Use ProcNumbers instead of direct Latch pointers to address other procs

This is in preparation for replacing Latches with a new abstraction.
That's still work in progress, but this seems a little tidier anyway,
so let's get this refactoring out of the way already.

Discussion: https://www.postgresql.org/message-id/391abe21-413e-4d91-a650-b663af49500c%40iki.fi

src/backend/access/transam/xlog.c

@@ -2671,8 +2671,14 @@ XLogSetAsyncXactLSN(XLogRecPtr asyncXactLSN)
 			wakeup = true;
 	}
 
-	if (wakeup && ProcGlobal->walwriterLatch)
-		SetLatch(ProcGlobal->walwriterLatch);
+	if (wakeup)
+	{
+		volatile PROC_HDR *procglobal = ProcGlobal;
+		ProcNumber	walwriterProc = procglobal->walwriterProc;
+
+		if (walwriterProc != INVALID_PROC_NUMBER)
+			SetLatch(&GetPGProcByNumber(walwriterProc)->procLatch);
+	}
 }
 
 /*

src/backend/access/transam/xlogwait.c

@@ -112,7 +112,7 @@ addLSNWaiter(XLogRecPtr lsn)
 
 	Assert(!procInfo->inHeap);
 
-	procInfo->latch = MyLatch;
+	procInfo->procno = MyProcNumber;
 	procInfo->waitLSN = lsn;
 
 	pairingheap_add(&waitLSNState->waitersHeap, &procInfo->phNode);
@@ -154,16 +154,17 @@ void
 WaitLSNSetLatches(XLogRecPtr currentLSN)
 {
 	int			i;
-	Latch	  **wakeUpProcLatches;
+	ProcNumber *wakeUpProcs;
 	int			numWakeUpProcs = 0;
 
-	wakeUpProcLatches = palloc(sizeof(Latch *) * MaxBackends);
+	wakeUpProcs = palloc(sizeof(ProcNumber) * MaxBackends);
 
 	LWLockAcquire(WaitLSNLock, LW_EXCLUSIVE);
 
 	/*
 	 * Iterate the pairing heap of waiting processes till we find LSN not yet
-	 * replayed.  Record the process latches to set them later.
+	 * replayed.  Record the process numbers to wake up, but to avoid holding
+	 * the lock for too long, send the wakeups only after releasing the lock.
 	 */
 	while (!pairingheap_is_empty(&waitLSNState->waitersHeap))
 	{
@@ -174,7 +175,7 @@ WaitLSNSetLatches(XLogRecPtr currentLSN)
 			procInfo->waitLSN > currentLSN)
 			break;
 
-		wakeUpProcLatches[numWakeUpProcs++] = procInfo->latch;
+		wakeUpProcs[numWakeUpProcs++] = procInfo->procno;
 		(void) pairingheap_remove_first(&waitLSNState->waitersHeap);
 		procInfo->inHeap = false;
 	}
@@ -191,9 +192,9 @@ WaitLSNSetLatches(XLogRecPtr currentLSN)
 	 */
 	for (i = 0; i < numWakeUpProcs; i++)
 	{
-		SetLatch(wakeUpProcLatches[i]);
+		SetLatch(&GetPGProcByNumber(wakeUpProcs[i])->procLatch);
 	}
-	pfree(wakeUpProcLatches);
+	pfree(wakeUpProcs);
 }
 
 /*

src/backend/postmaster/checkpointer.c

@@ -324,10 +324,10 @@ CheckpointerMain(char *startup_data, size_t startup_data_len)
 	UpdateSharedMemoryConfig();
 
 	/*
-	 * Advertise our latch that backends can use to wake us up while we're
-	 * sleeping.
+	 * Advertise our proc number that backends can use to wake us up while
+	 * we're sleeping.
 	 */
-	ProcGlobal->checkpointerLatch = &MyProc->procLatch;
+	ProcGlobal->checkpointerProc = MyProcNumber;
 
 	/*
 	 * Loop forever
@@ -1139,8 +1139,14 @@ ForwardSyncRequest(const FileTag *ftag, SyncRequestType type)
 	LWLockRelease(CheckpointerCommLock);
 
 	/* ... but not till after we release the lock */
-	if (too_full && ProcGlobal->checkpointerLatch)
-		SetLatch(ProcGlobal->checkpointerLatch);
+	if (too_full)
+	{
+		volatile PROC_HDR *procglobal = ProcGlobal;
+		ProcNumber	checkpointerProc = procglobal->checkpointerProc;
+
+		if (checkpointerProc != INVALID_PROC_NUMBER)
+			SetLatch(&GetPGProcByNumber(checkpointerProc)->procLatch);
+	}
 
 	return true;
 }

src/backend/postmaster/walwriter.c

@@ -208,10 +208,10 @@ WalWriterMain(char *startup_data, size_t startup_data_len)
 	SetWalWriterSleeping(false);
 
 	/*
-	 * Advertise our latch that backends can use to wake us up while we're
-	 * sleeping.
+	 * Advertise our proc number that backends can use to wake us up while
+	 * we're sleeping.
 	 */
-	ProcGlobal->walwriterLatch = &MyProc->procLatch;
+	ProcGlobal->walwriterProc = MyProcNumber;
 
 	/*
 	 * Loop forever

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -30,6 +30,7 @@
 #include "pgstat.h"
 #include "pqexpbuffer.h"
 #include "replication/walreceiver.h"
+#include "storage/latch.h"
 #include "utils/builtins.h"
 #include "utils/memutils.h"
 #include "utils/pg_lsn.h"

src/backend/replication/walreceiver.c

@@ -266,8 +266,8 @@ WalReceiverMain(char *startup_data, size_t startup_data_len)
 	walrcv->lastMsgSendTime =
 		walrcv->lastMsgReceiptTime = walrcv->latestWalEndTime = now;
 
-	/* Report the latch to use to awaken this process */
-	walrcv->latch = &MyProc->procLatch;
+	/* Report our proc number so that others can wake us up */
+	walrcv->procno = MyProcNumber;
 
 	SpinLockRelease(&walrcv->mutex);
 
@@ -819,8 +819,8 @@ WalRcvDie(int code, Datum arg)
 	Assert(walrcv->pid == MyProcPid);
 	walrcv->walRcvState = WALRCV_STOPPED;
 	walrcv->pid = 0;
+	walrcv->procno = INVALID_PROC_NUMBER;
 	walrcv->ready_to_display = false;
-	walrcv->latch = NULL;
 	SpinLockRelease(&walrcv->mutex);
 
 	ConditionVariableBroadcast(&walrcv->walRcvStoppedCV);
@@ -1358,15 +1358,15 @@ WalRcvComputeNextWakeup(WalRcvWakeupReason reason, TimestampTz now)
 void
 WalRcvForceReply(void)
 {
-	Latch	   *latch;
+	ProcNumber	procno;
 
 	WalRcv->force_reply = true;
-	/* fetching the latch pointer might not be atomic, so use spinlock */
+	/* fetching the proc number is probably atomic, but don't rely on it */
 	SpinLockAcquire(&WalRcv->mutex);
-	latch = WalRcv->latch;
+	procno = WalRcv->procno;
 	SpinLockRelease(&WalRcv->mutex);
-	if (latch)
-		SetLatch(latch);
+	if (procno != INVALID_PROC_NUMBER)
+		SetLatch(&GetPGProcByNumber(procno)->procLatch);
 }
 
 /*

src/backend/replication/walreceiverfuncs.c

@@ -27,6 +27,7 @@
 #include "pgstat.h"
 #include "replication/walreceiver.h"
 #include "storage/pmsignal.h"
+#include "storage/proc.h"
 #include "storage/shmem.h"
 #include "utils/timestamp.h"
 
@@ -66,7 +67,7 @@ WalRcvShmemInit(void)
 		ConditionVariableInit(&WalRcv->walRcvStoppedCV);
 		SpinLockInit(&WalRcv->mutex);
 		pg_atomic_init_u64(&WalRcv->writtenUpto, 0);
-		WalRcv->latch = NULL;
+		WalRcv->procno = INVALID_PROC_NUMBER;
 	}
 }
 
@@ -248,7 +249,7 @@ RequestXLogStreaming(TimeLineID tli, XLogRecPtr recptr, const char *conninfo,
 	WalRcvData *walrcv = WalRcv;
 	bool		launch = false;
 	pg_time_t	now = (pg_time_t) time(NULL);
-	Latch	   *latch;
+	ProcNumber	walrcv_proc;
 
 	/*
 	 * We always start at the beginning of the segment. That prevents a broken
@@ -309,14 +310,14 @@ RequestXLogStreaming(TimeLineID tli, XLogRecPtr recptr, const char *conninfo,
 	walrcv->receiveStart = recptr;
 	walrcv->receiveStartTLI = tli;
 
-	latch = walrcv->latch;
+	walrcv_proc = walrcv->procno;
 
 	SpinLockRelease(&walrcv->mutex);
 
 	if (launch)
 		SendPostmasterSignal(PMSIGNAL_START_WALRECEIVER);
-	else if (latch)
-		SetLatch(latch);
+	else if (walrcv_proc != INVALID_PROC_NUMBER)
+		SetLatch(&GetPGProcByNumber(walrcv_proc)->procLatch);
 }
 
 /*

src/backend/storage/lmgr/proc.c

@@ -194,8 +194,8 @@ InitProcGlobal(void)
 	dlist_init(&ProcGlobal->bgworkerFreeProcs);
 	dlist_init(&ProcGlobal->walsenderFreeProcs);
 	ProcGlobal->startupBufferPinWaitBufId = -1;
-	ProcGlobal->walwriterLatch = NULL;
-	ProcGlobal->checkpointerLatch = NULL;
+	ProcGlobal->walwriterProc = INVALID_PROC_NUMBER;
+	ProcGlobal->checkpointerProc = INVALID_PROC_NUMBER;
 	pg_atomic_init_u32(&ProcGlobal->procArrayGroupFirst, INVALID_PROC_NUMBER);
 	pg_atomic_init_u32(&ProcGlobal->clogGroupFirst, INVALID_PROC_NUMBER);