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Commitf6429bd

Browse files
author
Amit Kapila
committed
Fix xmin advancement during fast_forward decoding.
During logical decoding, we advance catalog_xmin of logical too early infast_forward mode, resulting in required catalog data being removed byvacuum. This mode is normally used to advance the slot without processingthe changes, but we still can't let the slot's xmin to advance to anincorrect value.Commitf49a80c fixed a similar issue where the logical slot'scatalog_xmin was getting advanced prematurely during non-fast-forwardmode. During xl_running_xacts processing, instead of directly advancingthe slot's xmin to the oldest running xid in the record, it allowed thexmin to be held back for snapshots that can be used fornot-yet-replayed transactions, as those might consider older txns asrunning too. However, it missed the fact that the same problem can happenduring fast_forward mode decoding, as we won't build a base snapshot inthat mode, and the future call to get_changes from the same slot can missseeing the required catalog changes leading to incorrect reslts.This commit allows building the base snapshot even in fast_forward mode toprevent the early advancement of xmin.Reported-by: Amit Kapila <amit.kapila16@gmail.com>Author: Zhijie Hou <houzj.fnst@fujitsu.com>Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>Reviewed-by: shveta malik <shveta.malik@gmail.com>Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>Backpatch-through: 13Discussion:https://postgr.es/m/CAA4eK1LqWncUOqKijiafe+Ypt1gQAQRjctKLMY953J79xDBgAg@mail.gmail.comDiscussion:https://postgr.es/m/OS0PR01MB57163087F86621D44D9A72BF94BB2@OS0PR01MB5716.jpnprd01.prod.outlook.com
1 parent90bc452 commitf6429bd

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3 files changed

+71
-13
lines changed

3 files changed

+71
-13
lines changed

‎contrib/test_decoding/expected/oldest_xmin.out

Lines changed: 41 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -38,3 +38,44 @@ COMMIT
3838
stop
3939
(1 row)
4040

41+
42+
starting permutation: s0_begin s0_getxid s1_begin s1_insert s0_alter s0_commit s0_checkpoint s0_advance_slot s0_advance_slot s1_commit s0_vacuum s0_get_changes
43+
step s0_begin: BEGIN;
44+
step s0_getxid: SELECT pg_current_xact_id() IS NULL;
45+
?column?
46+
--------
47+
f
48+
(1 row)
49+
50+
step s1_begin: BEGIN;
51+
step s1_insert: INSERT INTO harvest VALUES ((1, 2, 3));
52+
step s0_alter: ALTER TYPE basket DROP ATTRIBUTE mangos;
53+
step s0_commit: COMMIT;
54+
step s0_checkpoint: CHECKPOINT;
55+
step s0_advance_slot: SELECT slot_name FROM pg_replication_slot_advance('isolation_slot', pg_current_wal_lsn());
56+
slot_name
57+
--------------
58+
isolation_slot
59+
(1 row)
60+
61+
step s0_advance_slot: SELECT slot_name FROM pg_replication_slot_advance('isolation_slot', pg_current_wal_lsn());
62+
slot_name
63+
--------------
64+
isolation_slot
65+
(1 row)
66+
67+
step s1_commit: COMMIT;
68+
step s0_vacuum: VACUUM pg_attribute;
69+
step s0_get_changes: SELECT data FROM pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
70+
data
71+
------------------------------------------------------
72+
BEGIN
73+
table public.harvest: INSERT: fruits[basket]:'(1,2,3)'
74+
COMMIT
75+
(3 rows)
76+
77+
?column?
78+
--------
79+
stop
80+
(1 row)
81+

‎contrib/test_decoding/specs/oldest_xmin.spec

Lines changed: 5 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -25,6 +25,7 @@ step "s0_commit" { COMMIT; }
2525
step"s0_checkpoint" {CHECKPOINT; }
2626
step"s0_vacuum" {VACUUMpg_attribute; }
2727
step"s0_get_changes" {SELECTdataFROMpg_logical_slot_get_changes('isolation_slot',NULL,NULL,'include-xids','0','skip-empty-xacts','1'); }
28+
step"s0_advance_slot" {SELECTslot_nameFROMpg_replication_slot_advance('isolation_slot',pg_current_wal_lsn()); }
2829

2930
session"s1"
3031
setup {SETsynchronous_commit=on; }
@@ -40,3 +41,7 @@ step "s1_commit" { COMMIT; }
4041
# will be removed (xmax set) before T1 commits. That is, interlocking doesn't
4142
# forbid modifying catalog after someone read it (and didn't commit yet).
4243
permutation"s0_begin""s0_getxid""s1_begin""s1_insert""s0_alter""s0_commit""s0_checkpoint""s0_get_changes""s0_get_changes""s1_commit""s0_vacuum""s0_get_changes"
44+
45+
# Perform the same testing process as described above, but use advance_slot to
46+
# forces xmin advancement during fast forward decoding.
47+
permutation"s0_begin""s0_getxid""s1_begin""s1_insert""s0_alter""s0_commit""s0_checkpoint""s0_advance_slot""s0_advance_slot""s1_commit""s0_vacuum""s0_get_changes"

‎src/backend/replication/logical/decode.c

Lines changed: 25 additions & 13 deletions
Original file line numberDiff line numberDiff line change
@@ -385,20 +385,24 @@ heap2_decode(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
385385

386386
/*
387387
* If we don't have snapshot or we are just fast-forwarding, there is no
388-
* point in decoding changes.
388+
* point in decoding data changes. However, it's crucial to build the base
389+
* snapshot during fast-forward mode (as is done in
390+
* SnapBuildProcessChange()) because we require the snapshot's xmin when
391+
* determining the candidate catalog_xmin for the replication slot. See
392+
* SnapBuildProcessRunningXacts().
389393
*/
390-
if (SnapBuildCurrentState(builder)<SNAPBUILD_FULL_SNAPSHOT||
391-
ctx->fast_forward)
394+
if (SnapBuildCurrentState(builder)<SNAPBUILD_FULL_SNAPSHOT)
392395
return;
393396

394397
switch (info)
395398
{
396399
caseXLOG_HEAP2_MULTI_INSERT:
397-
if (!ctx->fast_forward&&
398-
SnapBuildProcessChange(builder,xid,buf->origptr))
400+
if (SnapBuildProcessChange(builder,xid,buf->origptr)&&
401+
!ctx->fast_forward)
399402
DecodeMultiInsert(ctx,buf);
400403
break;
401404
caseXLOG_HEAP2_NEW_CID:
405+
if (!ctx->fast_forward)
402406
{
403407
xl_heap_new_cid*xlrec;
404408

@@ -445,16 +449,20 @@ heap_decode(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
445449

446450
/*
447451
* If we don't have snapshot or we are just fast-forwarding, there is no
448-
* point in decoding data changes.
452+
* point in decoding data changes. However, it's crucial to build the base
453+
* snapshot during fast-forward mode (as is done in
454+
* SnapBuildProcessChange()) because we require the snapshot's xmin when
455+
* determining the candidate catalog_xmin for the replication slot. See
456+
* SnapBuildProcessRunningXacts().
449457
*/
450-
if (SnapBuildCurrentState(builder)<SNAPBUILD_FULL_SNAPSHOT||
451-
ctx->fast_forward)
458+
if (SnapBuildCurrentState(builder)<SNAPBUILD_FULL_SNAPSHOT)
452459
return;
453460

454461
switch (info)
455462
{
456463
caseXLOG_HEAP_INSERT:
457-
if (SnapBuildProcessChange(builder,xid,buf->origptr))
464+
if (SnapBuildProcessChange(builder,xid,buf->origptr)&&
465+
!ctx->fast_forward)
458466
DecodeInsert(ctx,buf);
459467
break;
460468

@@ -465,17 +473,20 @@ heap_decode(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
465473
*/
466474
caseXLOG_HEAP_HOT_UPDATE:
467475
caseXLOG_HEAP_UPDATE:
468-
if (SnapBuildProcessChange(builder,xid,buf->origptr))
476+
if (SnapBuildProcessChange(builder,xid,buf->origptr)&&
477+
!ctx->fast_forward)
469478
DecodeUpdate(ctx,buf);
470479
break;
471480

472481
caseXLOG_HEAP_DELETE:
473-
if (SnapBuildProcessChange(builder,xid,buf->origptr))
482+
if (SnapBuildProcessChange(builder,xid,buf->origptr)&&
483+
!ctx->fast_forward)
474484
DecodeDelete(ctx,buf);
475485
break;
476486

477487
caseXLOG_HEAP_TRUNCATE:
478-
if (SnapBuildProcessChange(builder,xid,buf->origptr))
488+
if (SnapBuildProcessChange(builder,xid,buf->origptr)&&
489+
!ctx->fast_forward)
479490
DecodeTruncate(ctx,buf);
480491
break;
481492

@@ -503,7 +514,8 @@ heap_decode(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
503514
break;
504515

505516
caseXLOG_HEAP_CONFIRM:
506-
if (SnapBuildProcessChange(builder,xid,buf->origptr))
517+
if (SnapBuildProcessChange(builder,xid,buf->origptr)&&
518+
!ctx->fast_forward)
507519
DecodeSpecConfirm(ctx,buf);
508520
break;
509521

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