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Commita4d1ce0

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Don't lose walreceiver start requests due to race condition in postmaster.
When a walreceiver dies, the startup process will notice that and senda PMSIGNAL_START_WALRECEIVER signal to the postmaster, asking for a newwalreceiver to be launched. There's a race condition, which at leastin HEAD is very easy to hit, whereby the postmaster might see thatsignal before it processes the SIGCHLD from the walreceiver process.In that situation, sigusr1_handler() just dropped the start requeston the floor, reasoning that it must be redundant. Eventually, after10 seconds (WALRCV_STARTUP_TIMEOUT), the startup process would make afresh request --- but that's a long time if the connection could havebeen re-established almost immediately.Fix it by setting a state flag inside the postmaster that we won'tclear until we do launch a walreceiver. In cases where that resultsin an extra walreceiver launch, it's up to the walreceiver to realizeit's unwanted and go away --- but we have, and need, that logic anywayfor the opposite race case.I came across this through investigating unexpected delays in thesrc/test/recovery TAP tests: it manifests there in test cases wherea master server is stopped and restarted while leaving streamingslaves active.This logic has been broken all along, so back-patch to all supportedbranches.Discussion:https://postgr.es/m/21344.1498494720@sss.pgh.pa.us
1 parentf6af9c7 commita4d1ce0

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1 file changed

+32
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‎src/backend/postmaster/postmaster.c

Lines changed: 32 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -354,6 +354,9 @@ static volatile sig_atomic_t start_autovac_launcher = false;
354354
/* the launcher needs to be signalled to communicate some condition */
355355
staticvolatileboolavlauncher_needs_signal= false;
356356

357+
/* received START_WALRECEIVER signal */
358+
staticvolatilesig_atomic_tWalReceiverRequested= false;
359+
357360
/* set when there's a worker that needs to be started up */
358361
staticvolatileboolStartWorkerNeeded= true;
359362
staticvolatileboolHaveCrashedWorker= false;
@@ -417,6 +420,7 @@ static void maybe_start_bgworkers(void);
417420
staticboolCreateOptsFile(intargc,char*argv[],char*fullprogname);
418421
staticpid_tStartChildProcess(AuxProcTypetype);
419422
staticvoidStartAutovacuumWorker(void);
423+
staticvoidMaybeStartWalReceiver(void);
420424
staticvoidInitPostmasterDeathWatchHandle(void);
421425

422426
/*
@@ -1783,6 +1787,10 @@ ServerLoop(void)
17831787
kill(AutoVacPID,SIGUSR2);
17841788
}
17851789

1790+
/* If we need to start a WAL receiver, try to do that now */
1791+
if (WalReceiverRequested)
1792+
MaybeStartWalReceiver();
1793+
17861794
/* Get other worker processes running, if needed */
17871795
if (StartWorkerNeeded||HaveCrashedWorker)
17881796
maybe_start_bgworkers();
@@ -2923,7 +2931,8 @@ reaper(SIGNAL_ARGS)
29232931
/*
29242932
* Was it the wal receiver? If exit status is zero (normal) or one
29252933
* (FATAL exit), we assume everything is all right just like normal
2926-
* backends.
2934+
* backends. (If we need a new wal receiver, we'll start one at the
2935+
* next iteration of the postmaster's main loop.)
29272936
*/
29282937
if (pid==WalReceiverPID)
29292938
{
@@ -5011,14 +5020,12 @@ sigusr1_handler(SIGNAL_ARGS)
50115020
StartAutovacuumWorker();
50125021
}
50135022

5014-
if (CheckPostmasterSignal(PMSIGNAL_START_WALRECEIVER)&&
5015-
WalReceiverPID==0&&
5016-
(pmState==PM_STARTUP||pmState==PM_RECOVERY||
5017-
pmState==PM_HOT_STANDBY||pmState==PM_WAIT_READONLY)&&
5018-
Shutdown==NoShutdown)
5023+
if (CheckPostmasterSignal(PMSIGNAL_START_WALRECEIVER))
50195024
{
50205025
/* Startup Process wants us to start the walreceiver process. */
5021-
WalReceiverPID=StartWalReceiver();
5026+
/* Start immediately if possible, else remember request for later. */
5027+
WalReceiverRequested= true;
5028+
MaybeStartWalReceiver();
50225029
}
50235030

50245031
if (CheckPostmasterSignal(PMSIGNAL_ADVANCE_STATE_MACHINE)&&
@@ -5369,6 +5376,24 @@ StartAutovacuumWorker(void)
53695376
}
53705377
}
53715378

5379+
/*
5380+
* MaybeStartWalReceiver
5381+
*Start the WAL receiver process, if not running and our state allows.
5382+
*/
5383+
staticvoid
5384+
MaybeStartWalReceiver(void)
5385+
{
5386+
if (WalReceiverPID==0&&
5387+
(pmState==PM_STARTUP||pmState==PM_RECOVERY||
5388+
pmState==PM_HOT_STANDBY||pmState==PM_WAIT_READONLY)&&
5389+
Shutdown==NoShutdown)
5390+
{
5391+
WalReceiverPID=StartWalReceiver();
5392+
WalReceiverRequested= false;
5393+
}
5394+
}
5395+
5396+
53725397
/*
53735398
* Create the opts file
53745399
*/

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