Use an shmem_exit callback to remove backend from PMChildFlags on exit

This seems nicer than having to duplicate the logic between
InitProcess() and ProcKill() for which child processes have a
PMChildFlags slot.

Move the MarkPostmasterChildActive() call earlier in InitProcess(),
out of the section protected by the spinlock.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/a102f15f-eac4-4ff2-af02-f9ff209ec66f@iki.fi

src/backend/storage/ipc/pmsignal.c

@@ -24,6 +24,7 @@
 #include "miscadmin.h"
 #include "postmaster/postmaster.h"
 #include "replication/walsender.h"
+#include "storage/ipc.h"
 #include "storage/pmsignal.h"
 #include "storage/shmem.h"
 #include "utils/memutils.h"
@@ -121,6 +122,8 @@ postmaster_death_handler(SIGNAL_ARGS)
 
 #endif							/* USE_POSTMASTER_DEATH_SIGNAL */
 
+static void MarkPostmasterChildInactive(int code, Datum arg);
+
 /*
  * PMSignalShmemSize
  *		Compute space needed for pmsignal.c's shared memory
@@ -316,11 +319,14 @@ IsPostmasterChildWalSender(int slot)
 }
 
 /*
- * MarkPostmasterChildActive - mark a postmaster child as about to begin
+ * RegisterPostmasterChildActive - mark a postmaster child as about to begin
  * actively using shared memory.  This is called in the child process.
+ *
+ * This register an shmem exit hook to mark us as inactive again when the
+ * process exits normally.
  */
 void
-MarkPostmasterChildActive(void)
+RegisterPostmasterChildActive(void)
 {
 	int			slot = MyPMChildSlot;
 
@@ -328,6 +334,9 @@ MarkPostmasterChildActive(void)
 	slot--;
 	Assert(PMSignalState->PMChildFlags[slot] == PM_CHILD_ASSIGNED);
 	PMSignalState->PMChildFlags[slot] = PM_CHILD_ACTIVE;
+
+	/* Arrange to clean up at exit. */
+	on_shmem_exit(MarkPostmasterChildInactive, 0);
 }
 
 /*
@@ -352,8 +361,8 @@ MarkPostmasterChildWalSender(void)
  * MarkPostmasterChildInactive - mark a postmaster child as done using
  * shared memory.  This is called in the child process.
  */
-void
-MarkPostmasterChildInactive(void)
+static void
+MarkPostmasterChildInactive(int code, Datum arg)
 {
 	int			slot = MyPMChildSlot;
 

src/backend/storage/lmgr/proc.c

@@ -354,6 +354,19 @@ InitProcess(void)
 	if (MyProc != NULL)
 		elog(ERROR, "you already exist");
 
+	/*
+	 * Before we start accessing the shared memory in a serious way, mark
+	 * ourselves as an active postmaster child; this is so that the postmaster
+	 * can detect it if we exit without cleaning up.  (XXX autovac launcher
+	 * currently doesn't participate in this; it probably should.)
+	 *
+	 * Slot sync worker also does not participate in it, see comments atop
+	 * 'struct bkend' in postmaster.c.
+	 */
+	if (IsUnderPostmaster && !AmAutoVacuumLauncherProcess() &&
+		!AmLogicalSlotSyncWorkerProcess())
+		RegisterPostmasterChildActive();
+
 	/* Decide which list should supply our PGPROC. */
 	if (AmAutoVacuumLauncherProcess() || AmAutoVacuumWorkerProcess())
 		procgloballist = &ProcGlobal->autovacFreeProcs;
@@ -406,19 +419,6 @@ InitProcess(void)
 	 */
 	Assert(MyProc->procgloballist == procgloballist);
 
-	/*
-	 * Now that we have a PGPROC, mark ourselves as an active postmaster
-	 * child; this is so that the postmaster can detect it if we exit without
-	 * cleaning up.  (XXX autovac launcher currently doesn't participate in
-	 * this; it probably should.)
-	 *
-	 * Slot sync worker also does not participate in it, see comments atop
-	 * 'struct bkend' in postmaster.c.
-	 */
-	if (IsUnderPostmaster && !AmAutoVacuumLauncherProcess() &&
-		!AmLogicalSlotSyncWorkerProcess())
-		MarkPostmasterChildActive();
-
 	/*
 	 * Initialize all fields of MyProc, except for those previously
 	 * initialized by InitProcGlobal.
@@ -993,18 +993,6 @@ ProcKill(int code, Datum arg)
 
 	SpinLockRelease(ProcStructLock);
 
-	/*
-	 * This process is no longer present in shared memory in any meaningful
-	 * way, so tell the postmaster we've cleaned up acceptably well. (XXX
-	 * autovac launcher should be included here someday)
-	 *
-	 * Slot sync worker is also not a postmaster child, so skip this shared
-	 * memory related processing here.
-	 */
-	if (IsUnderPostmaster && !AmAutoVacuumLauncherProcess() &&
-		!AmLogicalSlotSyncWorkerProcess())
-		MarkPostmasterChildInactive();
-
 	/* wake autovac launcher if needed -- see comments in FreeWorkerInfo */
 	if (AutovacuumLauncherPid != 0)
 		kill(AutovacuumLauncherPid, SIGUSR2);

src/include/storage/pmsignal.h

@@ -73,8 +73,7 @@ extern QuitSignalReason GetQuitSignalReason(void);
 extern int	AssignPostmasterChildSlot(void);
 extern bool ReleasePostmasterChildSlot(int slot);
 extern bool IsPostmasterChildWalSender(int slot);
-extern void MarkPostmasterChildActive(void);
-extern void MarkPostmasterChildInactive(void);
+extern void RegisterPostmasterChildActive(void);
 extern void MarkPostmasterChildWalSender(void);
 extern bool PostmasterIsAliveInternal(void);
 extern void PostmasterDeathSignalInit(void);