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Commit8eace46

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Fix race condition in reading commit timestamps
If a user requests the commit timestamp for a transaction old enoughthat its data is concurrently being truncated away by vacuum at just theright time, they would receive an ugly internal file-not-found errormessage from slru.c rather than the expected NULL return value.In a primary server, the window for the race is very small: the lookuphas to occur exactly between the two calls by vacuum, and there's not alot that happens between them (mostly just a multixact truncate). In astandby server, however, the window is larger because the truncation isexecuted as soon as the WAL record for it is replayed, but the advanceof the oldest-Xid is not executed until the next checkpoint record.To fix in the primary, simply reverse the order of operations invac_truncate_clog. To fix in the standby, augment the WAL truncationrecord so that the standby is aware of the new oldest-XID value and canapply the update immediately. WAL version bumped because of this.No backpatch, because of the low importance of the bug and its rarity.Author: Craig RingerReviewed-By: Petr Jelínek, Peter EisentrautDiscussion:https://postgr.es/m/CAMsr+YFhVtRQT1VAwC+WGbbxZZRzNou=N9Ed-FrCqkwQ8H8oJQ@mail.gmail.com
1 parent8b0fec9 commit8eace46

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5 files changed

+34
-13
lines changed

5 files changed

+34
-13
lines changed

‎src/backend/access/rmgrdesc/committsdesc.c

Lines changed: 3 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -33,10 +33,10 @@ commit_ts_desc(StringInfo buf, XLogReaderState *record)
3333
}
3434
elseif (info==COMMIT_TS_TRUNCATE)
3535
{
36-
intpageno;
36+
xl_commit_ts_truncate*trunc= (xl_commit_ts_truncate*)rec;
3737

38-
memcpy(&pageno,rec,sizeof(int));
39-
appendStringInfo(buf,"%d",pageno);
38+
appendStringInfo(buf,"pageno %d, oldestXid %u",
39+
trunc->pageno,trunc->oldestXid);
4040
}
4141
elseif (info==COMMIT_TS_SETTS)
4242
{

‎src/backend/access/transam/commit_ts.c

Lines changed: 13 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -113,7 +113,7 @@ static bool CommitTsPagePrecedes(int page1, int page2);
113113
staticvoidActivateCommitTs(void);
114114
staticvoidDeactivateCommitTs(void);
115115
staticvoidWriteZeroPageXlogRec(intpageno);
116-
staticvoidWriteTruncateXlogRec(intpageno);
116+
staticvoidWriteTruncateXlogRec(intpageno,TransactionIdoldestXid);
117117
staticvoidWriteSetTimestampXlogRec(TransactionIdmainxid,intnsubxids,
118118
TransactionId*subxids,TimestampTztimestamp,
119119
RepOriginIdnodeid);
@@ -824,7 +824,7 @@ TruncateCommitTs(TransactionId oldestXact)
824824
return;/* nothing to remove */
825825

826826
/* Write XLOG record */
827-
WriteTruncateXlogRec(cutoffPage);
827+
WriteTruncateXlogRec(cutoffPage,oldestXact);
828828

829829
/* Now we can remove the old CommitTs segment(s) */
830830
SimpleLruTruncate(CommitTsCtl,cutoffPage);
@@ -910,10 +910,15 @@ WriteZeroPageXlogRec(int pageno)
910910
* Write a TRUNCATE xlog record
911911
*/
912912
staticvoid
913-
WriteTruncateXlogRec(intpageno)
913+
WriteTruncateXlogRec(intpageno,TransactionIdoldestXid)
914914
{
915+
xl_commit_ts_truncatexlrec;
916+
917+
xlrec.pageno=pageno;
918+
xlrec.oldestXid=oldestXid;
919+
915920
XLogBeginInsert();
916-
XLogRegisterData((char*) (&pageno),sizeof(int));
921+
XLogRegisterData((char*) (&xlrec),SizeOfCommitTsTruncate);
917922
(void)XLogInsert(RM_COMMIT_TS_ID,COMMIT_TS_TRUNCATE);
918923
}
919924

@@ -967,17 +972,17 @@ commit_ts_redo(XLogReaderState *record)
967972
}
968973
elseif (info==COMMIT_TS_TRUNCATE)
969974
{
970-
intpageno;
975+
xl_commit_ts_truncate*trunc= (xl_commit_ts_truncate*)XLogRecGetData(record);
971976

972-
memcpy(&pageno,XLogRecGetData(record),sizeof(int));
977+
AdvanceOldestCommitTsXid(trunc->oldestXid);
973978

974979
/*
975980
* During XLOG replay, latest_page_number isn't set up yet; insert a
976981
* suitable value to bypass the sanity test in SimpleLruTruncate.
977982
*/
978-
CommitTsCtl->shared->latest_page_number=pageno;
983+
CommitTsCtl->shared->latest_page_number=trunc->pageno;
979984

980-
SimpleLruTruncate(CommitTsCtl,pageno);
985+
SimpleLruTruncate(CommitTsCtl,trunc->pageno);
981986
}
982987
elseif (info==COMMIT_TS_SETTS)
983988
{

‎src/backend/commands/vacuum.c

Lines changed: 9 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1152,6 +1152,15 @@ vac_truncate_clog(TransactionId frozenXID,
11521152
if (bogus)
11531153
return;
11541154

1155+
/*
1156+
* Advance the oldest value for commit timestamps before truncating, so
1157+
* that if a user requests a timestamp for a transaction we're truncating
1158+
* away right after this point, they get NULL instead of an ugly "file not
1159+
* found" error from slru.c. This doesn't matter for xact/multixact
1160+
* because they are not subject to arbitrary lookups from users.
1161+
*/
1162+
AdvanceOldestCommitTsXid(frozenXID);
1163+
11551164
/*
11561165
* Truncate CLOG, multixact and CommitTs to the oldest computed value.
11571166
*/
@@ -1167,7 +1176,6 @@ vac_truncate_clog(TransactionId frozenXID,
11671176
*/
11681177
SetTransactionIdLimit(frozenXID,oldestxid_datoid);
11691178
SetMultiXactIdLimit(minMulti,minmulti_datoid);
1170-
AdvanceOldestCommitTsXid(frozenXID);
11711179
}
11721180

11731181

‎src/include/access/commit_ts.h

Lines changed: 8 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -61,6 +61,14 @@ typedef struct xl_commit_ts_set
6161
#defineSizeOfCommitTsSet(offsetof(xl_commit_ts_set, mainxid) + \
6262
sizeof(TransactionId))
6363

64+
typedefstructxl_commit_ts_truncate
65+
{
66+
intpageno;
67+
TransactionIdoldestXid;
68+
}xl_commit_ts_truncate;
69+
70+
#defineSizeOfCommitTsTruncate(offsetof(xl_commit_ts_truncate, oldestXid) + \
71+
sizeof(TransactionId))
6472

6573
externvoidcommit_ts_redo(XLogReaderState*record);
6674
externvoidcommit_ts_desc(StringInfobuf,XLogReaderState*record);

‎src/include/access/xlog_internal.h

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -31,7 +31,7 @@
3131
/*
3232
* Each page of XLOG file has a header like this:
3333
*/
34-
#defineXLOG_PAGE_MAGIC0xD093/* can be used as WAL version indicator */
34+
#defineXLOG_PAGE_MAGIC0xD094/* can be used as WAL version indicator */
3535

3636
typedefstructXLogPageHeaderData
3737
{

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