Add condition variable for walreceiver shutdown.

Use this new CV to wait for walreceiver shutdown without a sleep/poll
loop, while also benefiting from standard postmaster death handling.

Discussion: https://postgr.es/m/CA%2BhUKGK1607VmtrDUHQXrsooU%3Dap4g4R2yaoByWOOA3m8xevUQ%40mail.gmail.com

doc/src/sgml/monitoring.sgml

@@ -1766,6 +1766,10 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
       <entry>Waiting for confirmation from a remote server during synchronous
        replication.</entry>
      </row>
+     <row>
+      <entry><literal>WalrcvExit</literal></entry>
+      <entry>Waiting for the walreceiver to exit.</entry>
+     </row>
      <row>
       <entry><literal>XactGroupUpdate</literal></entry>
       <entry>Waiting for the group leader to update transaction status at

src/backend/postmaster/pgstat.c

@@ -4124,6 +4124,9 @@ pgstat_get_wait_ipc(WaitEventIPC w)
 		case WAIT_EVENT_SYNC_REP:
 			event_name = "SyncRep";
 			break;
+		case WAIT_EVENT_WALRCV_EXIT:
+			event_name = "WalrcvExit";
+			break;
 		case WAIT_EVENT_XACT_GROUP_UPDATE:
 			event_name = "XactGroupUpdate";
 			break;

src/backend/replication/walreceiver.c

@@ -207,6 +207,7 @@ WalReceiverMain(void)
 
 		case WALRCV_STOPPED:
 			SpinLockRelease(&walrcv->mutex);
+			ConditionVariableBroadcast(&walrcv->walRcvStoppedCV);
 			proc_exit(1);
 			break;
 
@@ -784,6 +785,8 @@ WalRcvDie(int code, Datum arg)
 	walrcv->latch = NULL;
 	SpinLockRelease(&walrcv->mutex);
 
+	ConditionVariableBroadcast(&walrcv->walRcvStoppedCV);
+
 	/* Terminate the connection gracefully. */
 	if (wrconn != NULL)
 		walrcv_disconnect(wrconn);

src/backend/replication/walreceiverfuncs.c

@@ -23,6 +23,7 @@
 #include <signal.h>
 
 #include "access/xlog_internal.h"
+#include "pgstat.h"
 #include "postmaster/startup.h"
 #include "replication/walreceiver.h"
 #include "storage/pmsignal.h"
@@ -62,6 +63,7 @@ WalRcvShmemInit(void)
 		/* First time through, so initialize */
 		MemSet(WalRcv, 0, WalRcvShmemSize());
 		WalRcv->walRcvState = WALRCV_STOPPED;
+		ConditionVariableInit(&WalRcv->walRcvStoppedCV);
 		SpinLockInit(&WalRcv->mutex);
 		pg_atomic_init_u64(&WalRcv->writtenUpto, 0);
 		WalRcv->latch = NULL;
@@ -95,12 +97,18 @@ WalRcvRunning(void)
 
 		if ((now - startTime) > WALRCV_STARTUP_TIMEOUT)
 		{
+			bool		stopped = false;
+
 			SpinLockAcquire(&walrcv->mutex);
-
 			if (walrcv->walRcvState == WALRCV_STARTING)
+			{
 				state = walrcv->walRcvState = WALRCV_STOPPED;
-
+				stopped = true;
+			}
 			SpinLockRelease(&walrcv->mutex);
+
+			if (stopped)
+				ConditionVariableBroadcast(&walrcv->walRcvStoppedCV);
 		}
 	}
 
@@ -140,12 +148,18 @@ WalRcvStreaming(void)
 
 		if ((now - startTime) > WALRCV_STARTUP_TIMEOUT)
 		{
+			bool		stopped = false;
+
 			SpinLockAcquire(&walrcv->mutex);
-
 			if (walrcv->walRcvState == WALRCV_STARTING)
+			{
 				state = walrcv->walRcvState = WALRCV_STOPPED;
-
+				stopped = true;
+			}
 			SpinLockRelease(&walrcv->mutex);
+
+			if (stopped)
+				ConditionVariableBroadcast(&walrcv->walRcvStoppedCV);
 		}
 	}
 
@@ -165,6 +179,7 @@ ShutdownWalRcv(void)
 {
 	WalRcvData *walrcv = WalRcv;
 	pid_t		walrcvpid = 0;
+	bool		stopped = false;
 
 	/*
 	 * Request walreceiver to stop. Walreceiver will switch to WALRCV_STOPPED
@@ -178,6 +193,7 @@ ShutdownWalRcv(void)
 			break;
 		case WALRCV_STARTING:
 			walrcv->walRcvState = WALRCV_STOPPED;
+			stopped = true;
 			break;
 
 		case WALRCV_STREAMING:
@@ -191,6 +207,10 @@ ShutdownWalRcv(void)
 	}
 	SpinLockRelease(&walrcv->mutex);
 
+	/* Unnecessary but consistent. */
+	if (stopped)
+		ConditionVariableBroadcast(&walrcv->walRcvStoppedCV);
+
 	/*
 	 * Signal walreceiver process if it was still running.
 	 */
@@ -201,16 +221,11 @@ ShutdownWalRcv(void)
 	 * Wait for walreceiver to acknowledge its death by setting state to
 	 * WALRCV_STOPPED.
 	 */
+	ConditionVariablePrepareToSleep(&walrcv->walRcvStoppedCV);
 	while (WalRcvRunning())
-	{
+		ConditionVariableSleep(&walrcv->walRcvStoppedCV,
-		/*
+							   WAIT_EVENT_WALRCV_EXIT);
-		 * This possibly-long loop needs to handle interrupts of startup
+	ConditionVariableCancelSleep();
-		 * process.
-		 */
-		HandleStartupProcInterrupts();
-
-		pg_usleep(100000);		/* 100ms */
-	}
 }
 
 /*

src/include/pgstat.h

@@ -1009,6 +1009,7 @@ typedef enum
 	WAIT_EVENT_REPLICATION_SLOT_DROP,
 	WAIT_EVENT_SAFE_SNAPSHOT,
 	WAIT_EVENT_SYNC_REP,
+	WAIT_EVENT_WALRCV_EXIT,
 	WAIT_EVENT_XACT_GROUP_UPDATE
 } WaitEventIPC;
 

src/include/replication/walreceiver.h

@@ -19,6 +19,7 @@
 #include "port/atomics.h"
 #include "replication/logicalproto.h"
 #include "replication/walsender.h"
+#include "storage/condition_variable.h"
 #include "storage/latch.h"
 #include "storage/spin.h"
 #include "utils/tuplestore.h"
@@ -62,6 +63,7 @@ typedef struct
 	 */
 	pid_t		pid;
 	WalRcvState walRcvState;
+	ConditionVariable walRcvStoppedCV;
 	pg_time_t	startTime;
 
 	/*