0)
+ 			{
+ 				if (!ImportSnapshot(ExtractSetVariableArgs(stmt)))
+ 					ereport(ERROR,
+ 							(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ 							 errmsg("could not import the requested snapshot")));
+ 				break;
+ 			}
+ 			/* fallthrough */
  		case VAR_SET_CURRENT:
  			set_config_option(stmt->name,
  							  ExtractSetVariableArgs(stmt),
diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index 518aaf1..d047ba1 100644
*** a/src/backend/utils/time/snapmgr.c
--- b/src/backend/utils/time/snapmgr.c
***************
*** 33,44 ****
   */
  #include "postgres.h"
  
  #include "access/transam.h"
  #include "access/xact.h"
  #include "storage/predicate.h"
  #include "storage/proc.h"
  #include "storage/procarray.h"
! #include "utils/memutils.h"
  #include "utils/memutils.h"
  #include "utils/snapmgr.h"
  #include "utils/tqual.h"
--- 33,50 ----
   */
  #include "postgres.h"
  
+ #include <sys/types.h>
+ #include <sys/stat.h>
+ #include <unistd.h>
+ 
  #include "access/transam.h"
  #include "access/xact.h"
+ #include "miscadmin.h"
+ #include "storage/fd.h"
  #include "storage/predicate.h"
  #include "storage/proc.h"
  #include "storage/procarray.h"
! #include "utils/builtins.h"
  #include "utils/memutils.h"
  #include "utils/snapmgr.h"
  #include "utils/tqual.h"
*************** static Snapshot CopySnapshot(Snapshot sn
*** 116,133 ****
  static void FreeSnapshot(Snapshot snapshot);
  static void SnapshotResetXmin(void);
  
  
  /*
!  * GetTransactionSnapshot
   *		Get the appropriate snapshot for a new query in a transaction.
   *
!  * Note that the return value may point at static storage that will be modified
!  * by future calls and by CommandCounterIncrement().  Callers should call
!  * RegisterSnapshot or PushActiveSnapshot on the returned snap if it is to be
!  * used very long.
   */
! Snapshot
! GetTransactionSnapshot(void)
  {
  	/* First call in transaction? */
  	if (!FirstSnapshotSet)
--- 122,145 ----
  static void FreeSnapshot(Snapshot snapshot);
  static void SnapshotResetXmin(void);
  
+ /* What we need for exporting snapshots */
+ #define SNAPSHOT_EXPORT_DIR "pg_snapshots"
+ #define XactExportFilePath(path, xid, num, suffix) \
+     snprintf(path, MAXPGPATH, SNAPSHOT_EXPORT_DIR "/%08X-%d%s", xid, num, suffix)
+ 
+ List *exportedSnapshots = NIL;
  
  /*
!  * GetTransactionSnapshotFromTemplate
   *		Get the appropriate snapshot for a new query in a transaction.
   *
!  * A template snapshot is passed for the synchronized snapshots feature.
!  * In that case we want to have a snapshot back that has the template's
!  * values. We just pass it along and the lower level functions take care
!  * of it.
   */
! static Snapshot
! GetTransactionSnapshotFromTemplate(Snapshot stemplate)
  {
  	/* First call in transaction? */
  	if (!FirstSnapshotSet)
*************** GetTransactionSnapshot(void)
*** 145,153 ****
  		{
  			/* First, create the snapshot in CurrentSnapshotData */
  			if (IsolationIsSerializable())
! 				CurrentSnapshot = GetSerializableTransactionSnapshot(&CurrentSnapshotData);
  			else
! 				CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);
  			/* Make a saved copy */
  			CurrentSnapshot = CopySnapshot(CurrentSnapshot);
  			FirstXactSnapshot = CurrentSnapshot;
--- 157,166 ----
  		{
  			/* First, create the snapshot in CurrentSnapshotData */
  			if (IsolationIsSerializable())
! 				CurrentSnapshot = GetSerializableTransactionSnapshot(&CurrentSnapshotData,
! 																	 stemplate);
  			else
! 				CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData, stemplate);
  			/* Make a saved copy */
  			CurrentSnapshot = CopySnapshot(CurrentSnapshot);
  			FirstXactSnapshot = CurrentSnapshot;
*************** GetTransactionSnapshot(void)
*** 156,162 ****
  			RegisteredSnapshots++;
  		}
  		else
! 			CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);
  
  		FirstSnapshotSet = true;
  		return CurrentSnapshot;
--- 169,182 ----
  			RegisteredSnapshots++;
  		}
  		else
! 		{
! 			/*
! 			 * template is only used for the synchronized snapshot feature. Which in
! 			 * turn is only allowed for IsolationUsesXactSnapshot() == true transactions
! 			 */
! 			Assert(stemplate == InvalidSnapshot);
! 			CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData, InvalidSnapshot);
! 		}
  
  		FirstSnapshotSet = true;
  		return CurrentSnapshot;
*************** GetTransactionSnapshot(void)
*** 165,176 ****
  	if (IsolationUsesXactSnapshot())
  		return CurrentSnapshot;
  
! 	CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);
  
  	return CurrentSnapshot;
  }
  
  /*
   * GetLatestSnapshot
   *		Get a snapshot that is up-to-date as of the current instant,
   *		even if we are executing in transaction-snapshot mode.
--- 185,217 ----
  	if (IsolationUsesXactSnapshot())
  		return CurrentSnapshot;
  
! 	/* see comment above */
! 	Assert(stemplate == InvalidSnapshot);
! 	CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData, InvalidSnapshot);
  
  	return CurrentSnapshot;
  }
  
  /*
+  * GetTransactionSnapshot
+  *		Get the appropriate snapshot for a new query in a transaction.
+  *
+  * This is the public interface for anything different than the snapshot
+  * synchronization feature.
+  *
+  * Note that the return value may point at static storage that will be modified
+  * by future calls and by CommandCounterIncrement().  Callers should call
+  * RegisterSnapshot or PushActiveSnapshot on the returned snap if it is to be
+  * used very long.
+  */
+ Snapshot
+ GetTransactionSnapshot(void)
+ {
+ 	return GetTransactionSnapshotFromTemplate(InvalidSnapshot);
+ }
+ 
+ 
+ /*
   * GetLatestSnapshot
   *		Get a snapshot that is up-to-date as of the current instant,
   *		even if we are executing in transaction-snapshot mode.
*************** GetLatestSnapshot(void)
*** 182,188 ****
  	if (!FirstSnapshotSet)
  		elog(ERROR, "no snapshot has been set");
  
! 	SecondarySnapshot = GetSnapshotData(&SecondarySnapshotData);
  
  	return SecondarySnapshot;
  }
--- 223,229 ----
  	if (!FirstSnapshotSet)
  		elog(ERROR, "no snapshot has been set");
  
! 	SecondarySnapshot = GetSnapshotData(&SecondarySnapshotData, InvalidSnapshot);
  
  	return SecondarySnapshot;
  }
*************** SnapshotSetCommandId(CommandId curcid)
*** 204,246 ****
  }
  
  /*
!  * CopySnapshot
!  *		Copy the given snapshot.
   *
!  * The copy is palloc'd in TopTransactionContext and has initial refcounts set
!  * to 0.  The returned snapshot has the copied flag set.
   */
! static Snapshot
! CopySnapshot(Snapshot snapshot)
  {
- 	Snapshot	newsnap;
  	Size		subxipoff;
- 	Size		size;
- 
- 	Assert(snapshot != InvalidSnapshot);
- 
- 	/* We allocate any XID arrays needed in the same palloc block. */
- 	size = subxipoff = sizeof(SnapshotData) +
- 		snapshot->xcnt * sizeof(TransactionId);
- 	if (snapshot->subxcnt > 0)
- 		size += snapshot->subxcnt * sizeof(TransactionId);
  
! 	newsnap = (Snapshot) MemoryContextAlloc(TopTransactionContext, size);
! 	memcpy(newsnap, snapshot, sizeof(SnapshotData));
  
! 	newsnap->regd_count = 0;
! 	newsnap->active_count = 0;
! 	newsnap->copied = true;
  
  	/* setup XID array */
  	if (snapshot->xcnt > 0)
  	{
! 		newsnap->xip = (TransactionId *) (newsnap + 1);
! 		memcpy(newsnap->xip, snapshot->xip,
  			   snapshot->xcnt * sizeof(TransactionId));
  	}
  	else
! 		newsnap->xip = NULL;
  
  	/*
  	 * Setup subXID array. Don't bother to copy it if it had overflowed,
--- 245,277 ----
  }
  
  /*
!  * CopySnapshotOnto
!  *      Copy the given snapshot onto an already sufficiently allocated other
!  *      snapshot.
   *
!  * Return the modified snapshot (onto).
   */
! Snapshot
! CopySnapshotOnto(Snapshot snapshot, Snapshot onto)
  {
  	Size		subxipoff;
  
! 	subxipoff = sizeof(SnapshotData) + snapshot->xcnt * sizeof(TransactionId);
! 	memcpy(onto, snapshot, sizeof(SnapshotData));
  
! 	onto->regd_count = 0;
! 	onto->active_count = 0;
! 	onto->copied = true;
  
  	/* setup XID array */
  	if (snapshot->xcnt > 0)
  	{
! 		onto->xip = (TransactionId *) (onto + 1);
! 		memcpy(onto->xip, snapshot->xip,
  			   snapshot->xcnt * sizeof(TransactionId));
  	}
  	else
! 		onto->xip = NULL;
  
  	/*
  	 * Setup subXID array. Don't bother to copy it if it had overflowed,
*************** CopySnapshot(Snapshot snapshot)
*** 251,264 ****
  	if (snapshot->subxcnt > 0 &&
  		(!snapshot->suboverflowed || snapshot->takenDuringRecovery))
  	{
! 		newsnap->subxip = (TransactionId *) ((char *) newsnap + subxipoff);
! 		memcpy(newsnap->subxip, snapshot->subxip,
  			   snapshot->subxcnt * sizeof(TransactionId));
  	}
  	else
! 		newsnap->subxip = NULL;
  
! 	return newsnap;
  }
  
  /*
--- 282,320 ----
  	if (snapshot->subxcnt > 0 &&
  		(!snapshot->suboverflowed || snapshot->takenDuringRecovery))
  	{
! 		onto->subxip = (TransactionId *) ((char *) onto + subxipoff);
! 		memcpy(onto->subxip, snapshot->subxip,
  			   snapshot->subxcnt * sizeof(TransactionId));
  	}
  	else
! 		onto->subxip = NULL;
  
! 	return onto;
! }
! 
! /*
!  * CopySnapshot
!  *		Copy the given snapshot.
!  *
!  * The copy is palloc'd in TopTransactionContext and has initial refcounts set
!  * to 0.  The returned snapshot has the copied flag set.
!  */
! static Snapshot
! CopySnapshot(Snapshot snapshot)
! {
! 	Snapshot	newsnap;
! 	Size		size;
! 
! 	Assert(snapshot != InvalidSnapshot);
! 
! 	/* We allocate any XID arrays needed in the same palloc block. */
! 	size = sizeof(SnapshotData) + snapshot->xcnt * sizeof(TransactionId);
! 	if (snapshot->subxcnt > 0)
! 		size += snapshot->subxcnt * sizeof(TransactionId);
! 
! 	newsnap = (Snapshot) MemoryContextAlloc(TopTransactionContext, size);
! 
! 	return CopySnapshotOnto(snapshot, newsnap);
  }
  
  /*
*************** AtEOXact_Snapshot(bool isCommit)
*** 586,588 ****
--- 642,1039 ----
  
  	SnapshotResetXmin();
  }
+ 
+ /*
+  * PreCommit_Snapshot
+  *		Cleans up exported snapshots (this needs to happen before we update
+  *		our MyProc entry, hence it is in PreCommit).
+  */
+ void
+ InvalidateExportedSnapshots(void)
+ {
+ 	ListCell   *snapshot;
+ 	int			i;
+ 	char		buf[MAXPGPATH];
+ 
+ 	if (exportedSnapshots == NIL)
+ 		return;
+ 
+ 	Assert(list_length(exportedSnapshots) > 0);
+ 	Assert(TransactionIdIsValid(GetTopTransactionIdIfAny()));
+ 
+ 	for(i = 1; i <= list_length(exportedSnapshots); i++)
+ 	{
+ 		XactExportFilePath(buf, GetTopTransactionId(), i, "");
+ 		unlink(buf);
+ 	}
+ 
+ 	foreach(snapshot, exportedSnapshots)
+ 		UnregisterSnapshotFromOwner(lfirst(snapshot), TopTransactionResourceOwner);
+ 
+ 	exportedSnapshots = NIL;
+ }
+ 
+ /*
+  * DeleteAllExportedSnapshotFiles
+  *		Cleans up any files that have been left behind by a crashed backend
+  *		that had exported snapshots before it died.
+  */
+ void
+ DeleteAllExportedSnapshotFiles(void)
+ {
+ 	char		buf[MAXPGPATH];
+ 	DIR		   *s_dir;
+ 	struct dirent *s_de;
+ 
+ 	if (!(s_dir = AllocateDir(SNAPSHOT_EXPORT_DIR)))
+ 	{
+ 		/*
+ 		 * We really should have that directory in a sane cluster setup. But
+ 		 * then again if we don't it's not fatal enough to make it FATAL.
+ 		 */
+ 		elog(WARNING,
+ 			"could not open directory \"%s\": %m",
+ 			SNAPSHOT_EXPORT_DIR);
+ 		return;
+ 	}
+ 
+ 	while ((s_de = ReadDir(s_dir, SNAPSHOT_EXPORT_DIR)) != NULL)
+ 	{
+ 		if (strcmp(s_de->d_name, ".") == 0 ||
+ 			strcmp(s_de->d_name, "..") == 0)
+ 			continue;
+ 
+ 		snprintf(buf, MAXPGPATH, SNAPSHOT_EXPORT_DIR "/%s", s_de->d_name);
+ 		unlink(buf);
+ 	}
+ 	FreeDir(s_dir);
+ }
+ 
+ /*
+  * ExportSnapshot
+  *		Export the snapshot to a file so that other backends can import the same
+  *		snapshot.
+  *		Returns the token (the file name) that can be used to import this
+  *		snapshot.
+  */
+ static char *
+ ExportSnapshot(Snapshot snapshot)
+ {
+ #define SNAPSHOT_APPEND(x, y) (appendStringInfo(&buf, (x), (y)))
+ 	TransactionId *children, topXid;
+ 	FILE	   *f;
+ 	int			i;
+ 	int			nchildren;
+ 	MemoryContext oldcxt;
+ 	char		path[MAXPGPATH];
+ 	char		pathtmp[MAXPGPATH];
+ 	StringInfoData buf;
+ 
+ 	Assert(IsTransactionState());
+ 
+ 	/*
+ 	 * This will also assign a transaction id if we do not yet have one.
+ 	 */
+ 	topXid = GetTopTransactionId();
+ 
+ 	Assert(TransactionIdIsValid(GetTopTransactionIdIfAny()));
+ 
+ 	/*
+ 	 * We cannot export a snapshot from a subtransaction because in a
+ 	 * subtransaction we don't see our open subxip values in the snapshot so
+ 	 * they would be missing in the backend applying it.
+ 	 */
+ 	if (GetCurrentTransactionNestLevel() != 1)
+ 		ereport(ERROR,
+ 				(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
+ 				 errmsg("cannot export a snapshot from a subtransaction")));
+ 
+ 	/*
+ 	 * We do however see our already committed subxip values and add them to
+ 	 * the subxip array.
+ 	 */
+ 	nchildren = xactGetCommittedChildren(&children);
+ 
+ 	initStringInfo(&buf);
+ 
+ 	/* Write up all the data that we return */
+ 	SNAPSHOT_APPEND("xid:%d ", topXid);
+ 	SNAPSHOT_APPEND("xmi:%d ", snapshot->xmin);
+ 	SNAPSHOT_APPEND("xma:%d ", snapshot->xmax);
+ 	/* Include our own transaction ID into the count. */
+ 	SNAPSHOT_APPEND("xcnt:%d ", snapshot->xcnt + 1);
+ 	for (i = 0; i < snapshot->xcnt; i++)
+ 		SNAPSHOT_APPEND("xip:%d ", snapshot->xip[i]);
+ 	/*
+ 	 * Finally add our own XID, since by definition we will still be running
+ 	 * when the other transaction takes over the snapshot.
+ 	 */
+ 	SNAPSHOT_APPEND("xip:%d ", topXid);
+ 	if (snapshot->suboverflowed || snapshot->subxcnt + nchildren > TOTAL_MAX_CACHED_SUBXIDS)
+ 		SNAPSHOT_APPEND("sof:%d ", 1);
+ 	else
+ 	{
+ 		SNAPSHOT_APPEND("sxcnt:%d ", snapshot->subxcnt + nchildren);
+ 		for (i = 0; i < snapshot->subxcnt; i++)
+ 			SNAPSHOT_APPEND("sxp:%d ", snapshot->subxip[i]);
+ 		/* Add already committed subtransactions. */
+ 		for (i = 0; i < nchildren; i++)
+ 			SNAPSHOT_APPEND("sxp:%d ", children[i]);
+ 	}
+ 
+ 	/*
+ 	 * buf ends with a trailing space but we leave it in for simplicity. The
+ 	 * parsing routines also depend on it.
+ 	 */
+ 
+ 	/* Register the snapshot and add it to the list of exported snapshots */
+ 	snapshot = RegisterSnapshotOnOwner(snapshot, TopTransactionResourceOwner);
+ 
+ 	oldcxt = MemoryContextSwitchTo(TopTransactionContext);
+ 	exportedSnapshots = lappend(exportedSnapshots, snapshot);
+ 	MemoryContextSwitchTo(oldcxt);
+ 
+ 	XactExportFilePath(pathtmp, topXid, list_length(exportedSnapshots), ".tmp");
+ 	if (!(f = AllocateFile(pathtmp, PG_BINARY_W)))
+ 		ereport(ERROR,
+ 				(errcode_for_file_access(),
+ 				 errmsg("could not create file \"%s\": %m", pathtmp)));
+ 
+ 	if (fwrite(buf.data, buf.len, 1, f) != 1)
+ 		/* Aborting the transaction will also call FreeFile() */
+ 		ereport(ERROR,
+ 				(errcode_for_file_access(),
+ 				 errmsg("could not write to file \"%s\": %m", pathtmp)));
+ 
+ 	if (FreeFile(f))
+ 		ereport(ERROR,
+ 				(errcode_for_file_access(),
+ 				 errmsg("could not write to file \"%s\": %m", pathtmp)));
+ 
+ 	/*
+ 	 * Now that we have written everything into a .tmp file we rename the file
+ 	 * and remove the .tmp suffix. Our filename is predictable and we're
+ 	 * paranoid enough to not let us read a partially written file (we can't
+ 	 * read a .tmp file because this would fail the valid characters check in
+ 	 * ImportSnapshot).
+ 	 */
+ 	XactExportFilePath(path, topXid, list_length(exportedSnapshots), "");
+ 
+ 	if (rename(pathtmp, path) < 0)
+ 		ereport(WARNING,
+ 				(errcode_for_file_access(),
+ 				 errmsg("could not rename file \"%s\" to \"%s\": %m",
+ 						pathtmp, path)));
+ 
+ 	/*
+ 	 * The basename of the file is what we return from pg_export_snapshot().
+ 	 * It's already in path in a textual format and we know that the path
+ 	 * starts with SNAPSHOT_EXPORT_DIR. Skip over the prefix and over the
+ 	 * slash and pstrdup it to not return a local variable.
+ 	 */
+ 	return pstrdup(path + strlen(SNAPSHOT_EXPORT_DIR) + 1);
+ #undef SNAPSHOT_APPEND
+ }
+ 
+ /*
+  * Poor man's type independent parser. We only use it in the three functions
+  * below so there's no need to get ambitious about putting extra (x) around the
+  * arguments.
+  */
+ #define SNAPSHOT_PARSE(valFunc, inFunc, type, strpp, prfx, notfound)		\
+ 	do {																	\
+ 		char	   *n, *p = strstr(*strpp, prfx);							\
+ 		type		v;														\
+ 																			\
+ 		if (!p)																\
+ 			return notfound;												\
+ 		p += strlen(prfx);													\
+ 		n = strchr(p, ' ');													\
+ 		if (!n)																\
+ 			return notfound;												\
+ 		*n = '\0';															\
+ 		v = valFunc(DirectFunctionCall1(inFunc, CStringGetDatum(p)));		\
+ 		*strpp = n + 1;														\
+ 		return v;															\
+ 	} while (0);
+ 
+ static int
+ parseIntFromText(char **s, const char *prefix)
+ {
+ 	SNAPSHOT_PARSE(DatumGetInt32, int4in, int, s, prefix, 0);
+ }
+ 
+ static bool
+ parseBoolFromText(char **s, const char *prefix)
+ {
+ 	SNAPSHOT_PARSE(DatumGetInt32, int4in, bool, s, prefix, false);
+ }
+ 
+ static TransactionId
+ parseXactFromText(char **s, const char *prefix)
+ {
+ 	SNAPSHOT_PARSE(DatumGetTransactionId, xidin, TransactionId,
+ 				   s, prefix, InvalidTransactionId);
+ }
+ 
+ #undef SNAPSHOT_PARSE
+ 
+ /*
+  * ImportSnapshot
+  *      Import a previously exported snapshot. We expect that whatever we get
+  *      is a filename in SNAPSHOT_EXPORT_DIR. Load the snapshot from that file.
+  *      This is called from "SET TRANSACTION SNAPSHOT 'foo'" and we always
+  *      start fresh from zero with respect to the transaction state that we
+  *      work on. Returns true on success and false on failure.
+  */
+ bool
+ ImportSnapshot(char *idstr)
+ {
+ 	char		path[MAXPGPATH];
+ 	FILE	   *f;
+ 	int			i;
+ 	char	   *s;
+ 	struct stat	stat_buf;
+ 	int			sxcnt, xcnt;
+ 	TransactionId xid, origXid, myXid;
+ 	SnapshotData snapshot = {HeapTupleSatisfiesMVCC};
+ 
+ 	if (FirstSnapshotSet || GetCurrentTransactionNestLevel() != 1)
+ 		ereport(ERROR,
+ 			(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
+ 			 errmsg("SET TRANSACTION SNAPSHOT must be called before any query")));
+ 
+ 	/*
+ 	 * If we were in read committed mode then the next query would execute with a
+ 	 * new snapshot thus making this function call quite useless.
+ 	 */
+ 	if (!IsolationUsesXactSnapshot())
+ 		ereport(ERROR,
+ 			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+ 			 errmsg("A snapshot importing transaction must have ISOLATION "
+ 					"LEVEL SERIALIZABLE or ISOLATION LEVEL REPEATABLE READ")));
+ 
+ 	/* We're lucky to always start off from a pretty clean state */
+ 	Assert(IsTransactionState());
+ 	Assert(GetCurrentTransactionNestLevel() == 1);
+ 	Assert(GetTopTransactionIdIfAny() == InvalidTransactionId);
+ 	Assert(CurrentSnapshot == NULL);
+ 	Assert(SecondarySnapshot == NULL);
+ 	Assert(RegisteredSnapshots == 0);
+ 
+ 	/* verify the identifier, only 0-9,A-F and a hyphen are allowed... */
+ 	s = idstr;
+ 	while (*s)
+ 	{
+ 		if (!isdigit(*s) && !(*s >= 'A' && *s <= 'F') && *s != '-')
+ 			ereport(ERROR,
+ 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ 				 errmsg("could not import the requested snapshot"),
+ 				 errhint("the given snapshot identifier contains invalid characters")));
+ 		s++;
+ 	}
+ 
+ 	/*
+ 	 * Assign a transaction id. We only do this to detect a possible
+ 	 * transaction id wraparound which is somewhere between unlikely
+ 	 * and impossible...
+ 	 */
+ 	myXid = GetTopTransactionId();
+ 
+ 	snprintf(path, MAXPGPATH, SNAPSHOT_EXPORT_DIR "/%s", idstr);
+ 
+ 	/* get the size of the file so that we know how much memory we need */
+ 	if (stat(path, &stat_buf) != 0 || !(f = AllocateFile(path, PG_BINARY_R)))
+ 		ereport(ERROR,
+ 			(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ 			 errmsg("could not import the requested snapshot"),
+ 			 errhint("snapshot has not been exported or does not exist anymore")));
+ 
+ 	s = palloc(stat_buf.st_size + 1);
+ 	if (fread(s, stat_buf.st_size, 1, f) != 1)
+ 		return false;
+ 
+ 	s[stat_buf.st_size] = '\0';
+ 
+ 	FreeFile(f);
+ 
+ 	origXid = parseXactFromText(&s, "xid:");
+ 
+ 	snapshot.xmin = parseXactFromText(&s, "xmi:");
+ 	Assert(snapshot.xmin != InvalidTransactionId);
+ 	snapshot.xmax = parseXactFromText(&s, "xma:");
+ 	Assert(snapshot.xmax != InvalidTransactionId);
+ 
+ 	xcnt = parseIntFromText(&s, "xcnt:");
+ 	/*
+ 	 * This snapshot only serves as a template, there is no need for it to have
+ 	 * maxProcs entries, so let's make it just as large as we need it.
+ 	 */
+ 	snapshot.xip = palloc(xcnt * sizeof(TransactionId));
+ 
+ 	i = 0;
+ 	while ((xid = parseXactFromText(&s, "xip:")) != InvalidTransactionId)
+ 		snapshot.xip[i++] = xid;
+ 	snapshot.xcnt = i;
+ 	Assert(snapshot.xcnt == xcnt);
+ 
+ 	/*
+ 	 * We only write "sof:1" if the snapshot overflowed. If not, then there is
+ 	 * no "sof:x" entry at all and parseBoolFromText() will return false.
+ 	 */
+ 	snapshot.suboverflowed = parseBoolFromText(&s, "sof:");
+ 
+ 	if (!snapshot.suboverflowed)
+ 	{
+ 		sxcnt = parseIntFromText(&s, "sxcnt:");
+ 		snapshot.subxip = palloc(sxcnt * sizeof(TransactionId));
+ 
+ 		i = 0;
+ 		while ((xid = parseXactFromText(&s, "sxp:")) != InvalidTransactionId)
+ 			snapshot.subxip[i++] = xid;
+ 		snapshot.subxcnt = i;
+ 		Assert(snapshot.subxcnt == sxcnt);
+ 	} else {
+ 		snapshot.subxip = NULL;
+ 		snapshot.subxcnt = 0;
+ 	}
+ 
+ 	/* complete the snapshot data structure */
+ 	snapshot.curcid = 0;
+ 	snapshot.takenDuringRecovery = RecoveryInProgress();
+ 
+ 	/*
+ 	 * Note that MyProc->xmin can go backwards here. However this is safe
+ 	 * because the xmin we set here is the same as in the backend's proc->xmin
+ 	 * whose snapshot we are copying. At this very moment, anybody computing a
+ 	 * minimum will calculate at least this xmin as the overall xmin with or
+ 	 * without us setting MyProc->xmin to this value.
+ 	 */
+ 	LWLockAcquire(ProcArrayLock, LW_SHARED);
+ 	MyProc->xmin = snapshot.xmin;
+ 	LWLockRelease(ProcArrayLock);
+ 
+ 	/* bail out if the original transaction is not running anymore... */
+ 	if (!TransactionIdIsInProgress(origXid) || TransactionIdPrecedes(myXid, origXid))
+ 		return false;
+ 
+ 	/*
+ 	 * Install the snapshot as if we got it through GetTransactionSnapshot().
+ 	 * This will set up CurrentSnapshot and also set up the predicate locks for a
+ 	 * serializable transaction.
+ 	 */
+ 	GetTransactionSnapshotFromTemplate(&snapshot);
+ 	return true;
+ }
+ 
+ Datum
+ pg_export_snapshot(PG_FUNCTION_ARGS)
+ {
+ 	char	   *snapshotData;
+ 
+ 	RequireTransactionChain(true, "pg_export_snapshot()");
+ 
+ 	snapshotData = ExportSnapshot(GetTransactionSnapshot());
+ 	PG_RETURN_TEXT_P(cstring_to_text(snapshotData));
+ }
+ 
diff --git a/src/bin/initdb/initdb.c b/src/bin/initdb/initdb.c
index e535fda..0e981cd 100644
*** a/src/bin/initdb/initdb.c
--- b/src/bin/initdb/initdb.c
*************** main(int argc, char *argv[])
*** 2557,2562 ****
--- 2557,2563 ----
  		"pg_serial",
  		"pg_subtrans",
  		"pg_twophase",
+ 		"pg_snapshots",
  		"pg_multixact/members",
  		"pg_multixact/offsets",
  		"base",
diff --git a/src/include/access/twophase.h b/src/include/access/twophase.h
index 0019df5..4706f10 100644
*** a/src/include/access/twophase.h
--- b/src/include/access/twophase.h
***************
*** 22,30 ****
   */
  typedef struct GlobalTransactionData *GlobalTransaction;
  
- /* GUC variable */
- extern int	max_prepared_xacts;
- 
  extern Size TwoPhaseShmemSize(void);
  extern void TwoPhaseShmemInit(void);
  
--- 22,27 ----
diff --git a/src/include/catalog/pg_proc.h b/src/include/catalog/pg_proc.h
index 96f43fe..a4e0387 100644
*** a/src/include/catalog/pg_proc.h
--- b/src/include/catalog/pg_proc.h
*************** DATA(insert OID = 2171 ( pg_cancel_backe
*** 2853,2858 ****
--- 2853,2860 ----
  DESCR("cancel a server process' current query");
  DATA(insert OID = 2096 ( pg_terminate_backend		PGNSP PGUID 12 1 0 0 0 f f f t f v 1 0 16 "23" _null_ _null_ _null_ _null_ pg_terminate_backend _null_ _null_ _null_ ));
  DESCR("terminate a server process");
+ DATA(insert OID = 3122 ( pg_export_snapshot		PGNSP PGUID 12 1 0 0 0 f f f t f v 0 0 25 "" _null_ _null_ _null_ _null_ pg_export_snapshot _null_ _null_ _null_ ));
+ DESCR("export a snapshot");
  DATA(insert OID = 2172 ( pg_start_backup		PGNSP PGUID 12 1 0 0 0 f f f t f v 2 0 25 "25 16" _null_ _null_ _null_ _null_ pg_start_backup _null_ _null_ _null_ ));
  DESCR("prepare for taking an online backup");
  DATA(insert OID = 2173 ( pg_stop_backup			PGNSP PGUID 12 1 0 0 0 f f f t f v 0 0 25 "" _null_ _null_ _null_ _null_ pg_stop_backup _null_ _null_ _null_ ));
diff --git a/src/include/miscadmin.h b/src/include/miscadmin.h
index 9d19417..15326cf 100644
*** a/src/include/miscadmin.h
--- b/src/include/miscadmin.h
*************** extern PGDLLIMPORT int NBuffers;
*** 134,139 ****
--- 134,142 ----
  extern int	MaxBackends;
  extern int	MaxConnections;
  
+ /* GUC variable */
+ extern int	max_prepared_xacts;
+ 
  extern PGDLLIMPORT int MyProcPid;
  extern PGDLLIMPORT pg_time_t MyStartTime;
  extern PGDLLIMPORT struct Port *MyProcPort;
diff --git a/src/include/parser/kwlist.h b/src/include/parser/kwlist.h
index 12c2faf..3d170bc 100644
*** a/src/include/parser/kwlist.h
--- b/src/include/parser/kwlist.h
*************** PG_KEYWORD("show", SHOW, UNRESERVED_KEYW
*** 337,342 ****
--- 337,343 ----
  PG_KEYWORD("similar", SIMILAR, TYPE_FUNC_NAME_KEYWORD)
  PG_KEYWORD("simple", SIMPLE, UNRESERVED_KEYWORD)
  PG_KEYWORD("smallint", SMALLINT, COL_NAME_KEYWORD)
+ PG_KEYWORD("snapshot", SNAPSHOT, UNRESERVED_KEYWORD)
  PG_KEYWORD("some", SOME, RESERVED_KEYWORD)
  PG_KEYWORD("stable", STABLE, UNRESERVED_KEYWORD)
  PG_KEYWORD("standalone", STANDALONE_P, UNRESERVED_KEYWORD)
diff --git a/src/include/storage/predicate.h b/src/include/storage/predicate.h
index 9603b10..50b046e 100644
*** a/src/include/storage/predicate.h
--- b/src/include/storage/predicate.h
*************** extern void CheckPointPredicate(void);
*** 42,48 ****
  extern bool PageIsPredicateLocked(Relation relation, BlockNumber blkno);
  
  /* predicate lock maintenance */
! extern Snapshot GetSerializableTransactionSnapshot(Snapshot snapshot);
  extern void RegisterPredicateLockingXid(TransactionId xid);
  extern void PredicateLockRelation(Relation relation, Snapshot snapshot);
  extern void PredicateLockPage(Relation relation, BlockNumber blkno, Snapshot snapshot);
--- 42,48 ----
  extern bool PageIsPredicateLocked(Relation relation, BlockNumber blkno);
  
  /* predicate lock maintenance */
! extern Snapshot GetSerializableTransactionSnapshot(Snapshot snapshot, Snapshot stemplate);
  extern void RegisterPredicateLockingXid(TransactionId xid);
  extern void PredicateLockRelation(Relation relation, Snapshot snapshot);
  extern void PredicateLockPage(Relation relation, BlockNumber blkno, Snapshot snapshot);
diff --git a/src/include/storage/procarray.h b/src/include/storage/procarray.h
index a11d438..dfe57aa 100644
*** a/src/include/storage/procarray.h
--- b/src/include/storage/procarray.h
*************** extern void ExpireOldKnownAssignedTransa
*** 39,45 ****
  
  extern RunningTransactions GetRunningTransactionData(void);
  
! extern Snapshot GetSnapshotData(Snapshot snapshot);
  
  extern bool TransactionIdIsInProgress(TransactionId xid);
  extern bool TransactionIdIsActive(TransactionId xid);
--- 39,45 ----
  
  extern RunningTransactions GetRunningTransactionData(void);
  
! extern Snapshot GetSnapshotData(Snapshot snapshot, Snapshot stemplate);
  
  extern bool TransactionIdIsInProgress(TransactionId xid);
  extern bool TransactionIdIsActive(TransactionId xid);
*************** extern void XidCacheRemoveRunningXids(Tr
*** 69,72 ****
--- 69,78 ----
  						  int nxids, const TransactionId *xids,
  						  TransactionId latestXid);
  
+ /* Size of the ProcArray structure itself */
+ #define PROCARRAY_MAXPROCS	(MaxBackends + max_prepared_xacts)
+ 
+ #define TOTAL_MAX_CACHED_SUBXIDS \
+ 	((PGPROC_MAX_CACHED_SUBXIDS + 1) * PROCARRAY_MAXPROCS)
+ 
  #endif   /* PROCARRAY_H */
diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h
index e665a28..a26e2fe 100644
*** a/src/include/utils/snapmgr.h
--- b/src/include/utils/snapmgr.h
*************** extern TransactionId TransactionXmin;
*** 22,27 ****
--- 22,29 ----
  extern TransactionId RecentXmin;
  extern TransactionId RecentGlobalXmin;
  
+ extern List *exportedSnapshots;
+ 
  extern Snapshot GetTransactionSnapshot(void);
  extern Snapshot GetLatestSnapshot(void);
  extern void SnapshotSetCommandId(CommandId curcid);
*************** extern void UpdateActiveSnapshotCommandI
*** 32,37 ****
--- 34,40 ----
  extern void PopActiveSnapshot(void);
  extern Snapshot GetActiveSnapshot(void);
  extern bool ActiveSnapshotSet(void);
+ extern Snapshot CopySnapshotOnto(Snapshot onto, Snapshot snapshot);
  
  extern Snapshot RegisterSnapshot(Snapshot snapshot);
  extern void UnregisterSnapshot(Snapshot snapshot);
*************** extern void AtSubCommit_Snapshot(int lev
*** 42,45 ****
--- 45,53 ----
  extern void AtSubAbort_Snapshot(int level);
  extern void AtEOXact_Snapshot(bool isCommit);
  
+ extern Datum pg_export_snapshot(PG_FUNCTION_ARGS);
+ extern bool ImportSnapshot(char *idstr);
+ extern void InvalidateExportedSnapshots(void);
+ extern void DeleteAllExportedSnapshotFiles(void);
+ 
  #endif   /* SNAPMGR_H */