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Prevent concurrent SimpleLruTruncate() for any given SLRU.

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The SimpleLruTruncate() header comment states the new coding rule.  To
achieve this, add locktype "frozenid" and two LWLocks.  This closes a
rare opportunity for data loss, which manifested as "apparent
wraparound" or "could not access status of transaction" errors.  Data
loss is more likely in pg_multixact, due to released branches' thin
margin between multiStopLimit and multiWrapLimit.  If a user's physical
replication primary logged ":  apparent wraparound" messages, the user
should rebuild standbys of that primary regardless of symptoms.  At less
risk is a cluster having emitted "not accepting commands" errors or
"must be vacuumed" warnings at some point.  One can test a cluster for
this data loss by running VACUUM FREEZE in every database.  Back-patch
to 9.5 (all supported versions).

Discussion: https://postgr.es/m/20190218073103.GA1434723@rfd.leadboat.com
This commit is contained in:
Noah Misch
2020-08-15 10:15:53 -07:00
parent d4d443b3bb
commit 566372b3d6
11 changed files with 117 additions and 13 deletions
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8
src/backend/access/transam/slru.c
View File

@ -1191,6 +1191,14 @@ SimpleLruFlush(SlruCtl ctl, bool allow_redirtied)
/*
* Remove all segments before the one holding the passed page number
*
* All SLRUs prevent concurrent calls to this function, either with an LWLock
* or by calling it only as part of a checkpoint. Mutual exclusion must begin
* before computing cutoffPage. Mutual exclusion must end after any limit
* update that would permit other backends to write fresh data into the
* segment immediately preceding the one containing cutoffPage. Otherwise,
* when the SLRU is quite full, SimpleLruTruncate() might delete that segment
* after it has accrued freshly-written data.
*/
void
SimpleLruTruncate(SlruCtl ctl, int cutoffPage)

4
src/backend/access/transam/subtrans.c
View File

@ -349,8 +349,8 @@ ExtendSUBTRANS(TransactionId newestXact)
/*
* Remove all SUBTRANS segments before the one holding the passed transaction ID
*
* This is normally called during checkpoint, with oldestXact being the
* oldest TransactionXmin of any running transaction.
* oldestXact is the oldest TransactionXmin of any running transaction. This
* is called only during checkpoint.
*/
void
TruncateSUBTRANS(TransactionId oldestXact)