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Commitcbd9619

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Block signals while computing the sleep time in postmaster's main loop.
DetermineSleepTime() was previously called without blockedsignals. That's not good, because it allows signal handlers tointerrupt its workings.DetermineSleepTime() was added in 9.3 with the addition of backgroundworkers (da07a1e), where it only read fromBackgroundWorkerList.Since 9.4, where dynamic background workers were added (7f7485a),the list is also manipulated in DetermineSleepTime(). That's badbecause the list now can be persistently corrupted if modified by botha signal handler and DetermineSleepTime().This was discovered during the investigation of hangs on buildfarmmember anole. It's unclear whether this bug is the source of thesehangs or not, but it's worth fixing either way. I have confirmed thatit can cause crashes.It luckily looks like this only can cause problems when bgworkers areactively used.Discussion: 20140929193733.GB14400@awork2.anarazel.deBackpatch to 9.3 where background workers were introduced.
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‎src/backend/postmaster/postmaster.c

Lines changed: 16 additions & 10 deletions
Original file line numberDiff line numberDiff line change
@@ -1502,6 +1502,8 @@ DetermineSleepTime(struct timeval * timeout)
15021502

15031503
/*
15041504
* Main idle loop of postmaster
1505+
*
1506+
* NB: Needs to be called with signals blocked
15051507
*/
15061508
staticint
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ServerLoop(void)
@@ -1523,34 +1525,38 @@ ServerLoop(void)
15231525
/*
15241526
* Wait for a connection request to arrive.
15251527
*
1528+
* We block all signals except while sleeping. That makes it safe for
1529+
* signal handlers, which again block all signals while executing, to
1530+
* do nontrivial work.
1531+
*
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* If we are in PM_WAIT_DEAD_END state, then we don't want to accept
1527-
* any new connections, so we don't call select() at all; just sleep
1528-
* for a little bit with signals unblocked.
1533+
* any new connections, so we don't call select(), and just sleep.
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*/
15301535
memcpy((char*)&rmask, (char*)&readmask,sizeof(fd_set));
15311536

1532-
PG_SETMASK(&UnBlockSig);
1533-
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if (pmState==PM_WAIT_DEAD_END)
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{
1539+
PG_SETMASK(&UnBlockSig);
1540+
15361541
pg_usleep(100000L);/* 100 msec seems reasonable */
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selres=0;
1543+
1544+
PG_SETMASK(&BlockSig);
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}
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else
15401547
{
15411548
/* must set timeout each time; some OSes change it! */
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structtimevaltimeout;
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1551+
/* Needs to run with blocked signals! */
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DetermineSleepTime(&timeout);
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1554+
PG_SETMASK(&UnBlockSig);
1555+
15461556
selres=select(nSockets,&rmask,NULL,NULL,&timeout);
1547-
}
15481557

1549-
/*
1550-
* Block all signals until we wait again. (This makes it safe for our
1551-
* signal handlers to do nontrivial work.)
1552-
*/
1553-
PG_SETMASK(&BlockSig);
1558+
PG_SETMASK(&BlockSig);
1559+
}
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15551561
/* Now check the select() result */
15561562
if (selres<0)

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