Remove MaxBackends variable in favor of GetMaxBackends() function.

Previously, it was really easy to write code that accessed MaxBackends
before we'd actually initialized it, especially when coding up an
extension. To make this less error-prune, introduce a new function
GetMaxBackends() which should be used to obtain the correct value.
This will ERROR if called too early. Demote the global variable to
a file-level static, so that nobody can peak at it directly.

Nathan Bossart. Idea by Andres Freund. Review by Greg Sabino Mullane,
by Michael Paquier (who had doubts about the approach), and by me.

Discussion: http://postgr.es/m/20210802224204.bckcikl45uezv5e4@alap3.anarazel.de

8 files changed, 85 insertions, 63 deletions

diff --git a/src/backend/storage/ipc/dsm.c b/src/backend/storage/ipc/dsm.c
index ae75141592..e9e9fae3eb 100644
--- a/src/backend/storage/ipc/dsm.c
+++ b/src/backend/storage/ipc/dsm.c
@@ -166,7 +166,7 @@ dsm_postmaster_startup(PGShmemHeader *shim)
 
 	/* Determine size for new control segment. */
 	maxitems = PG_DYNSHMEM_FIXED_SLOTS
-		+ PG_DYNSHMEM_SLOTS_PER_BACKEND * MaxBackends;
+		+ PG_DYNSHMEM_SLOTS_PER_BACKEND * GetMaxBackends();
 	elog(DEBUG2, "dynamic shared memory system will support %u segments",
 		 maxitems);
 	segsize = dsm_control_bytes_needed(maxitems);
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 9d3efb7d80..13d192ec2b 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -97,7 +97,7 @@ typedef struct ProcArrayStruct
 	/* oldest catalog xmin of any replication slot */
 	TransactionId replication_slot_catalog_xmin;
 
-	/* indexes into allProcs[], has PROCARRAY_MAXPROCS entries */
+	/* indexes into allProcs[], has ProcArrayMaxProcs entries */
 	int			pgprocnos[FLEXIBLE_ARRAY_MEMBER];
 } ProcArrayStruct;
 
@@ -355,6 +355,17 @@ static void MaintainLatestCompletedXidRecovery(TransactionId latestXid);
 static inline FullTransactionId FullXidRelativeTo(FullTransactionId rel,
 												  TransactionId xid);
 static void GlobalVisUpdateApply(ComputeXidHorizonsResult *horizons);
+static inline int GetProcArrayMaxProcs(void);
+
+
+/*
+ * Retrieve the number of slots in the ProcArray structure.
+ */
+static inline int
+GetProcArrayMaxProcs(void)
+{
+	return GetMaxBackends() + max_prepared_xacts;
+}
 
 /*
  * Report shared-memory space needed by CreateSharedProcArray.
@@ -365,10 +376,8 @@ ProcArrayShmemSize(void)
 	Size		size;
 
 	/* Size of the ProcArray structure itself */
-#define PROCARRAY_MAXPROCS	(MaxBackends + max_prepared_xacts)
-
 	size = offsetof(ProcArrayStruct, pgprocnos);
-	size = add_size(size, mul_size(sizeof(int), PROCARRAY_MAXPROCS));
+	size = add_size(size, mul_size(sizeof(int), GetProcArrayMaxProcs()));
 
 	/*
 	 * During Hot Standby processing we have a data structure called
@@ -384,7 +393,7 @@ ProcArrayShmemSize(void)
 	 * shared memory is being set up.
 	 */
 #define TOTAL_MAX_CACHED_SUBXIDS \
-	((PGPROC_MAX_CACHED_SUBXIDS + 1) * PROCARRAY_MAXPROCS)
+	((PGPROC_MAX_CACHED_SUBXIDS + 1) * GetProcArrayMaxProcs())
 
 	if (EnableHotStandby)
 	{
@@ -411,7 +420,7 @@ CreateSharedProcArray(void)
 		ShmemInitStruct("Proc Array",
 						add_size(offsetof(ProcArrayStruct, pgprocnos),
 								 mul_size(sizeof(int),
-										  PROCARRAY_MAXPROCS)),
+										  GetProcArrayMaxProcs())),
 						&found);
 
 	if (!found)
@@ -420,7 +429,7 @@ CreateSharedProcArray(void)
 		 * We're the first - initialize.
 		 */
 		procArray->numProcs = 0;
-		procArray->maxProcs = PROCARRAY_MAXPROCS;
+		procArray->maxProcs = GetProcArrayMaxProcs();
 		procArray->maxKnownAssignedXids = TOTAL_MAX_CACHED_SUBXIDS;
 		procArray->numKnownAssignedXids = 0;
 		procArray->tailKnownAssignedXids = 0;
@@ -4623,7 +4632,7 @@ KnownAssignedXidsCompress(bool force)
 		 */
 		int			nelements = head - tail;
 
-		if (nelements < 4 * PROCARRAY_MAXPROCS ||
+		if (nelements < 4 * GetProcArrayMaxProcs() ||
 			nelements < 2 * pArray->numKnownAssignedXids)
 			return;
 	}
diff --git a/src/backend/storage/ipc/procsignal.c b/src/backend/storage/ipc/procsignal.c
index f1c8ff8f9e..d158bb7a19 100644
--- a/src/backend/storage/ipc/procsignal.c
+++ b/src/backend/storage/ipc/procsignal.c
@@ -80,13 +80,6 @@ typedef struct
 	ProcSignalSlot psh_slot[FLEXIBLE_ARRAY_MEMBER];
 } ProcSignalHeader;
 
-/*
- * We reserve a slot for each possible BackendId, plus one for each
- * possible auxiliary process type.  (This scheme assumes there is not
- * more than one of any auxiliary process type at a time.)
- */
-#define NumProcSignalSlots	(MaxBackends + NUM_AUXPROCTYPES)
-
 /* Check whether the relevant type bit is set in the flags. */
 #define BARRIER_SHOULD_CHECK(flags, type) \
 	(((flags) & (((uint32) 1) << (uint32) (type))) != 0)
@@ -102,6 +95,20 @@ static bool CheckProcSignal(ProcSignalReason reason);
 static void CleanupProcSignalState(int status, Datum arg);
 static void ResetProcSignalBarrierBits(uint32 flags);
 static bool ProcessBarrierPlaceholder(void);
+static inline int GetNumProcSignalSlots(void);
+
+/*
+ * GetNumProcSignalSlots
+ *
+ * We reserve a slot for each possible BackendId, plus one for each possible
+ * auxiliary process type.  (This scheme assume there is not more than one of
+ * any auxiliary process type at a time.)
+ */
+static inline int
+GetNumProcSignalSlots(void)
+{
+	return GetMaxBackends() + NUM_AUXPROCTYPES;
+}
 
 /*
  * ProcSignalShmemSize
@@ -112,7 +119,7 @@ ProcSignalShmemSize(void)
 {
 	Size		size;
 
-	size = mul_size(NumProcSignalSlots, sizeof(ProcSignalSlot));
+	size = mul_size(GetNumProcSignalSlots(), sizeof(ProcSignalSlot));
 	size = add_size(size, offsetof(ProcSignalHeader, psh_slot));
 	return size;
 }
@@ -126,6 +133,7 @@ ProcSignalShmemInit(void)
 {
 	Size		size = ProcSignalShmemSize();
 	bool		found;
+	int			numProcSignalSlots = GetNumProcSignalSlots();
 
 	ProcSignal = (ProcSignalHeader *)
 		ShmemInitStruct("ProcSignal", size, &found);
@@ -137,7 +145,7 @@ ProcSignalShmemInit(void)
 
 		pg_atomic_init_u64(&ProcSignal->psh_barrierGeneration, 0);
 
-		for (i = 0; i < NumProcSignalSlots; ++i)
+		for (i = 0; i < numProcSignalSlots; ++i)
 		{
 			ProcSignalSlot *slot = &ProcSignal->psh_slot[i];
 
@@ -163,7 +171,7 @@ ProcSignalInit(int pss_idx)
 	ProcSignalSlot *slot;
 	uint64		barrier_generation;
 
-	Assert(pss_idx >= 1 && pss_idx <= NumProcSignalSlots);
+	Assert(pss_idx >= 1 && pss_idx <= GetNumProcSignalSlots());
 
 	slot = &ProcSignal->psh_slot[pss_idx - 1];
 
@@ -292,7 +300,7 @@ SendProcSignal(pid_t pid, ProcSignalReason reason, BackendId backendId)
 		 */
 		int			i;
 
-		for (i = NumProcSignalSlots - 1; i >= 0; i--)
+		for (i = GetNumProcSignalSlots() - 1; i >= 0; i--)
 		{
 			slot = &ProcSignal->psh_slot[i];
 
@@ -333,6 +341,7 @@ EmitProcSignalBarrier(ProcSignalBarrierType type)
 {
 	uint32		flagbit = 1 << (uint32) type;
 	uint64		generation;
+	int			numProcSignalSlots = GetNumProcSignalSlots();
 
 	/*
 	 * Set all the flags.
@@ -342,7 +351,7 @@ EmitProcSignalBarrier(ProcSignalBarrierType type)
 	 * anything that we do afterwards. (This is also true of the later call to
 	 * pg_atomic_add_fetch_u64.)
 	 */
-	for (int i = 0; i < NumProcSignalSlots; i++)
+	for (int i = 0; i < numProcSignalSlots; i++)
 	{
 		volatile ProcSignalSlot *slot = &ProcSignal->psh_slot[i];
 
@@ -368,7 +377,7 @@ EmitProcSignalBarrier(ProcSignalBarrierType type)
 	 * backends that need to update state - but they won't actually need to
 	 * change any state.
 	 */
-	for (int i = NumProcSignalSlots - 1; i >= 0; i--)
+	for (int i = numProcSignalSlots - 1; i >= 0; i--)
 	{
 		volatile ProcSignalSlot *slot = &ProcSignal->psh_slot[i];
 		pid_t		pid = slot->pss_pid;
@@ -393,7 +402,7 @@ WaitForProcSignalBarrier(uint64 generation)
 {
 	Assert(generation <= pg_atomic_read_u64(&ProcSignal->psh_barrierGeneration));
 
-	for (int i = NumProcSignalSlots - 1; i >= 0; i--)
+	for (int i = GetNumProcSignalSlots() - 1; i >= 0; i--)
 	{
 		ProcSignalSlot *slot = &ProcSignal->psh_slot[i];
 		uint64		oldval;
diff --git a/src/backend/storage/ipc/sinvaladt.c b/src/backend/storage/ipc/sinvaladt.c
index cb3ee82046..68e7160b30 100644
--- a/src/backend/storage/ipc/sinvaladt.c
+++ b/src/backend/storage/ipc/sinvaladt.c
@@ -205,7 +205,7 @@ SInvalShmemSize(void)
 	Size		size;
 
 	size = offsetof(SISeg, procState);
-	size = add_size(size, mul_size(sizeof(ProcState), MaxBackends));
+	size = add_size(size, mul_size(sizeof(ProcState), GetMaxBackends()));
 
 	return size;
 }
@@ -231,7 +231,7 @@ CreateSharedInvalidationState(void)
 	shmInvalBuffer->maxMsgNum = 0;
 	shmInvalBuffer->nextThreshold = CLEANUP_MIN;
 	shmInvalBuffer->lastBackend = 0;
-	shmInvalBuffer->maxBackends = MaxBackends;
+	shmInvalBuffer->maxBackends = GetMaxBackends();
 	SpinLockInit(&shmInvalBuffer->msgnumLock);
 
 	/* The buffer[] array is initially all unused, so we need not fill it */
diff --git a/src/backend/storage/lmgr/deadlock.c b/src/backend/storage/lmgr/deadlock.c
index cd9c0418ec..b5d539ba5d 100644
--- a/src/backend/storage/lmgr/deadlock.c
+++ b/src/backend/storage/lmgr/deadlock.c
@@ -143,6 +143,7 @@ void
 InitDeadLockChecking(void)
 {
 	MemoryContext oldcxt;
+	int max_backends = GetMaxBackends();
 
 	/* Make sure allocations are permanent */
 	oldcxt = MemoryContextSwitchTo(TopMemoryContext);
@@ -151,16 +152,16 @@ InitDeadLockChecking(void)
 	 * FindLockCycle needs at most MaxBackends entries in visitedProcs[] and
 	 * deadlockDetails[].
 	 */
-	visitedProcs = (PGPROC **) palloc(MaxBackends * sizeof(PGPROC *));
-	deadlockDetails = (DEADLOCK_INFO *) palloc(MaxBackends * sizeof(DEADLOCK_INFO));
+	visitedProcs = (PGPROC **) palloc(max_backends * sizeof(PGPROC *));
+	deadlockDetails = (DEADLOCK_INFO *) palloc(max_backends * sizeof(DEADLOCK_INFO));
 
 	/*
 	 * TopoSort needs to consider at most MaxBackends wait-queue entries, and
 	 * it needn't run concurrently with FindLockCycle.
 	 */
 	topoProcs = visitedProcs;	/* re-use this space */
-	beforeConstraints = (int *) palloc(MaxBackends * sizeof(int));
-	afterConstraints = (int *) palloc(MaxBackends * sizeof(int));
+	beforeConstraints = (int *) palloc(max_backends * sizeof(int));
+	afterConstraints = (int *) palloc(max_backends * sizeof(int));
 
 	/*
 	 * We need to consider rearranging at most MaxBackends/2 wait queues
@@ -169,8 +170,8 @@ InitDeadLockChecking(void)
 	 * MaxBackends total waiters.
 	 */
 	waitOrders = (WAIT_ORDER *)
-		palloc((MaxBackends / 2) * sizeof(WAIT_ORDER));
-	waitOrderProcs = (PGPROC **) palloc(MaxBackends * sizeof(PGPROC *));
+		palloc((max_backends / 2) * sizeof(WAIT_ORDER));
+	waitOrderProcs = (PGPROC **) palloc(max_backends * sizeof(PGPROC *));
 
 	/*
 	 * Allow at most MaxBackends distinct constraints in a configuration. (Is
@@ -180,7 +181,7 @@ InitDeadLockChecking(void)
 	 * limits the maximum recursion depth of DeadLockCheckRecurse. Making it
 	 * really big might potentially allow a stack-overflow problem.
 	 */
-	maxCurConstraints = MaxBackends;
+	maxCurConstraints = max_backends;
 	curConstraints = (EDGE *) palloc(maxCurConstraints * sizeof(EDGE));
 
 	/*
@@ -191,7 +192,7 @@ InitDeadLockChecking(void)
 	 * last MaxBackends entries in possibleConstraints[] are reserved as
 	 * output workspace for FindLockCycle.
 	 */
-	maxPossibleConstraints = MaxBackends * 4;
+	maxPossibleConstraints = max_backends * 4;
 	possibleConstraints =
 		(EDGE *) palloc(maxPossibleConstraints * sizeof(EDGE));
 
@@ -327,7 +328,7 @@ DeadLockCheckRecurse(PGPROC *proc)
 	if (nCurConstraints >= maxCurConstraints)
 		return true;			/* out of room for active constraints? */
 	oldPossibleConstraints = nPossibleConstraints;
-	if (nPossibleConstraints + nEdges + MaxBackends <= maxPossibleConstraints)
+	if (nPossibleConstraints + nEdges + GetMaxBackends() <= maxPossibleConstraints)
 	{
 		/* We can save the edge list in possibleConstraints[] */
 		nPossibleConstraints += nEdges;
@@ -388,7 +389,7 @@ TestConfiguration(PGPROC *startProc)
 	/*
 	 * Make sure we have room for FindLockCycle's output.
 	 */
-	if (nPossibleConstraints + MaxBackends > maxPossibleConstraints)
+	if (nPossibleConstraints + GetMaxBackends() > maxPossibleConstraints)
 		return -1;
 
 	/*
@@ -486,7 +487,7 @@ FindLockCycleRecurse(PGPROC *checkProc,
 				 * record total length of cycle --- outer levels will now fill
 				 * deadlockDetails[]
 				 */
-				Assert(depth <= MaxBackends);
+				Assert(depth <= GetMaxBackends());
 				nDeadlockDetails = depth;
 
 				return true;
@@ -500,7 +501,7 @@ FindLockCycleRecurse(PGPROC *checkProc,
 		}
 	}
 	/* Mark proc as seen */
-	Assert(nVisitedProcs < MaxBackends);
+	Assert(nVisitedProcs < GetMaxBackends());
 	visitedProcs[nVisitedProcs++] = checkProc;
 
 	/*
@@ -698,7 +699,7 @@ FindLockCycleRecurseMember(PGPROC *checkProc,
 					/*
 					 * Add this edge to the list of soft edges in the cycle
 					 */
-					Assert(*nSoftEdges < MaxBackends);
+					Assert(*nSoftEdges < GetMaxBackends());
 					softEdges[*nSoftEdges].waiter = checkProcLeader;
 					softEdges[*nSoftEdges].blocker = leader;
 					softEdges[*nSoftEdges].lock = lock;
@@ -771,7 +772,7 @@ FindLockCycleRecurseMember(PGPROC *checkProc,
 					/*
 					 * Add this edge to the list of soft edges in the cycle
 					 */
-					Assert(*nSoftEdges < MaxBackends);
+					Assert(*nSoftEdges < GetMaxBackends());
 					softEdges[*nSoftEdges].waiter = checkProcLeader;
 					softEdges[*nSoftEdges].blocker = leader;
 					softEdges[*nSoftEdges].lock = lock;
@@ -834,7 +835,7 @@ ExpandConstraints(EDGE *constraints,
 		waitOrders[nWaitOrders].procs = waitOrderProcs + nWaitOrderProcs;
 		waitOrders[nWaitOrders].nProcs = lock->waitProcs.size;
 		nWaitOrderProcs += lock->waitProcs.size;
-		Assert(nWaitOrderProcs <= MaxBackends);
+		Assert(nWaitOrderProcs <= GetMaxBackends());
 
 		/*
 		 * Do the topo sort.  TopoSort need not examine constraints after this
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index 5f5803f681..1528d788d0 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -55,7 +55,7 @@
 int			max_locks_per_xact; /* set by guc.c */
 
 #define NLOCKENTS() \
-	mul_size(max_locks_per_xact, add_size(MaxBackends, max_prepared_xacts))
+	mul_size(max_locks_per_xact, add_size(GetMaxBackends(), max_prepared_xacts))
 
 
 /*
@@ -2942,12 +2942,12 @@ GetLockConflicts(const LOCKTAG *locktag, LOCKMODE lockmode, int *countp)
 			vxids = (VirtualTransactionId *)
 				MemoryContextAlloc(TopMemoryContext,
 								   sizeof(VirtualTransactionId) *
-								   (MaxBackends + max_prepared_xacts + 1));
+								   (GetMaxBackends() + max_prepared_xacts + 1));
 	}
 	else
 		vxids = (VirtualTransactionId *)
 			palloc0(sizeof(VirtualTransactionId) *
-					(MaxBackends + max_prepared_xacts + 1));
+					(GetMaxBackends() + max_prepared_xacts + 1));
 
 	/* Compute hash code and partition lock, and look up conflicting modes. */
 	hashcode = LockTagHashCode(locktag);
@@ -3104,7 +3104,7 @@ GetLockConflicts(const LOCKTAG *locktag, LOCKMODE lockmode, int *countp)
 
 	LWLockRelease(partitionLock);
 
-	if (count > MaxBackends + max_prepared_xacts)	/* should never happen */
+	if (count > GetMaxBackends() + max_prepared_xacts)	/* should never happen */
 		elog(PANIC, "too many conflicting locks found");
 
 	vxids[count].backendId = InvalidBackendId;
@@ -3651,11 +3651,12 @@ GetLockStatusData(void)
 	int			els;
 	int			el;
 	int			i;
+	int			max_backends = GetMaxBackends();
 
 	data = (LockData *) palloc(sizeof(LockData));
 
 	/* Guess how much space we'll need. */
-	els = MaxBackends;
+	els = max_backends;
 	el = 0;
 	data->locks = (LockInstanceData *) palloc(sizeof(LockInstanceData) * els);
 
@@ -3689,7 +3690,7 @@ GetLockStatusData(void)
 
 			if (el >= els)
 			{
-				els += MaxBackends;
+				els += max_backends;
 				data->locks = (LockInstanceData *)
 					repalloc(data->locks, sizeof(LockInstanceData) * els);
 			}
@@ -3721,7 +3722,7 @@ GetLockStatusData(void)
 
 			if (el >= els)
 			{
-				els += MaxBackends;
+				els += max_backends;
 				data->locks = (LockInstanceData *)
 					repalloc(data->locks, sizeof(LockInstanceData) * els);
 			}
@@ -3850,7 +3851,7 @@ GetBlockerStatusData(int blocked_pid)
 	 * for the procs[] array; the other two could need enlargement, though.)
 	 */
 	data->nprocs = data->nlocks = data->npids = 0;
-	data->maxprocs = data->maxlocks = data->maxpids = MaxBackends;
+	data->maxprocs = data->maxlocks = data->maxpids = GetMaxBackends();
 	data->procs = (BlockedProcData *) palloc(sizeof(BlockedProcData) * data->maxprocs);
 	data->locks = (LockInstanceData *) palloc(sizeof(LockInstanceData) * data->maxlocks);
 	data->waiter_pids = (int *) palloc(sizeof(int) * data->maxpids);
@@ -3925,6 +3926,7 @@ GetSingleProcBlockerStatusData(PGPROC *blocked_proc, BlockedProcsData *data)
 	PGPROC	   *proc;
 	int			queue_size;
 	int			i;
+	int			max_backends = GetMaxBackends();
 
 	/* Nothing to do if this proc is not blocked */
 	if (theLock == NULL)
@@ -3953,7 +3955,7 @@ GetSingleProcBlockerStatusData(PGPROC *blocked_proc, BlockedProcsData *data)
 
 		if (data->nlocks >= data->maxlocks)
 		{
-			data->maxlocks += MaxBackends;
+			data->maxlocks += max_backends;
 			data->locks = (LockInstanceData *)
 				repalloc(data->locks, sizeof(LockInstanceData) * data->maxlocks);
 		}
@@ -3982,7 +3984,7 @@ GetSingleProcBlockerStatusData(PGPROC *blocked_proc, BlockedProcsData *data)
 
 	if (queue_size > data->maxpids - data->npids)
 	{
-		data->maxpids = Max(data->maxpids + MaxBackends,
+		data->maxpids = Max(data->maxpids + max_backends,
 							data->npids + queue_size);
 		data->waiter_pids = (int *) repalloc(data->waiter_pids,
 											 sizeof(int) * data->maxpids);
diff --git a/src/backend/storage/lmgr/predicate.c b/src/backend/storage/lmgr/predicate.c
index 25e7e4e37b..e337aad5b2 100644
--- a/src/backend/storage/lmgr/predicate.c
+++ b/src/backend/storage/lmgr/predicate.c
@@ -257,7 +257,7 @@
 	(&MainLWLockArray[PREDICATELOCK_MANAGER_LWLOCK_OFFSET + (i)].lock)
 
 #define NPREDICATELOCKTARGETENTS() \
-	mul_size(max_predicate_locks_per_xact, add_size(MaxBackends, max_prepared_xacts))
+	mul_size(max_predicate_locks_per_xact, add_size(GetMaxBackends(), max_prepared_xacts))
 
 #define SxactIsOnFinishedList(sxact) (!SHMQueueIsDetached(&((sxact)->finishedLink)))
 
@@ -1222,7 +1222,7 @@ InitPredicateLocks(void)
 	 * Compute size for serializable transaction hashtable. Note these
 	 * calculations must agree with PredicateLockShmemSize!
 	 */
-	max_table_size = (MaxBackends + max_prepared_xacts);
+	max_table_size = (GetMaxBackends() + max_prepared_xacts);
 
 	/*
 	 * Allocate a list to hold information on transactions participating in
@@ -1375,7 +1375,7 @@ PredicateLockShmemSize(void)
 	size = add_size(size, size / 10);
 
 	/* transaction list */
-	max_table_size = MaxBackends + max_prepared_xacts;
+	max_table_size = GetMaxBackends() + max_prepared_xacts;
 	max_table_size *= 10;
 	size = add_size(size, PredXactListDataSize);
 	size = add_size(size, mul_size((Size) max_table_size,
@@ -1907,7 +1907,7 @@ GetSerializableTransactionSnapshotInt(Snapshot snapshot,
 	{
 		++(PredXact->WritableSxactCount);
 		Assert(PredXact->WritableSxactCount <=
-			   (MaxBackends + max_prepared_xacts));
+			   (GetMaxBackends() + max_prepared_xacts));
 	}
 
 	MySerializableXact = sxact;
@@ -5111,7 +5111,7 @@ predicatelock_twophase_recover(TransactionId xid, uint16 info,
 		{
 			++(PredXact->WritableSxactCount);
 			Assert(PredXact->WritableSxactCount <=
-				   (MaxBackends + max_prepared_xacts));
+				   (GetMaxBackends() + max_prepared_xacts));
 		}
 
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index e306b04738..37f032e7b9 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -103,7 +103,7 @@ ProcGlobalShmemSize(void)
 {
 	Size		size = 0;
 	Size		TotalProcs =
-	add_size(MaxBackends, add_size(NUM_AUXILIARY_PROCS, max_prepared_xacts));
+	add_size(GetMaxBackends(), add_size(NUM_AUXILIARY_PROCS, max_prepared_xacts));
 
 	/* ProcGlobal */
 	size = add_size(size, sizeof(PROC_HDR));
@@ -127,7 +127,7 @@ ProcGlobalSemas(void)
 	 * We need a sema per backend (including autovacuum), plus one for each
 	 * auxiliary process.
 	 */
-	return MaxBackends + NUM_AUXILIARY_PROCS;
+	return GetMaxBackends() + NUM_AUXILIARY_PROCS;
 }
 
 /*
@@ -162,7 +162,8 @@ InitProcGlobal(void)
 	int			i,
 				j;
 	bool		found;
-	uint32		TotalProcs = MaxBackends + NUM_AUXILIARY_PROCS + max_prepared_xacts;
+	int			max_backends = GetMaxBackends();
+	uint32		TotalProcs = max_backends + NUM_AUXILIARY_PROCS + max_prepared_xacts;
 
 	/* Create the ProcGlobal shared structure */
 	ProcGlobal = (PROC_HDR *)
@@ -195,7 +196,7 @@ InitProcGlobal(void)
 	MemSet(procs, 0, TotalProcs * sizeof(PGPROC));
 	ProcGlobal->allProcs = procs;
 	/* XXX allProcCount isn't really all of them; it excludes prepared xacts */
-	ProcGlobal->allProcCount = MaxBackends + NUM_AUXILIARY_PROCS;
+	ProcGlobal->allProcCount = max_backends + NUM_AUXILIARY_PROCS;
 
 	/*
 	 * Allocate arrays mirroring PGPROC fields in a dense manner. See
@@ -221,7 +222,7 @@ InitProcGlobal(void)
 		 * dummy PGPROCs don't need these though - they're never associated
 		 * with a real process
 		 */
-		if (i < MaxBackends + NUM_AUXILIARY_PROCS)
+		if (i < max_backends + NUM_AUXILIARY_PROCS)
 		{
 			procs[i].sem = PGSemaphoreCreate();
 			InitSharedLatch(&(procs[i].procLatch));
@@ -258,7 +259,7 @@ InitProcGlobal(void)
 			ProcGlobal->bgworkerFreeProcs = &procs[i];
 			procs[i].procgloballist = &ProcGlobal->bgworkerFreeProcs;
 		}
-		else if (i < MaxBackends)
+		else if (i < max_backends)
 		{
 			/* PGPROC for walsender, add to walsenderFreeProcs list */
 			procs[i].links.next = (SHM_QUEUE *) ProcGlobal->walsenderFreeProcs;
@@ -286,8 +287,8 @@ InitProcGlobal(void)
 	 * Save pointers to the blocks of PGPROC structures reserved for auxiliary
 	 * processes and prepared transactions.
 	 */
-	AuxiliaryProcs = &procs[MaxBackends];
-	PreparedXactProcs = &procs[MaxBackends + NUM_AUXILIARY_PROCS];
+	AuxiliaryProcs = &procs[max_backends];
+	PreparedXactProcs = &procs[max_backends + NUM_AUXILIARY_PROCS];
 
 	/* Create ProcStructLock spinlock, too */
 	ProcStructLock = (slock_t *) ShmemAlloc(sizeof(slock_t));