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Fix NOTIFY to cope with I/O problems, such as out-of-disk-space.

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The LISTEN/NOTIFY subsystem got confused if SimpleLruZeroPage failed,
which would typically happen as a result of a write() failure while
attempting to dump a dirty pg_notify page out of memory.  Subsequently,
all attempts to send more NOTIFY messages would fail with messages like
"Could not read from file "pg_notify/nnnn" at offset nnnnn: Success".
Only restarting the server would clear this condition.  Per reports from
Kevin Grittner and Christoph Berg.

Back-patch to 9.0, where the problem was introduced during the
LISTEN/NOTIFY rewrite.
This commit is contained in:
Tom Lane 2012-06-29 00:51:34 -04:00
parent 9e26326ad6
commit ae90128dc5
1 changed files with 22 additions and 5 deletions
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27
src/backend/commands/async.c
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@ -1285,6 +1285,7 @@ static ListCell *
asyncQueueAddEntries(ListCell *nextNotify)
{
AsyncQueueEntry qe;
QueuePosition queue_head;
int pageno;
int offset;
int slotno;
@ -1292,8 +1293,21 @@ asyncQueueAddEntries(ListCell *nextNotify)
/* We hold both AsyncQueueLock and AsyncCtlLock during this operation */
LWLockAcquire(AsyncCtlLock, LW_EXCLUSIVE);
/*
* We work with a local copy of QUEUE_HEAD, which we write back to shared
* memory upon exiting. The reason for this is that if we have to advance
* to a new page, SimpleLruZeroPage might fail (out of disk space, for
* instance), and we must not advance QUEUE_HEAD if it does. (Otherwise,
* subsequent insertions would try to put entries into a page that slru.c
* thinks doesn't exist yet.) So, use a local position variable. Note
* that if we do fail, any already-inserted queue entries are forgotten;
* this is okay, since they'd be useless anyway after our transaction
* rolls back.
*/
queue_head = QUEUE_HEAD;
/* Fetch the current page */
pageno = QUEUE_POS_PAGE(QUEUE_HEAD);
pageno = QUEUE_POS_PAGE(queue_head);
slotno = SimpleLruReadPage(AsyncCtl, pageno, true, InvalidTransactionId);
/* Note we mark the page dirty before writing in it */
AsyncCtl->shared->page_dirty[slotno] = true;
@ -1305,7 +1319,7 @@ asyncQueueAddEntries(ListCell *nextNotify)
/* Construct a valid queue entry in local variable qe */
asyncQueueNotificationToEntry(n, &qe);
offset = QUEUE_POS_OFFSET(QUEUE_HEAD);
offset = QUEUE_POS_OFFSET(queue_head);
/* Check whether the entry really fits on the current page */
if (offset + qe.length <= QUEUE_PAGESIZE)
@ -1331,8 +1345,8 @@ asyncQueueAddEntries(ListCell *nextNotify)
&qe,
qe.length);
/* Advance QUEUE_HEAD appropriately, and note if page is full */
if (asyncQueueAdvance(&(QUEUE_HEAD), qe.length))
/* Advance queue_head appropriately, and detect if page is full */
if (asyncQueueAdvance(&(queue_head), qe.length))
{
/*
* Page is full, so we're done here, but first fill the next page
@ -1342,12 +1356,15 @@ asyncQueueAddEntries(ListCell *nextNotify)
* asyncQueueIsFull() ensured that there is room to create this
* page without overrunning the queue.
*/
slotno = SimpleLruZeroPage(AsyncCtl, QUEUE_POS_PAGE(QUEUE_HEAD));
slotno = SimpleLruZeroPage(AsyncCtl, QUEUE_POS_PAGE(queue_head));
/* And exit the loop */
break;
}
}
/* Success, so update the global QUEUE_HEAD */
QUEUE_HEAD = queue_head;
LWLockRelease(AsyncCtlLock);
return nextNotify;