Fix incorrect initialization of ProcGlobal->startupBufferPinWaitBufId.

It was initialized in the wrong place and to the wrong value.  With bad
luck this could result in incorrect query-cancellation failures in hot
standby sessions, should a HS backend be holding pin on buffer number 1
while trying to acquire a lock.

src/backend/storage/buffer/bufmgr.c

@@ -2449,10 +2449,11 @@ LockBufferForCleanup(Buffer buffer)
 		/* Wait to be signaled by UnpinBuffer() */
 		if (InHotStandby)
 		{
-			/* Share the bufid that Startup process waits on */
+			/* Publish the bufid that Startup process waits on */
 			SetStartupBufferPinWaitBufId(buffer - 1);
 			/* Set alarm and then wait to be signaled by UnpinBuffer() */
 			ResolveRecoveryConflictWithBufferPin();
+			/* Reset the published bufid */
 			SetStartupBufferPinWaitBufId(-1);
 		}
 		else

src/backend/storage/lmgr/proc.c

@@ -171,6 +171,9 @@ InitProcGlobal(void)
 	ProcGlobal->spins_per_delay = DEFAULT_SPINS_PER_DELAY;
 	ProcGlobal->freeProcs = NULL;
 	ProcGlobal->autovacFreeProcs = NULL;
+	ProcGlobal->startupProc = NULL;
+	ProcGlobal->startupProcPid = 0;
+	ProcGlobal->startupBufferPinWaitBufId = -1;
 
 	/*
 	 * Create and initialize all the PGPROC structures we'll need (except for
@@ -493,7 +496,6 @@ PublishStartupProcessInformation(void)
 
 	procglobal->startupProc = MyProc;
 	procglobal->startupProcPid = MyProcPid;
-	procglobal->startupBufferPinWaitBufId = 0;
 
 	SpinLockRelease(ProcStructLock);
 }
@@ -520,14 +522,10 @@ SetStartupBufferPinWaitBufId(int bufid)
 int
 GetStartupBufferPinWaitBufId(void)
 {
-	int			bufid;
-
 	/* use volatile pointer to prevent code rearrangement */
 	volatile PROC_HDR *procglobal = ProcGlobal;
 
-	bufid = procglobal->startupBufferPinWaitBufId;
-
-	return bufid;
+	return procglobal->startupBufferPinWaitBufId;
 }
 
 /*

src/include/storage/proc.h

@@ -178,7 +178,7 @@ typedef struct PROC_HDR
 	/* The proc of the Startup process, since not in ProcArray */
 	PGPROC	   *startupProc;
 	int			startupProcPid;
-	/* Buffer id of the buffer that Startup process waits for pin on */
+	/* Buffer id of the buffer that Startup process waits for pin on, or -1 */
 	int			startupBufferPinWaitBufId;
 } PROC_HDR;