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Fix race condition in reading commit timestamps

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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
This commit is contained in:
Alvaro Herrera
2017-01-19 18:23:09 -03:00
parent 8b0fec93ec
commit 8eace46d34
5 changed files with 34 additions and 13 deletions
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8
src/include/access/commit_ts.h
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@ -61,6 +61,14 @@ typedef struct xl_commit_ts_set
#define SizeOfCommitTsSet (offsetof(xl_commit_ts_set, mainxid) + \
sizeof(TransactionId))
typedef struct xl_commit_ts_truncate
{
int pageno;
TransactionId oldestXid;
} xl_commit_ts_truncate;
#define SizeOfCommitTsTruncate (offsetof(xl_commit_ts_truncate, oldestXid) + \
sizeof(TransactionId))
extern void commit_ts_redo(XLogReaderState *record);
extern void commit_ts_desc(StringInfo buf, XLogReaderState *record);