commit 526bb78facd8791921adfe37e307f8cb8e249c37
Author: Andres Freund <andres@anarazel.de>
Date:   Mon Feb 14 16:44:28 2022 -0800

    Move replication slot release to before_shmem_exit().
    
    Previously, replication slots were released in ProcKill() on error, resulting
    in reporting replication slot drop of ephemeral slots after the stats
    subsystem was already shut down.
    
    To fix this problem, move replication slot release to a before_shmem_exit()
    hook that is called before the stats collector shuts down. There wasn't really
    a good reason for the slot handling to be in ProcKill() anyway.
    
    Patch by Masahiko Sawada, with very minor polishing by me.
    
    I, Andres, wrote a test for dropping slots during process exit, but there may
    be some OS dependent issues around the number of times FATAL error messages
    are displayed due to a still debated libpq issue. So that test will be
    committed separately / later.
    
    Reviewed-By: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
    Reviewed-By: Andres Freund <andres@anarazel.de>
    Author: Masahiko Sawada <sawada.mshk@gmail.com>
    Discussion: https://postgr.es/m/CAD21AoDAeEpAbZEyYJsPZJUmSPaRicVSBObaL7sPaofnKz+9zg@mail.gmail.com

diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index 037a347cba..27f802427a 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -46,6 +46,7 @@
 #include "pgstat.h"
 #include "replication/slot.h"
 #include "storage/fd.h"
+#include "storage/ipc.h"
 #include "storage/proc.h"
 #include "storage/procarray.h"
 #include "utils/builtins.h"
@@ -99,6 +100,7 @@ ReplicationSlot *MyReplicationSlot = NULL;
 int			max_replication_slots = 0;	/* the maximum number of replication
 										 * slots */
 
+static void ReplicationSlotShmemExit(int code, Datum arg);
 static void ReplicationSlotDropAcquired(void);
 static void ReplicationSlotDropPtr(ReplicationSlot *slot);
 
@@ -160,6 +162,29 @@ ReplicationSlotsShmemInit(void)
 	}
 }
 
+/*
+ * Register the callback for replication slot cleanup and releasing.
+ */
+void
+ReplicationSlotInitialize(void)
+{
+	before_shmem_exit(ReplicationSlotShmemExit, 0);
+}
+
+/*
+ * Release and cleanup replication slots.
+ */
+static void
+ReplicationSlotShmemExit(int code, Datum arg)
+{
+	/* Make sure active replication slots are released */
+	if (MyReplicationSlot != NULL)
+		ReplicationSlotRelease();
+
+	/* Also cleanup all the temporary slots. */
+	ReplicationSlotCleanup();
+}
+
 /*
  * Check whether the passed slot name is valid and report errors at elevel.
  *
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index f62e622a7b..27d4966c70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -853,13 +853,6 @@ ProcKill(int code, Datum arg)
 	/* Cancel any pending condition variable sleep, too */
 	ConditionVariableCancelSleep();
 
-	/* Make sure active replication slots are released */
-	if (MyReplicationSlot != NULL)
-		ReplicationSlotRelease();
-
-	/* Also cleanup all the temporary slots. */
-	ReplicationSlotCleanup();
-
 	/*
 	 * Detach from any lock group of which we are a member.  If the leader
 	 * exist before all other group members, its PGPROC will remain allocated
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index c613495ee2..8d034844d6 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -4273,8 +4273,8 @@ PostgresMain(int argc, char *argv[],
 		 * We can't release replication slots inside AbortTransaction() as we
 		 * need to be able to start and abort transactions while having a slot
 		 * acquired. But we never need to hold them across top level errors,
-		 * so releasing here is fine. There's another cleanup in ProcKill()
-		 * ensuring we'll correctly cleanup on FATAL errors as well.
+		 * so releasing here is fine. There also is a before_shmem_exit()
+		 * callback ensuring correct cleanup on FATAL errors.
 		 */
 		if (MyReplicationSlot != NULL)
 			ReplicationSlotRelease();
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 0b5a3050f1..d756de19f3 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -40,6 +40,7 @@
 #include "pgstat.h"
 #include "postmaster/autovacuum.h"
 #include "postmaster/postmaster.h"
+#include "replication/slot.h"
 #include "replication/walsender.h"
 #include "storage/bufmgr.h"
 #include "storage/fd.h"
@@ -548,6 +549,12 @@ BaseInit(void)
 	InitSync();
 	smgrinit();
 	InitBufferPoolAccess();
+
+	/*
+	 * Initialize replication slots after pgstat. The exit hook might need to
+	 * drop ephemeral slots, which in turn triggers stats reporting.
+	 */
+	ReplicationSlotInitialize();
 }
 
 
diff --git a/src/include/replication/slot.h b/src/include/replication/slot.h
index c0fca56bee..bad07e4ffc 100644
--- a/src/include/replication/slot.h
+++ b/src/include/replication/slot.h
@@ -207,6 +207,7 @@ extern void ReplicationSlotSave(void);
 extern void ReplicationSlotMarkDirty(void);
 
 /* misc stuff */
+extern void ReplicationSlotInitialize(void);
 extern bool ReplicationSlotValidateName(const char *name, int elevel);
 extern void ReplicationSlotReserveWal(void);
 extern void ReplicationSlotsComputeRequiredXmin(bool already_locked);