From 8eace46d34ab6ac0d887aa4d3504bc4222c2e448 Mon Sep 17 00:00:00 2001 From: Alvaro Herrera Date: Thu, 19 Jan 2017 18:23:09 -0300 Subject: Fix race condition in reading commit timestamps MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 --- src/backend/access/transam/commit_ts.c | 21 +++++++++++++-------- 1 file changed, 13 insertions(+), 8 deletions(-) (limited to 'src/backend/access/transam/commit_ts.c') diff --git a/src/backend/access/transam/commit_ts.c b/src/backend/access/transam/commit_ts.c index 2403de3ae36..18a5f5602c7 100644 --- a/src/backend/access/transam/commit_ts.c +++ b/src/backend/access/transam/commit_ts.c @@ -113,7 +113,7 @@ static bool CommitTsPagePrecedes(int page1, int page2); static void ActivateCommitTs(void); static void DeactivateCommitTs(void); static void WriteZeroPageXlogRec(int pageno); -static void WriteTruncateXlogRec(int pageno); +static void WriteTruncateXlogRec(int pageno, TransactionId oldestXid); static void WriteSetTimestampXlogRec(TransactionId mainxid, int nsubxids, TransactionId *subxids, TimestampTz timestamp, RepOriginId nodeid); @@ -824,7 +824,7 @@ TruncateCommitTs(TransactionId oldestXact) return; /* nothing to remove */ /* Write XLOG record */ - WriteTruncateXlogRec(cutoffPage); + WriteTruncateXlogRec(cutoffPage, oldestXact); /* Now we can remove the old CommitTs segment(s) */ SimpleLruTruncate(CommitTsCtl, cutoffPage); @@ -910,10 +910,15 @@ WriteZeroPageXlogRec(int pageno) * Write a TRUNCATE xlog record */ static void -WriteTruncateXlogRec(int pageno) +WriteTruncateXlogRec(int pageno, TransactionId oldestXid) { + xl_commit_ts_truncate xlrec; + + xlrec.pageno = pageno; + xlrec.oldestXid = oldestXid; + XLogBeginInsert(); - XLogRegisterData((char *) (&pageno), sizeof(int)); + XLogRegisterData((char *) (&xlrec), SizeOfCommitTsTruncate); (void) XLogInsert(RM_COMMIT_TS_ID, COMMIT_TS_TRUNCATE); } @@ -967,17 +972,17 @@ commit_ts_redo(XLogReaderState *record) } else if (info == COMMIT_TS_TRUNCATE) { - int pageno; + xl_commit_ts_truncate *trunc = (xl_commit_ts_truncate *) XLogRecGetData(record); - memcpy(&pageno, XLogRecGetData(record), sizeof(int)); + AdvanceOldestCommitTsXid(trunc->oldestXid); /* * During XLOG replay, latest_page_number isn't set up yet; insert a * suitable value to bypass the sanity test in SimpleLruTruncate. */ - CommitTsCtl->shared->latest_page_number = pageno; + CommitTsCtl->shared->latest_page_number = trunc->pageno; - SimpleLruTruncate(CommitTsCtl, pageno); + SimpleLruTruncate(CommitTsCtl, trunc->pageno); } else if (info == COMMIT_TS_SETTS) { -- cgit v1.2.3