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Commitae90128

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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 parent9e26326 commitae90128

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+22
-5
lines changed

1 file changed

+22
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‎src/backend/commands/async.c

Lines changed: 22 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -1285,15 +1285,29 @@ static ListCell *
12851285
asyncQueueAddEntries(ListCell*nextNotify)
12861286
{
12871287
AsyncQueueEntryqe;
1288+
QueuePositionqueue_head;
12881289
intpageno;
12891290
intoffset;
12901291
intslotno;
12911292

12921293
/* We hold both AsyncQueueLock and AsyncCtlLock during this operation */
12931294
LWLockAcquire(AsyncCtlLock,LW_EXCLUSIVE);
12941295

1296+
/*
1297+
* We work with a local copy of QUEUE_HEAD, which we write back to shared
1298+
* memory upon exiting. The reason for this is that if we have to advance
1299+
* to a new page, SimpleLruZeroPage might fail (out of disk space, for
1300+
* instance), and we must not advance QUEUE_HEAD if it does. (Otherwise,
1301+
* subsequent insertions would try to put entries into a page that slru.c
1302+
* thinks doesn't exist yet.) So, use a local position variable. Note
1303+
* that if we do fail, any already-inserted queue entries are forgotten;
1304+
* this is okay, since they'd be useless anyway after our transaction
1305+
* rolls back.
1306+
*/
1307+
queue_head=QUEUE_HEAD;
1308+
12951309
/* Fetch the current page */
1296-
pageno=QUEUE_POS_PAGE(QUEUE_HEAD);
1310+
pageno=QUEUE_POS_PAGE(queue_head);
12971311
slotno=SimpleLruReadPage(AsyncCtl,pageno, true,InvalidTransactionId);
12981312
/* Note we mark the page dirty before writing in it */
12991313
AsyncCtl->shared->page_dirty[slotno]= true;
@@ -1305,7 +1319,7 @@ asyncQueueAddEntries(ListCell *nextNotify)
13051319
/* Construct a valid queue entry in local variable qe */
13061320
asyncQueueNotificationToEntry(n,&qe);
13071321

1308-
offset=QUEUE_POS_OFFSET(QUEUE_HEAD);
1322+
offset=QUEUE_POS_OFFSET(queue_head);
13091323

13101324
/* Check whether the entry really fits on the current page */
13111325
if (offset+qe.length <=QUEUE_PAGESIZE)
@@ -1331,8 +1345,8 @@ asyncQueueAddEntries(ListCell *nextNotify)
13311345
&qe,
13321346
qe.length);
13331347

1334-
/* AdvanceQUEUE_HEAD appropriately, andnote if page is full */
1335-
if (asyncQueueAdvance(&(QUEUE_HEAD),qe.length))
1348+
/* Advancequeue_head appropriately, anddetect if page is full */
1349+
if (asyncQueueAdvance(&(queue_head),qe.length))
13361350
{
13371351
/*
13381352
* Page is full, so we're done here, but first fill the next page
@@ -1342,12 +1356,15 @@ asyncQueueAddEntries(ListCell *nextNotify)
13421356
* asyncQueueIsFull() ensured that there is room to create this
13431357
* page without overrunning the queue.
13441358
*/
1345-
slotno=SimpleLruZeroPage(AsyncCtl,QUEUE_POS_PAGE(QUEUE_HEAD));
1359+
slotno=SimpleLruZeroPage(AsyncCtl,QUEUE_POS_PAGE(queue_head));
13461360
/* And exit the loop */
13471361
break;
13481362
}
13491363
}
13501364

1365+
/* Success, so update the global QUEUE_HEAD */
1366+
QUEUE_HEAD=queue_head;
1367+
13511368
LWLockRelease(AsyncCtlLock);
13521369

13531370
returnnextNotify;

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