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Commite5d494d

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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
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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
@@ -357,6 +357,9 @@ static volatile sig_atomic_t start_autovac_launcher = false;
357357
/* the launcher needs to be signalled to communicate some condition */
358358
staticvolatileboolavlauncher_needs_signal= false;
359359

360+
/* received START_WALRECEIVER signal */
361+
staticvolatilesig_atomic_tWalReceiverRequested= false;
362+
360363
/* set when there's a worker that needs to be started up */
361364
staticvolatileboolStartWorkerNeeded= true;
362365
staticvolatileboolHaveCrashedWorker= false;
@@ -426,6 +429,7 @@ static void maybe_start_bgworkers(void);
426429
staticboolCreateOptsFile(intargc,char*argv[],char*fullprogname);
427430
staticpid_tStartChildProcess(AuxProcTypetype);
428431
staticvoidStartAutovacuumWorker(void);
432+
staticvoidMaybeStartWalReceiver(void);
429433
staticvoidInitPostmasterDeathWatchHandle(void);
430434

431435
/*
@@ -1810,6 +1814,10 @@ ServerLoop(void)
18101814
kill(AutoVacPID,SIGUSR2);
18111815
}
18121816

1817+
/* If we need to start a WAL receiver, try to do that now */
1818+
if (WalReceiverRequested)
1819+
MaybeStartWalReceiver();
1820+
18131821
/* Get other worker processes running, if needed */
18141822
if (StartWorkerNeeded||HaveCrashedWorker)
18151823
maybe_start_bgworkers();
@@ -2958,7 +2966,8 @@ reaper(SIGNAL_ARGS)
29582966
/*
29592967
* Was it the wal receiver? If exit status is zero (normal) or one
29602968
* (FATAL exit), we assume everything is all right just like normal
2961-
* backends.
2969+
* backends. (If we need a new wal receiver, we'll start one at the
2970+
* next iteration of the postmaster's main loop.)
29622971
*/
29632972
if (pid==WalReceiverPID)
29642973
{
@@ -5066,14 +5075,12 @@ sigusr1_handler(SIGNAL_ARGS)
50665075
StartAutovacuumWorker();
50675076
}
50685077

5069-
if (CheckPostmasterSignal(PMSIGNAL_START_WALRECEIVER)&&
5070-
WalReceiverPID==0&&
5071-
(pmState==PM_STARTUP||pmState==PM_RECOVERY||
5072-
pmState==PM_HOT_STANDBY||pmState==PM_WAIT_READONLY)&&
5073-
Shutdown==NoShutdown)
5078+
if (CheckPostmasterSignal(PMSIGNAL_START_WALRECEIVER))
50745079
{
50755080
/* Startup Process wants us to start the walreceiver process. */
5076-
WalReceiverPID=StartWalReceiver();
5081+
/* Start immediately if possible, else remember request for later. */
5082+
WalReceiverRequested= true;
5083+
MaybeStartWalReceiver();
50775084
}
50785085

50795086
if (CheckPostmasterSignal(PMSIGNAL_ADVANCE_STATE_MACHINE)&&
@@ -5409,6 +5416,24 @@ StartAutovacuumWorker(void)
54095416
}
54105417
}
54115418

5419+
/*
5420+
* MaybeStartWalReceiver
5421+
*Start the WAL receiver process, if not running and our state allows.
5422+
*/
5423+
staticvoid
5424+
MaybeStartWalReceiver(void)
5425+
{
5426+
if (WalReceiverPID==0&&
5427+
(pmState==PM_STARTUP||pmState==PM_RECOVERY||
5428+
pmState==PM_HOT_STANDBY||pmState==PM_WAIT_READONLY)&&
5429+
Shutdown==NoShutdown)
5430+
{
5431+
WalReceiverPID=StartWalReceiver();
5432+
WalReceiverRequested= false;
5433+
}
5434+
}
5435+
5436+
54125437
/*
54135438
* Create the opts file
54145439
*/

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