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

1 files changed, 17 insertions, 8 deletions

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 9d3efb7d80e..13d192ec2b4 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;
 	}