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

1 files changed, 8 insertions, 0 deletions

diff --git a/src/include/access/commit_ts.h b/src/include/access/commit_ts.h
index 98bd938d5be..f172c91d8f3 100644
--- a/src/include/access/commit_ts.h
+++ b/src/include/access/commit_ts.h
@@ -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);