From 81163b9592f7762d83549962ad4e50e2abb80c07 Mon Sep 17 00:00:00 2001 From: Thomas Munro Date: Fri, 17 Apr 2020 15:18:49 +1200 Subject: [PATCH v2 5/6] Truncate old snapshot XIDs before truncating CLOG. --- contrib/old_snapshot/Makefile | 2 +- contrib/old_snapshot/t/001_truncate.pl | 80 ++++++++++++++++++++++++++ src/backend/commands/vacuum.c | 3 + src/backend/utils/time/snapmgr.c | 21 +++++++ src/include/utils/snapmgr.h | 1 + 5 files changed, 106 insertions(+), 1 deletion(-) create mode 100644 contrib/old_snapshot/t/001_truncate.pl diff --git a/contrib/old_snapshot/Makefile b/contrib/old_snapshot/Makefile index 091231f25f..c839d30346 100644 --- a/contrib/old_snapshot/Makefile +++ b/contrib/old_snapshot/Makefile @@ -10,7 +10,7 @@ EXTENSION = old_snapshot DATA = old_snapshot--1.0.sql PGFILEDESC = "old_snapshot - utilities in support of old_snapshot_threshold" -REGRESS = old_snapshot +TAP_TESTS = 1 ifdef USE_PGXS PG_CONFIG = pg_config diff --git a/contrib/old_snapshot/t/001_truncate.pl b/contrib/old_snapshot/t/001_truncate.pl new file mode 100644 index 0000000000..d6c0def00f --- /dev/null +++ b/contrib/old_snapshot/t/001_truncate.pl @@ -0,0 +1,80 @@ +# Test truncation of the old snapshot time mapping, to check +# that we can't get into trouble when xids wrap around. +use strict; +use warnings; +use PostgresNode; +use TestLib; +use Test::More tests => 6; + +my $node = get_new_node('master'); +$node->init; +$node->append_conf("postgresql.conf", "timezone = UTC"); +$node->append_conf("postgresql.conf", "old_snapshot_threshold=10"); +$node->append_conf("postgresql.conf", "max_prepared_transactions=10"); +$node->start; +$node->psql('postgres', 'update pg_database set datallowconn = true'); +$node->psql('postgres', 'create extension old_snapshot'); + +note "check time map is truncated when CLOG is"; + +# build up a time map with 4 entries +$node->psql('postgres', "select pg_clobber_current_snapshot_timestamp('3000-01-01 00:00:00Z')"); +$node->psql('postgres', "select pg_current_xact_id()"); +$node->psql('postgres', "select pg_clobber_current_snapshot_timestamp('3000-01-01 00:01:00Z')"); +$node->psql('postgres', "select pg_current_xact_id()"); +$node->psql('postgres', "select pg_clobber_current_snapshot_timestamp('3000-01-01 00:02:00Z')"); +$node->psql('postgres', "select pg_current_xact_id()"); +$node->psql('postgres', "select pg_clobber_current_snapshot_timestamp('3000-01-01 00:03:00Z')"); +$node->psql('postgres', "select pg_current_xact_id()"); +my $count; +$node->psql('postgres', "select count(*) from pg_old_snapshot_time_mapping()", stdout => \$count); +is($count, 4, "expected to have 4 entries in the old snapshot time map"); + +# now cause frozen XID to advance +$node->psql('postgres', 'vacuum freeze'); +$node->psql('template0', 'vacuum freeze'); +$node->psql('template1', 'vacuum freeze'); + +# we expect all XIDs to have been truncated +$node->psql('postgres', "select count(*) from pg_old_snapshot_time_mapping()", stdout => \$count); +is($count, 0, "expected to have 0 entries in the old snapshot time map"); + +# put two more in the map +$node->psql('postgres', "select pg_clobber_current_snapshot_timestamp('3000-01-01 00:04:00Z')"); +$node->psql('postgres', "select pg_current_xact_id()"); +$node->psql('postgres', "select pg_clobber_current_snapshot_timestamp('3000-01-01 00:05:00Z')"); +$node->psql('postgres', "select pg_current_xact_id()"); +$node->psql('postgres', "select count(*) from pg_old_snapshot_time_mapping()", stdout => \$count); +is($count, 2, "expected to have 2 entries in the old snapshot time map"); + +# prepare a transaction, to stop xmin from getting further ahead +$node->psql('postgres', "begin; select pg_current_xact_id(); prepare transaction 'tx1'"); + +# add 16 more minutes (this tests wrapping around the mapping array, which is of size 10 + 10)... +$node->psql('postgres', "select pg_clobber_current_snapshot_timestamp('3000-01-01 00:21:00Z')"); +$node->psql('postgres', "select pg_current_xact_id()"); +$node->psql('postgres', "select count(*) from pg_old_snapshot_time_mapping()", stdout => \$count); +is($count, 18, "expected to have 18 entries in the old snapshot time map"); + +# now cause frozen XID to advance +$node->psql('postgres', 'vacuum freeze'); +$node->psql('template0', 'vacuum freeze'); +$node->psql('template1', 'vacuum freeze'); + +# this should leave just 16 +$node->psql('postgres', "select count(*) from pg_old_snapshot_time_mapping()", stdout => \$count); +is($count, 16, "expected to have 16 entries in the old snapshot time map"); + +# commit tx1, and then freeze again to get rid of all of them +$node->psql('postgres', "commit prepared 'tx1'"); + +# now cause frozen XID to advance +$node->psql('postgres', 'vacuum freeze'); +$node->psql('template0', 'vacuum freeze'); +$node->psql('template1', 'vacuum freeze'); + +# we should now be back to empty +$node->psql('postgres', "select count(*) from pg_old_snapshot_time_mapping()", stdout => \$count); +is($count, 0, "expected to have 0 entries in the old snapshot time map"); + +$node->stop; diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c index 5a110edb07..37ead45fa5 100644 --- a/src/backend/commands/vacuum.c +++ b/src/backend/commands/vacuum.c @@ -1627,6 +1627,9 @@ vac_truncate_clog(TransactionId frozenXID, */ AdvanceOldestCommitTsXid(frozenXID); + /* Make sure snapshot_too_old drops old XIDs. */ + TruncateOldSnapshotTimeMapping(frozenXID); + /* * Truncate CLOG, multixact and CommitTs to the oldest computed value. */ diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 72b2c61a07..d604e69270 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -1998,6 +1998,27 @@ MaintainOldSnapshotTimeMapping(TimestampTz whenTaken, TransactionId xmin) } +/* + * Remove old xids from the timing map, so the CLOG can be truncated. + */ +void +TruncateOldSnapshotTimeMapping(TransactionId frozenXID) +{ + LWLockAcquire(OldSnapshotTimeMapLock, LW_EXCLUSIVE); + while (oldSnapshotControl->count_used > 0 && + TransactionIdPrecedes(oldSnapshotControl->xid_by_minute[oldSnapshotControl->head_offset], + frozenXID)) + { + oldSnapshotControl->head_timestamp += USECS_PER_MINUTE; + oldSnapshotControl->head_offset = + (oldSnapshotControl->head_offset + 1) % + OLD_SNAPSHOT_TIME_MAP_ENTRIES; + oldSnapshotControl->count_used--; + } + LWLockRelease(OldSnapshotTimeMapLock); +} + + /* * Setup a snapshot that replaces normal catalog snapshots that allows catalog * access to behave just like it did at a certain point in the past. diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b28d13ce84..4f53aad956 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -135,6 +135,7 @@ extern TransactionId TransactionIdLimitedForOldSnapshots(TransactionId recentXmi Relation relation); extern void MaintainOldSnapshotTimeMapping(TimestampTz whenTaken, TransactionId xmin); +extern void TruncateOldSnapshotTimeMapping(TransactionId frozenXID); extern char *ExportSnapshot(Snapshot snapshot); -- 2.20.1