From db5bdd8e1a350cf77b77884ec445b8f0f18f135a Mon Sep 17 00:00:00 2001
From: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Date: Fri, 15 Oct 2021 14:23:52 +0900
Subject: [PATCH v2 5/5] Get rid of the old BackendId at all

Since we have got rid of the annoyging O(N) behavior in
SIInsertDataEntries, sinvaladt.c can say good-by to the old packed
Backend ID. Still CleanupInvalidationState and SICleanupQueue has O(N)
behavior but they are executed rarely or ends in a short time in the
most cases.
---
 src/backend/storage/ipc/sinvaladt.c | 67 ++++++-----------------------
 1 file changed, 14 insertions(+), 53 deletions(-)

diff --git a/src/backend/storage/ipc/sinvaladt.c b/src/backend/storage/ipc/sinvaladt.c
index ee3a3accfd..d56c77400f 100644
--- a/src/backend/storage/ipc/sinvaladt.c
+++ b/src/backend/storage/ipc/sinvaladt.c
@@ -193,8 +193,6 @@ static LocalTransactionId nextLocalTransactionId;
 
 static void CleanupInvalidationState(int status, Datum arg);
 
-static int siindex;
-
 /*
  * SInvalShmemSize --- return shared-memory space needed
  */
@@ -254,7 +252,6 @@ CreateSharedInvalidationState(void)
 void
 SharedInvalBackendInit(bool sendOnly)
 {
-	int			index;
 	ProcState  *stateP = NULL;
 	SISeg	   *segP = shmInvalBuffer;
 
@@ -265,38 +262,15 @@ SharedInvalBackendInit(bool sendOnly)
 	 */
 	LWLockAcquire(SInvalWriteLock, LW_EXCLUSIVE);
 
-	/* Look for a free entry in the procState array */
-	for (index = 0; index < segP->lastBackend; index++)
-	{
-		if (segP->procState[index].procPid == 0)	/* inactive slot? */
-		{
-			stateP = &segP->procState[index];
-			break;
-		}
-	}
+	Assert (MyBackendId < MaxBackends);
+	stateP = &segP->procState[MyBackendId];
 
-	if (stateP == NULL)
-	{
-		if (segP->lastBackend < segP->maxBackends)
-		{
-			stateP = &segP->procState[segP->lastBackend];
-			Assert(stateP->procPid == 0);
-			segP->lastBackend++;
-		}
-		else
-		{
-			/*
-			 * out of procState slots: MaxBackends exceeded -- report normally
-			 */
-			siindex = -1;
-			LWLockRelease(SInvalWriteLock);
-			ereport(FATAL,
-					(errcode(ERRCODE_TOO_MANY_CONNECTIONS),
-					 errmsg("sorry, too many clients already")));
-		}
-	}
+	/* this entry should be free */
+	Assert (stateP->procPid == 0);
 
-	siindex = (stateP - &segP->procState[0]);
+	/* adjust lastBackend if needed  */
+	if (segP->lastBackend < MyBackendId)
+		segP->lastBackend = MyBackendId;
 
 	/* Fetch next local transaction ID into local memory */
 	nextLocalTransactionId = stateP->nextLXID;
@@ -313,8 +287,6 @@ SharedInvalBackendInit(bool sendOnly)
 
 	/* register exit routine to mark my entry inactive at exit */
 	on_shmem_exit(CleanupInvalidationState, PointerGetDatum(segP));
-
-	elog(DEBUG4, "my SI slot index is %d", siindex);
 }
 
 /*
@@ -336,7 +308,7 @@ CleanupInvalidationState(int status, Datum arg)
 
 	LWLockAcquire(SInvalWriteLock, LW_EXCLUSIVE);
 
-	stateP = &segP->procState[siindex];
+	stateP = &segP->procState[MyBackendId];
 
 	/* Update next local transaction ID for next holder of this backendID */
 	stateP->nextLXID = nextLocalTransactionId;
@@ -349,7 +321,7 @@ CleanupInvalidationState(int status, Datum arg)
 	stateP->signaled = false;
 
 	/* Recompute index of last active backend */
-	for (i = segP->lastBackend; i > 0; i--)
+	for (i = segP->lastBackend; i >= 0; i--)
 	{
 		if (segP->procState[i - 1].procPid != 0)
 			break;
@@ -368,27 +340,16 @@ CleanupInvalidationState(int status, Datum arg)
 void
 BackendIdGetTransactionIds(int backendID, TransactionId *xid, TransactionId *xmin)
 {
-	SISeg	   *segP = shmInvalBuffer;
-
 	*xid = InvalidTransactionId;
 	*xmin = InvalidTransactionId;
 
-	/* Need to lock out additions/removals of backends */
-	LWLockAcquire(SInvalWriteLock, LW_SHARED);
-
-	if (backendID > 0 && backendID <= segP->lastBackend)
+	if (backendID >= 0 && backendID < MaxBackends)
 	{
-		ProcState  *stateP = &segP->procState[backendID - 1];
-		PGPROC	   *proc = stateP->proc;
+		PGPROC	   *proc = &ProcGlobal->allProcs[backendID];
 
-		if (proc != NULL)
-		{
-			*xid = proc->xid;
-			*xmin = proc->xmin;
-		}
+		*xid = proc->xid;
+		*xmin = proc->xmin;
 	}
-
-	LWLockRelease(SInvalWriteLock);
 }
 
 /*
@@ -495,7 +456,7 @@ SIGetDataEntries(SharedInvalidationMessage *data, int datasize)
 	int			n;
 
 	segP = shmInvalBuffer;
-	stateP = &segP->procState[siindex];
+	stateP = &segP->procState[MyBackendId];
 
 	/*
 	 * Before starting to take locks, do a quick, unlocked test to see whether
-- 
2.27.0