PublishStartupProcessInformation() to avoid rare hang in recovery.

Bgwriter could cause hang in recovery during page concurrent cleaning.

Bug report and testing by Bernd Helmle, fix by me

src/backend/access/transam/xlog.c

@@ -44,6 +44,7 @@
 #include "storage/fd.h"
 #include "storage/ipc.h"
 #include "storage/pmsignal.h"
+#include "storage/proc.h"
 #include "storage/procarray.h"
 #include "storage/smgr.h"
 #include "storage/spin.h"
@@ -5569,6 +5570,7 @@ StartupXLOG(void)
 			 */
 			if (InArchiveRecovery && IsUnderPostmaster)
 			{
+				PublishStartupProcessInformation();
 				SetForwardFsyncRequests();
 				SendPostmasterSignal(PMSIGNAL_RECOVERY_STARTED);
 				bgwriterLaunched = true;

src/backend/storage/lmgr/proc.c

@@ -1287,12 +1287,53 @@ ProcWaitForSignal(void)
 void
 ProcSendSignal(int pid)
 {
-	PGPROC	   *proc = BackendPidGetProc(pid);
+	PGPROC	   *proc = NULL;
+
+	proc = BackendPidGetProc(pid);
+
+	if (proc == NULL)
+	{
+		/* use volatile pointer to prevent code rearrangement */
+		volatile PROC_HDR *procglobal = ProcGlobal;
+
+		SpinLockAcquire(ProcStructLock);
+
+		/*
+		 * Check to see whether it is the Startup process we wish to signal.
+		 * This call is made by the buffer manager when it wishes to wake up a
+		 * process that has been waiting for a pin in so it can obtain a
+		 * cleanup lock using LockBufferForCleanup(). Startup is not a normal
+		 * backend, so BackendPidGetProc() will not return any pid at all. So
+		 * we remember the information for this special case.
+		 */
+		if (pid == procglobal->startupProcPid)
+			proc = procglobal->startupProc;
+
+		SpinLockRelease(ProcStructLock);
+	}
 
 	if (proc != NULL)
 		PGSemaphoreUnlock(&proc->sem);
 }
 
+/*
+ * Record the PID and PGPROC structures for the Startup process, for use in
+ * ProcSendSignal().  See comments there for further explanation.
+ */
+void
+PublishStartupProcessInformation(void)
+{
+	/* use volatile pointer to prevent code rearrangement */
+	volatile PROC_HDR *procglobal = ProcGlobal;
+
+	SpinLockAcquire(ProcStructLock);
+
+	procglobal->startupProc = MyProc;
+	procglobal->startupProcPid = MyProcPid;
+
+	SpinLockRelease(ProcStructLock);
+}
+
 
 /*****************************************************************************
  * SIGALRM interrupt support

src/include/storage/proc.h

@@ -133,6 +133,11 @@ typedef struct PROC_HDR
 	PGPROC	   *autovacFreeProcs;
 	/* Current shared estimate of appropriate spins_per_delay value */
 	int			spins_per_delay;
+
+	/* PGPROC of Startup process */
+	PGPROC	   *startupProc;
+	/* Pid of Startup process */
+	int			startupProcPid;
 } PROC_HDR;
 
 /*
@@ -175,6 +180,7 @@ extern void LockWaitCancel(void);
 
 extern void ProcWaitForSignal(void);
 extern void ProcSendSignal(int pid);
+extern void PublishStartupProcessInformation(void);
 
 extern bool enable_sig_alarm(int delayms, bool is_statement_timeout);
 extern bool disable_sig_alarm(bool is_statement_timeout);