Make FP_LOCK_SLOTS_PER_BACKEND look like a function

The FP_LOCK_SLOTS_PER_BACKEND macro looks like a constant, but it
depends on the max_locks_per_transaction GUC, and thus can change. This
is non-obvious and confusing, so make it look more like a function by
renaming it to FastPathLockSlotsPerBackend().

While at it, use the macro when initializing fast-path shared memory,
instead of using the formula.

Reported-by: Andres Freund
Discussion: https://postgr.es/m/ffiwtzc6vedo6wb4gbwelon5nefqg675t5c7an2ta7pcz646cg%40qwmkdb3l4ett

Author: tvondra

src/backend/storage/lmgr/lock.c

@@ -226,10 +226,10 @@ int			FastPathLockGroupsPerBackend = 0;
  * the FAST_PATH_SLOT macro, split it into group and index (in the group).
  */
 #define FAST_PATH_GROUP(index)	\
-	(AssertMacro((uint32) (index) < FP_LOCK_SLOTS_PER_BACKEND), \
+	(AssertMacro((uint32) (index) < FastPathLockSlotsPerBackend()), \
 	 ((index) / FP_LOCK_SLOTS_PER_GROUP))
 #define FAST_PATH_INDEX(index)	\
-	(AssertMacro((uint32) (index) < FP_LOCK_SLOTS_PER_BACKEND), \
+	(AssertMacro((uint32) (index) < FastPathLockSlotsPerBackend()), \
 	 ((index) % FP_LOCK_SLOTS_PER_GROUP))
 
 /* Macros for manipulating proc->fpLockBits */
@@ -242,7 +242,7 @@ int			FastPathLockGroupsPerBackend = 0;
 #define FAST_PATH_BIT_POSITION(n, l) \
 	(AssertMacro((l) >= FAST_PATH_LOCKNUMBER_OFFSET), \
 	 AssertMacro((l) < FAST_PATH_BITS_PER_SLOT+FAST_PATH_LOCKNUMBER_OFFSET), \
-	 AssertMacro((n) < FP_LOCK_SLOTS_PER_BACKEND), \
+	 AssertMacro((n) < FastPathLockSlotsPerBackend()), \
 	 ((l) - FAST_PATH_LOCKNUMBER_OFFSET + FAST_PATH_BITS_PER_SLOT * (FAST_PATH_INDEX(n))))
 #define FAST_PATH_SET_LOCKMODE(proc, n, l) \
 	 FAST_PATH_BITS(proc, n) |= UINT64CONST(1) << FAST_PATH_BIT_POSITION(n, l)
@@ -2691,7 +2691,7 @@ static bool
 FastPathGrantRelationLock(Oid relid, LOCKMODE lockmode)
 {
 	uint32		i;
-	uint32		unused_slot = FP_LOCK_SLOTS_PER_BACKEND;
+	uint32		unused_slot = FastPathLockSlotsPerBackend();
 
 	/* fast-path group the lock belongs to */
 	uint32		group = FAST_PATH_REL_GROUP(relid);
@@ -2713,7 +2713,7 @@ FastPathGrantRelationLock(Oid relid, LOCKMODE lockmode)
 	}
 
 	/* If no existing entry, use any empty slot. */
-	if (unused_slot < FP_LOCK_SLOTS_PER_BACKEND)
+	if (unused_slot < FastPathLockSlotsPerBackend())
 	{
 		MyProc->fpRelId[unused_slot] = relid;
 		FAST_PATH_SET_LOCKMODE(MyProc, unused_slot, lockmode);

src/backend/storage/lmgr/proc.c

@@ -116,7 +116,7 @@ ProcGlobalShmemSize(void)
 	 * nicely aligned in each backend.
 	 */
 	fpLockBitsSize = MAXALIGN(FastPathLockGroupsPerBackend * sizeof(uint64));
-	fpRelIdSize = MAXALIGN(FastPathLockGroupsPerBackend * sizeof(Oid) * FP_LOCK_SLOTS_PER_GROUP);
+	fpRelIdSize = MAXALIGN(FastPathLockSlotsPerBackend() * sizeof(Oid));
 
 	size = add_size(size, mul_size(TotalProcs, (fpLockBitsSize + fpRelIdSize)));
 
@@ -231,7 +231,7 @@ InitProcGlobal(void)
 	 * shared memory and then divide that between backends.
 	 */
 	fpLockBitsSize = MAXALIGN(FastPathLockGroupsPerBackend * sizeof(uint64));
-	fpRelIdSize = MAXALIGN(FastPathLockGroupsPerBackend * sizeof(Oid) * FP_LOCK_SLOTS_PER_GROUP);
+	fpRelIdSize = MAXALIGN(FastPathLockSlotsPerBackend() * sizeof(Oid));
 
 	fpPtr = ShmemAlloc(TotalProcs * (fpLockBitsSize + fpRelIdSize));
 	MemSet(fpPtr, 0, TotalProcs * (fpLockBitsSize + fpRelIdSize));

src/backend/utils/init/postinit.c

@@ -587,7 +587,7 @@ InitializeFastPathLocks(void)
 	while (FastPathLockGroupsPerBackend < FP_LOCK_GROUPS_PER_BACKEND_MAX)
 	{
 		/* stop once we exceed max_locks_per_xact */
-		if (FastPathLockGroupsPerBackend * FP_LOCK_SLOTS_PER_GROUP >= max_locks_per_xact)
+		if (FastPathLockSlotsPerBackend() >= max_locks_per_xact)
 			break;
 
 		FastPathLockGroupsPerBackend *= 2;

src/include/storage/proc.h

@@ -88,7 +88,8 @@ extern PGDLLIMPORT int FastPathLockGroupsPerBackend;
 
 #define		FP_LOCK_GROUPS_PER_BACKEND_MAX	1024
 #define		FP_LOCK_SLOTS_PER_GROUP		16	/* don't change */
-#define		FP_LOCK_SLOTS_PER_BACKEND	(FP_LOCK_SLOTS_PER_GROUP * FastPathLockGroupsPerBackend)
+#define		FastPathLockSlotsPerBackend() \
+	(FP_LOCK_SLOTS_PER_GROUP * FastPathLockGroupsPerBackend)
 
 /*
  * Flags for PGPROC.delayChkptFlags