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Commit bb19b70

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MasahikoSawadaMasahikoSawada
committed
Fix possibility of logical decoding partial transaction changes.
When creating and initializing a logical slot, the restart_lsn is set to the latest WAL insertion point (or the latest replay point on standbys). Subsequently, WAL records are decoded from that point to find the start point for extracting changes in the DecodingContextFindStartpoint() function. Since the initial restart_lsn could be in the middle of a transaction, the start point must be a consistent point where we won't see the data for partial transactions. Previously, when not building a full snapshot, serialized snapshots were restored, and the SnapBuild jumps to the consistent state even while finding the start point. Consequently, the slot's restart_lsn and confirmed_flush could be set to the middle of a transaction. This could lead to various unexpected consequences. Specifically, there were reports of logical decoding decoding partial transactions, and assertion failures occurred because only subtransactions were decoded without decoding their top-level transaction until decoding the commit record. To resolve this issue, the changes prevent restoring the serialized snapshot and jumping to the consistent state while finding the start point. On v17 and HEAD, a flag indicating whether snapshot restores should be skipped has been added to the SnapBuild struct, and SNAPBUILD_VERSION has been bumpded. On backbranches, the flag is stored in the LogicalDecodingContext instead, preserving on-disk compatibility. Backpatch to all supported versions. Reported-by: Drew Callahan Reviewed-by: Amit Kapila, Hayato Kuroda Discussion: https://postgr.es/m/2444AA15-D21B-4CCE-8052-52C7C2DAFE5C%40amazon.com Backpatch-through: 12
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‎contrib/test_decoding/Makefile

Copy file name to clipboardExpand all lines: contrib/test_decoding/Makefile
+2-1Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,8 @@ REGRESS = ddl xact rewrite toast permissions decoding_in_xact \
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spill slot truncate stream stats twophase twophase_stream
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ISOLATION = mxact delayed_startup ondisk_startup concurrent_ddl_dml \
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oldest_xmin snapshot_transfer subxact_without_top concurrent_stream \
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twophase_snapshot slot_creation_error catalog_change_snapshot
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twophase_snapshot slot_creation_error catalog_change_snapshot \
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skip_snapshot_restore
1213

1314
REGRESS_OPTS = --temp-config $(top_srcdir)/contrib/test_decoding/logical.conf
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ISOLATION_OPTS = --temp-config $(top_srcdir)/contrib/test_decoding/logical.conf

‎contrib/test_decoding/expected/skip_snapshot_restore.out

Copy file name to clipboard
+45Lines changed: 45 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,45 @@
1+
Parsed test spec with 3 sessions
2+
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starting permutation: s0_init s0_begin s0_insert1 s1_init s2_checkpoint s2_get_changes_slot0 s0_insert2 s0_commit s1_get_changes_slot0 s1_get_changes_slot1
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step s0_init: SELECT 'init' FROM pg_create_logical_replication_slot('slot0', 'test_decoding');
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?column?
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--------
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init
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(1 row)
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step s0_begin: BEGIN;
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step s0_insert1: INSERT INTO tbl VALUES (1);
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step s1_init: SELECT 'init' FROM pg_create_logical_replication_slot('slot1', 'test_decoding'); <waiting ...>
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step s2_checkpoint: CHECKPOINT;
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step s2_get_changes_slot0: SELECT data FROM pg_logical_slot_get_changes('slot0', NULL, NULL, 'skip-empty-xacts', '1', 'include-xids', '0');
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data
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----
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(0 rows)
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step s0_insert2: INSERT INTO tbl VALUES (2);
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step s0_commit: COMMIT;
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step s1_init: <... completed>
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?column?
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--------
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init
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(1 row)
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step s1_get_changes_slot0: SELECT data FROM pg_logical_slot_get_changes('slot0', NULL, NULL, 'skip-empty-xacts', '1', 'include-xids', '0');
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data
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-----------------------------------------
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BEGIN
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table public.tbl: INSERT: val1[integer]:1
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table public.tbl: INSERT: val1[integer]:2
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COMMIT
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(4 rows)
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step s1_get_changes_slot1: SELECT data FROM pg_logical_slot_get_changes('slot1', NULL, NULL, 'skip-empty-xacts', '1', 'include-xids', '0');
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data
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----
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(0 rows)
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?column?
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--------
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stop
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(1 row)
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‎contrib/test_decoding/meson.build

Copy file name to clipboardExpand all lines: contrib/test_decoding/meson.build
+1Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -62,6 +62,7 @@ tests += {
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'concurrent_stream',
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'twophase_snapshot',
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'slot_creation_error',
65+
'skip_snapshot_restore',
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],
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'regress_args': [
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'--temp-config', files('logical.conf'),

‎contrib/test_decoding/specs/skip_snapshot_restore.spec

Copy file name to clipboard
+46Lines changed: 46 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,46 @@
1+
# Test that a slot creation skips to restore serialized snapshot to reach
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# the consistent state.
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setup
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{
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DROP TABLE IF EXISTS tbl;
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CREATE TABLE tbl (val1 integer);
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}
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teardown
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{
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DROP TABLE tbl;
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SELECT 'stop' FROM pg_drop_replication_slot('slot0');
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SELECT 'stop' FROM pg_drop_replication_slot('slot1');
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}
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session "s0"
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setup { SET synchronous_commit = on; }
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step "s0_init" { SELECT 'init' FROM pg_create_logical_replication_slot('slot0', 'test_decoding'); }
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step "s0_begin" { BEGIN; }
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step "s0_insert1" { INSERT INTO tbl VALUES (1); }
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step "s0_insert2" { INSERT INTO tbl VALUES (2); }
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step "s0_commit" { COMMIT; }
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session "s1"
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setup { SET synchronous_commit = on; }
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step "s1_init" { SELECT 'init' FROM pg_create_logical_replication_slot('slot1', 'test_decoding'); }
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step "s1_get_changes_slot0" { SELECT data FROM pg_logical_slot_get_changes('slot0', NULL, NULL, 'skip-empty-xacts', '1', 'include-xids', '0'); }
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step "s1_get_changes_slot1" { SELECT data FROM pg_logical_slot_get_changes('slot1', NULL, NULL, 'skip-empty-xacts', '1', 'include-xids', '0'); }
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session "s2"
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setup { SET synchronous_commit = on ;}
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step "s2_checkpoint" { CHECKPOINT; }
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step "s2_get_changes_slot0" { SELECT data FROM pg_logical_slot_get_changes('slot0', NULL, NULL, 'skip-empty-xacts', '1', 'include-xids', '0'); }
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# While 'slot1' creation by "s1_init" waits for s0-transaction to commit, the
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# RUNNING_XACTS record is written by "s2_checkpoint" and "s2_get_changes_slot1"
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# serializes consistent snapshots to the disk at LSNs where are before
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# s0-transaction's commit. After s0-transaction commits, "s1_init" resumes but
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# must not restore any serialized snapshots and will reach the consistent state
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# when decoding a RUNNING_XACT record generated after s0-transaction's commit.
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# We check if the get_changes on 'slot1' will not return any s0-transaction's
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# changes as its confirmed_flush_lsn will be after the s0-transaction's commit
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# record.
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permutation "s0_init" "s0_begin" "s0_insert1" "s1_init" "s2_checkpoint" "s2_get_changes_slot0" "s0_insert2" "s0_commit" "s1_get_changes_slot0" "s1_get_changes_slot1"

‎src/backend/replication/logical/logical.c

Copy file name to clipboardExpand all lines: src/backend/replication/logical/logical.c
+4-3Lines changed: 4 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -152,6 +152,7 @@ StartupDecodingContext(List *output_plugin_options,
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TransactionId xmin_horizon,
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bool need_full_snapshot,
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bool fast_forward,
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bool in_create,
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XLogReaderRoutine *xl_routine,
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LogicalOutputPluginWriterPrepareWrite prepare_write,
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LogicalOutputPluginWriterWrite do_write,
@@ -212,7 +213,7 @@ StartupDecodingContext(List *output_plugin_options,
212213
ctx->reorder = ReorderBufferAllocate();
213214
ctx->snapshot_builder =
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AllocateSnapshotBuilder(ctx->reorder, xmin_horizon, start_lsn,
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need_full_snapshot, slot->data.two_phase_at);
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need_full_snapshot, in_create, slot->data.two_phase_at);
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ctx->reorder->private_data = ctx;
218219

@@ -438,7 +439,7 @@ CreateInitDecodingContext(const char *plugin,
438439
ReplicationSlotSave();
439440

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ctx = StartupDecodingContext(NIL, restart_lsn, xmin_horizon,
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need_full_snapshot, false,
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need_full_snapshot, false, true,
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xl_routine, prepare_write, do_write,
443444
update_progress);
444445

@@ -592,7 +593,7 @@ CreateDecodingContext(XLogRecPtr start_lsn,
592593

593594
ctx = StartupDecodingContext(output_plugin_options,
594595
start_lsn, InvalidTransactionId, false,
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fast_forward, xl_routine, prepare_write,
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fast_forward, false, xl_routine, prepare_write,
596597
do_write, update_progress);
597598

598599
/* call output plugin initialization callback */

‎src/backend/replication/logical/snapbuild.c

Copy file name to clipboardExpand all lines: src/backend/replication/logical/snapbuild.c
+23-6Lines changed: 23 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -189,6 +189,14 @@ struct SnapBuild
189189
/* Indicates if we are building full snapshot or just catalog one. */
190190
bool building_full_snapshot;
191191

192+
/*
193+
* Indicates if we are using the snapshot builder for the creation of a
194+
* logical replication slot. If it's true, the start point for decoding
195+
* changes is not determined yet. So we skip snapshot restores to properly
196+
* find the start point. See SnapBuildFindSnapshot() for details.
197+
*/
198+
bool in_slot_creation;
199+
192200
/*
193201
* Snapshot that's valid to see the catalog state seen at this moment.
194202
*/
@@ -317,6 +325,7 @@ AllocateSnapshotBuilder(ReorderBuffer *reorder,
317325
TransactionId xmin_horizon,
318326
XLogRecPtr start_lsn,
319327
bool need_full_snapshot,
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bool in_slot_creation,
320329
XLogRecPtr two_phase_at)
321330
{
322331
MemoryContext context;
@@ -347,6 +356,7 @@ AllocateSnapshotBuilder(ReorderBuffer *reorder,
347356

348357
builder->initial_xmin_horizon = xmin_horizon;
349358
builder->start_decoding_at = start_lsn;
359+
builder->in_slot_creation = in_slot_creation;
350360
builder->building_full_snapshot = need_full_snapshot;
351361
builder->two_phase_at = two_phase_at;
352362

@@ -1327,10 +1337,12 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn
13271337
* state while waiting on c)'s sub-states.
13281338
*
13291339
* b) This (in a previous run) or another decoding slot serialized a
1330-
* snapshot to disk that we can use. Can't use this method for the
1331-
* initial snapshot when slot is being created and needs full snapshot
1332-
* for export or direct use, as that snapshot will only contain catalog
1333-
* modifying transactions.
1340+
* snapshot to disk that we can use. Can't use this method while finding
1341+
* the start point for decoding changes as the restart LSN would be an
1342+
* arbitrary LSN but we need to find the start point to extract changes
1343+
* where we won't see the data for partial transactions. Also, we cannot
1344+
* use this method when a slot needs a full snapshot for export or direct
1345+
* use, as that snapshot will only contain catalog modifying transactions.
13341346
*
13351347
* c) First incrementally build a snapshot for catalog tuples
13361348
* (BUILDING_SNAPSHOT), that requires all, already in-progress,
@@ -1395,8 +1407,13 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn
13951407

13961408
return false;
13971409
}
1398-
/* b) valid on disk state and not building full snapshot */
1410+
1411+
/*
1412+
* b) valid on disk state and while neither building full snapshot nor
1413+
* creating a slot.
1414+
*/
13991415
else if (!builder->building_full_snapshot &&
1416+
!builder->in_slot_creation &&
14001417
SnapBuildRestore(builder, lsn))
14011418
{
14021419
/* there won't be any state to cleanup */
@@ -1580,7 +1597,7 @@ typedef struct SnapBuildOnDisk
15801597
offsetof(SnapBuildOnDisk, version)
15811598

15821599
#define SNAPBUILD_MAGIC 0x51A1E001
1583-
#define SNAPBUILD_VERSION 5
1600+
#define SNAPBUILD_VERSION 6
15841601

15851602
/*
15861603
* Store/Load a snapshot from disk, depending on the snapshot builder's state.

‎src/include/replication/snapbuild.h

Copy file name to clipboardExpand all lines: src/include/replication/snapbuild.h
+1Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -62,6 +62,7 @@ extern void CheckPointSnapBuild(void);
6262
extern SnapBuild *AllocateSnapshotBuilder(struct ReorderBuffer *reorder,
6363
TransactionId xmin_horizon, XLogRecPtr start_lsn,
6464
bool need_full_snapshot,
65+
bool in_slot_creation,
6566
XLogRecPtr two_phase_at);
6667
extern void FreeSnapshotBuilder(SnapBuild *builder);
6768

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