Harden pmsignal.c against clobbered shared memory.

The postmaster is not supposed to do anything that depends
fundamentally on shared memory contents, because that creates
the risk that a backend crash that trashes shared memory will
take the postmaster down with it, preventing automatic recovery.
In commit 969d7cd4 I lost sight of this principle and coded
AssignPostmasterChildSlot() in such a way that it could fail
or even crash if the shared PMSignalState structure became
corrupted.  Remarkably, we've not seen field reports of such
crashes; but I managed to induce one while testing the recent
changes around palloc chunk headers.

To fix, make a semi-duplicative state array inside the postmaster
so that we need consult only local state while choosing a "child
slot" for a new backend.  Ensure that other postmaster-executed
routines in pmsignal.c don't have critical dependencies on the
shared state, either.  Corruption of PMSignalState might now
lead ReleasePostmasterChildSlot() to conclude that backend X
failed, when actually backend Y was the one that trashed things.
But that doesn't matter, because we'll force a cluster-wide reset
regardless.

Back-patch to all supported branches, since this is an old bug.

Discussion: https://postgr.es/m/3436789.1665187055@sss.pgh.pa.us

src/backend/storage/ipc/pmsignal.c

@@ -26,6 +26,7 @@
 #include "replication/walsender.h"
 #include "storage/pmsignal.h"
 #include "storage/shmem.h"
+#include "utils/memutils.h"
 
 
 /*
@@ -75,12 +76,21 @@ struct PMSignalData
 	QuitSignalReason sigquit_reason;	/* why SIGQUIT was sent */
 	/* per-child-process flags */
 	int			num_child_flags;	/* # of entries in PMChildFlags[] */
-	int			next_child_flag;	/* next slot to try to assign */
 	sig_atomic_t PMChildFlags[FLEXIBLE_ARRAY_MEMBER];
 };
 
+/* PMSignalState pointer is valid in both postmaster and child processes */
 NON_EXEC_STATIC volatile PMSignalData *PMSignalState = NULL;
 
+/*
+ * These static variables are valid only in the postmaster.  We keep a
+ * duplicative private array so that we can trust its state even if some
+ * failing child has clobbered the PMSignalData struct in shared memory.
+ */
+static int	num_child_inuse;	/* # of entries in PMChildInUse[] */
+static int	next_child_inuse;	/* next slot to try to assign */
+static bool *PMChildInUse;		/* true if i'th flag slot is assigned */
+
 /*
  * Signal handler to be notified if postmaster dies.
  */
@@ -142,7 +152,25 @@ PMSignalShmemInit(void)
 	{
 		/* initialize all flags to zeroes */
 		MemSet(unvolatize(PMSignalData *, PMSignalState), 0, PMSignalShmemSize());
-		PMSignalState->num_child_flags = MaxLivePostmasterChildren();
+		num_child_inuse = MaxLivePostmasterChildren();
+		PMSignalState->num_child_flags = num_child_inuse;
+
+		/*
+		 * Also allocate postmaster's private PMChildInUse[] array.  We
+		 * might've already done that in a previous shared-memory creation
+		 * cycle, in which case free the old array to avoid a leak.  (Do it
+		 * like this to support the possibility that MaxLivePostmasterChildren
+		 * changed.)  In a standalone backend, we do not need this.
+		 */
+		if (PostmasterContext != NULL)
+		{
+			if (PMChildInUse)
+				pfree(PMChildInUse);
+			PMChildInUse = (bool *)
+				MemoryContextAllocZero(PostmasterContext,
+									   num_child_inuse * sizeof(bool));
+		}
+		next_child_inuse = 0;
 	}
 }
 
@@ -218,21 +246,24 @@ GetQuitSignalReason(void)
 int
 AssignPostmasterChildSlot(void)
 {
-	int			slot = PMSignalState->next_child_flag;
+	int			slot = next_child_inuse;
 	int			n;
 
 	/*
-	 * Scan for a free slot.  We track the last slot assigned so as not to
-	 * waste time repeatedly rescanning low-numbered slots.
+	 * Scan for a free slot.  Notice that we trust nothing about the contents
+	 * of PMSignalState, but use only postmaster-local data for this decision.
+	 * We track the last slot assigned so as not to waste time repeatedly
+	 * rescanning low-numbered slots.
 	 */
-	for (n = PMSignalState->num_child_flags; n > 0; n--)
+	for (n = num_child_inuse; n > 0; n--)
 	{
 		if (--slot < 0)
-			slot = PMSignalState->num_child_flags - 1;
-		if (PMSignalState->PMChildFlags[slot] == PM_CHILD_UNUSED)
+			slot = num_child_inuse - 1;
+		if (!PMChildInUse[slot])
 		{
+			PMChildInUse[slot] = true;
 			PMSignalState->PMChildFlags[slot] = PM_CHILD_ASSIGNED;
-			PMSignalState->next_child_flag = slot;
+			next_child_inuse = slot;
 			return slot + 1;
 		}
 	}
@@ -254,7 +285,7 @@ ReleasePostmasterChildSlot(int slot)
 {
 	bool		result;
 
-	Assert(slot > 0 && slot <= PMSignalState->num_child_flags);
+	Assert(slot > 0 && slot <= num_child_inuse);
 	slot--;
 
 	/*
@@ -264,17 +295,18 @@ ReleasePostmasterChildSlot(int slot)
 	 */
 	result = (PMSignalState->PMChildFlags[slot] == PM_CHILD_ASSIGNED);
 	PMSignalState->PMChildFlags[slot] = PM_CHILD_UNUSED;
+	PMChildInUse[slot] = false;
 	return result;
 }
 
 /*
  * IsPostmasterChildWalSender - check if given slot is in use by a
- * walsender process.
+ * walsender process.  This is called only by the postmaster.
  */
 bool
 IsPostmasterChildWalSender(int slot)
 {
-	Assert(slot > 0 && slot <= PMSignalState->num_child_flags);
+	Assert(slot > 0 && slot <= num_child_inuse);
 	slot--;
 
 	if (PMSignalState->PMChildFlags[slot] == PM_CHILD_WALSENDER)