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Commit38a9052

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Fix NOTIFY to cope with I/O problems, such as out-of-disk-space.
The LISTEN/NOTIFY subsystem got confused if SimpleLruZeroPage failed,which would typically happen as a result of a write() failure whileattempting 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 fromKevin Grittner and Christoph Berg.Back-patch to 9.0, where the problem was introduced during theLISTEN/NOTIFY rewrite.
1 parent73a3b1d commit38a9052

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‎src/backend/commands/async.c‎

Lines changed: 22 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -1306,15 +1306,29 @@ static ListCell *
13061306
asyncQueueAddEntries(ListCell*nextNotify)
13071307
{
13081308
AsyncQueueEntryqe;
1309+
QueuePositionqueue_head;
13091310
intpageno;
13101311
intoffset;
13111312
intslotno;
13121313

13131314
/* We hold both AsyncQueueLock and AsyncCtlLock during this operation */
13141315
LWLockAcquire(AsyncCtlLock,LW_EXCLUSIVE);
13151316

1317+
/*
1318+
* We work with a local copy of QUEUE_HEAD, which we write back to shared
1319+
* memory upon exiting. The reason for this is that if we have to advance
1320+
* to a new page, SimpleLruZeroPage might fail (out of disk space, for
1321+
* instance), and we must not advance QUEUE_HEAD if it does. (Otherwise,
1322+
* subsequent insertions would try to put entries into a page that slru.c
1323+
* thinks doesn't exist yet.) So, use a local position variable. Note
1324+
* that if we do fail, any already-inserted queue entries are forgotten;
1325+
* this is okay, since they'd be useless anyway after our transaction
1326+
* rolls back.
1327+
*/
1328+
queue_head=QUEUE_HEAD;
1329+
13161330
/* Fetch the current page */
1317-
pageno=QUEUE_POS_PAGE(QUEUE_HEAD);
1331+
pageno=QUEUE_POS_PAGE(queue_head);
13181332
slotno=SimpleLruReadPage(AsyncCtl,pageno, true,InvalidTransactionId);
13191333
/* Note we mark the page dirty before writing in it */
13201334
AsyncCtl->shared->page_dirty[slotno]= true;
@@ -1326,7 +1340,7 @@ asyncQueueAddEntries(ListCell *nextNotify)
13261340
/* Construct a valid queue entry in local variable qe */
13271341
asyncQueueNotificationToEntry(n,&qe);
13281342

1329-
offset=QUEUE_POS_OFFSET(QUEUE_HEAD);
1343+
offset=QUEUE_POS_OFFSET(queue_head);
13301344

13311345
/* Check whether the entry really fits on the current page */
13321346
if (offset+qe.length <=QUEUE_PAGESIZE)
@@ -1352,8 +1366,8 @@ asyncQueueAddEntries(ListCell *nextNotify)
13521366
&qe,
13531367
qe.length);
13541368

1355-
/* AdvanceQUEUE_HEAD appropriately, andnote if page is full */
1356-
if (asyncQueueAdvance(&(QUEUE_HEAD),qe.length))
1369+
/* Advancequeue_head appropriately, anddetect if page is full */
1370+
if (asyncQueueAdvance(&(queue_head),qe.length))
13571371
{
13581372
/*
13591373
* Page is full, so we're done here, but first fill the next page
@@ -1363,12 +1377,15 @@ asyncQueueAddEntries(ListCell *nextNotify)
13631377
* asyncQueueIsFull() ensured that there is room to create this
13641378
* page without overrunning the queue.
13651379
*/
1366-
slotno=SimpleLruZeroPage(AsyncCtl,QUEUE_POS_PAGE(QUEUE_HEAD));
1380+
slotno=SimpleLruZeroPage(AsyncCtl,QUEUE_POS_PAGE(queue_head));
13671381
/* And exit the loop */
13681382
break;
13691383
}
13701384
}
13711385

1386+
/* Success, so update the global QUEUE_HEAD */
1387+
QUEUE_HEAD=queue_head;
1388+
13721389
LWLockRelease(AsyncCtlLock);
13731390

13741391
returnnextNotify;

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