Fix race condition in reading commit timestamps
If a user requests the commit timestamp for a transaction old enough that its data is concurrently being truncated away by vacuum at just the right time, they would receive an ugly internal file-not-found error message from slru.c rather than the expected NULL return value. In a primary server, the window for the race is very small: the lookup has to occur exactly between the two calls by vacuum, and there's not a lot that happens between them (mostly just a multixact truncate). In a standby server, however, the window is larger because the truncation is executed as soon as the WAL record for it is replayed, but the advance of the oldest-Xid is not executed until the next checkpoint record. To fix in the primary, simply reverse the order of operations in vac_truncate_clog. To fix in the standby, augment the WAL truncation record so that the standby is aware of the new oldest-XID value and can apply the update immediately. WAL version bumped because of this. No backpatch, because of the low importance of the bug and its rarity. Author: Craig Ringer Reviewed-By: Petr Jelínek, Peter Eisentraut Discussion: https://postgr.es/m/CAMsr+YFhVtRQT1VAwC+WGbbxZZRzNou=N9Ed-FrCqkwQ8H8oJQ@mail.gmail.com
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@ -33,10 +33,10 @@ commit_ts_desc(StringInfo buf, XLogReaderState *record)
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}
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else if (info == COMMIT_TS_TRUNCATE)
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{
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int pageno;
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xl_commit_ts_truncate *trunc = (xl_commit_ts_truncate *) rec;
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memcpy(&pageno, rec, sizeof(int));
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appendStringInfo(buf, "%d", pageno);
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appendStringInfo(buf, "pageno %d, oldestXid %u",
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trunc->pageno, trunc->oldestXid);
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}
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else if (info == COMMIT_TS_SETTS)
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{
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@ -113,7 +113,7 @@ static bool CommitTsPagePrecedes(int page1, int page2);
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static void ActivateCommitTs(void);
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static void DeactivateCommitTs(void);
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static void WriteZeroPageXlogRec(int pageno);
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static void WriteTruncateXlogRec(int pageno);
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static void WriteTruncateXlogRec(int pageno, TransactionId oldestXid);
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static void WriteSetTimestampXlogRec(TransactionId mainxid, int nsubxids,
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TransactionId *subxids, TimestampTz timestamp,
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RepOriginId nodeid);
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@ -824,7 +824,7 @@ TruncateCommitTs(TransactionId oldestXact)
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return; /* nothing to remove */
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/* Write XLOG record */
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WriteTruncateXlogRec(cutoffPage);
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WriteTruncateXlogRec(cutoffPage, oldestXact);
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/* Now we can remove the old CommitTs segment(s) */
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SimpleLruTruncate(CommitTsCtl, cutoffPage);
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@ -910,10 +910,15 @@ WriteZeroPageXlogRec(int pageno)
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* Write a TRUNCATE xlog record
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*/
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static void
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WriteTruncateXlogRec(int pageno)
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WriteTruncateXlogRec(int pageno, TransactionId oldestXid)
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{
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xl_commit_ts_truncate xlrec;
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xlrec.pageno = pageno;
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xlrec.oldestXid = oldestXid;
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XLogBeginInsert();
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XLogRegisterData((char *) (&pageno), sizeof(int));
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XLogRegisterData((char *) (&xlrec), SizeOfCommitTsTruncate);
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(void) XLogInsert(RM_COMMIT_TS_ID, COMMIT_TS_TRUNCATE);
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}
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@ -967,17 +972,17 @@ commit_ts_redo(XLogReaderState *record)
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}
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else if (info == COMMIT_TS_TRUNCATE)
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{
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int pageno;
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xl_commit_ts_truncate *trunc = (xl_commit_ts_truncate *) XLogRecGetData(record);
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memcpy(&pageno, XLogRecGetData(record), sizeof(int));
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AdvanceOldestCommitTsXid(trunc->oldestXid);
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/*
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* During XLOG replay, latest_page_number isn't set up yet; insert a
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* suitable value to bypass the sanity test in SimpleLruTruncate.
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*/
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CommitTsCtl->shared->latest_page_number = pageno;
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CommitTsCtl->shared->latest_page_number = trunc->pageno;
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SimpleLruTruncate(CommitTsCtl, pageno);
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SimpleLruTruncate(CommitTsCtl, trunc->pageno);
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}
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else if (info == COMMIT_TS_SETTS)
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{
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@ -1152,6 +1152,15 @@ vac_truncate_clog(TransactionId frozenXID,
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if (bogus)
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return;
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/*
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* Advance the oldest value for commit timestamps before truncating, so
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* that if a user requests a timestamp for a transaction we're truncating
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* away right after this point, they get NULL instead of an ugly "file not
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* found" error from slru.c. This doesn't matter for xact/multixact
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* because they are not subject to arbitrary lookups from users.
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*/
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AdvanceOldestCommitTsXid(frozenXID);
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/*
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* Truncate CLOG, multixact and CommitTs to the oldest computed value.
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*/
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@ -1167,7 +1176,6 @@ vac_truncate_clog(TransactionId frozenXID,
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*/
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SetTransactionIdLimit(frozenXID, oldestxid_datoid);
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SetMultiXactIdLimit(minMulti, minmulti_datoid);
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AdvanceOldestCommitTsXid(frozenXID);
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}
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@ -61,6 +61,14 @@ typedef struct xl_commit_ts_set
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#define SizeOfCommitTsSet (offsetof(xl_commit_ts_set, mainxid) + \
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sizeof(TransactionId))
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typedef struct xl_commit_ts_truncate
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{
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int pageno;
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TransactionId oldestXid;
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} xl_commit_ts_truncate;
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#define SizeOfCommitTsTruncate (offsetof(xl_commit_ts_truncate, oldestXid) + \
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sizeof(TransactionId))
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extern void commit_ts_redo(XLogReaderState *record);
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extern void commit_ts_desc(StringInfo buf, XLogReaderState *record);
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@ -31,7 +31,7 @@
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/*
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* Each page of XLOG file has a header like this:
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*/
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#define XLOG_PAGE_MAGIC 0xD093 /* can be used as WAL version indicator */
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#define XLOG_PAGE_MAGIC 0xD094 /* can be used as WAL version indicator */
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typedef struct XLogPageHeaderData
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{
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