From 787761b6f959b018f0af68861191ca67a6185b1d Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Fri, 23 Aug 2024 17:25:15 +0300
Subject: [PATCH 04/11] Address walwriter and checkpointer by proc number

instead of pointing directly to their latch
---
 src/backend/access/transam/xlog.c     | 11 +++++++++--
 src/backend/postmaster/checkpointer.c | 17 ++++++++++++-----
 src/backend/postmaster/walwriter.c    |  6 +++---
 src/backend/storage/lmgr/proc.c       |  4 ++--
 src/include/storage/proc.h            | 12 ++++++++----
 5 files changed, 34 insertions(+), 16 deletions(-)

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 45a4a40eca..63a8ab5a29 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -2669,8 +2669,15 @@ XLogSetAsyncXactLSN(XLogRecPtr asyncXactLSN)
 			wakeup = true;
 	}
 
-	if (wakeup && ProcGlobal->walwriterLatch)
-		SetLatch(ProcGlobal->walwriterLatch);
+	if (wakeup)
+	{
+		volatile PROC_HDR *procglobal = ProcGlobal;
+		ProcNumber	walwriterProc;
+
+		walwriterProc = procglobal->walwriterProc;
+		if (walwriterProc != INVALID_PROC_NUMBER)
+			SetLatch(&GetPGProcByNumber(walwriterProc)->procLatch);
+	}
 }
 
 /*
diff --git a/src/backend/postmaster/checkpointer.c b/src/backend/postmaster/checkpointer.c
index 199f008bcd..e556d7ecee 100644
--- a/src/backend/postmaster/checkpointer.c
+++ b/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
@@ -1128,8 +1128,15 @@ 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;
+
+		checkpointerProc = procglobal->checkpointerProc;
+		if (checkpointerProc != INVALID_PROC_NUMBER)
+			SetLatch(&GetPGProcByNumber(checkpointerProc)->procLatch);
+	}
 
 	return true;
 }
diff --git a/src/backend/postmaster/walwriter.c b/src/backend/postmaster/walwriter.c
index 6e7918a78d..0c55d9fa59 100644
--- a/src/backend/postmaster/walwriter.c
+++ b/src/backend/postmaster/walwriter.c
@@ -210,10 +210,10 @@ 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
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index ac66da8638..56c60704f5 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -177,8 +177,8 @@ 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);
 
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index deeb06c9e0..95710e416f 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -413,10 +413,14 @@ typedef struct PROC_HDR
 	pg_atomic_uint32 procArrayGroupFirst;
 	/* First pgproc waiting for group transaction status update */
 	pg_atomic_uint32 clogGroupFirst;
-	/* WALWriter process's latch */
-	Latch	   *walwriterLatch;
-	/* Checkpointer process's latch */
-	Latch	   *checkpointerLatch;
+
+	/*
+	 * Current slot numbers of some auxiliary processes. There can be only one
+	 * of each of these running at a time.
+	 */
+	ProcNumber	walwriterProc;
+	ProcNumber	checkpointerProc;
+
 	/* Current shared estimate of appropriate spins_per_delay value */
 	int			spins_per_delay;
 	/* Buffer id of the buffer that Startup process waits for pin on, or -1 */
-- 
2.39.2