Provide a way to predefine LWLock tranche IDs.

It's a bit cumbersome to use LWLockNewTrancheId(), because the returned
value needs to be shared between backends so that each backend can call
LWLockRegisterTranche() with the correct ID.  So, for built-in tranches,
use a hard-coded value instead.

This is motivated by an upcoming patch adding further built-in tranches.

Andres Freund and Robert Haas

src/backend/access/transam/xlog.c

@@ -512,7 +512,6 @@ typedef struct XLogCtlInsert
 	 */
 	WALInsertLockPadded *WALInsertLocks;
 	LWLockTranche WALInsertLockTranche;
-	int			WALInsertLockTrancheId;
 } XLogCtlInsert;
 
 /*
@@ -4653,7 +4652,7 @@ XLOGShmemInit(void)
 
 		/* Initialize local copy of WALInsertLocks and register the tranche */
 		WALInsertLocks = XLogCtl->Insert.WALInsertLocks;
-		LWLockRegisterTranche(XLogCtl->Insert.WALInsertLockTrancheId,
+		LWLockRegisterTranche(LWTRANCHE_WAL_INSERT,
 							  &XLogCtl->Insert.WALInsertLockTranche);
 		return;
 	}
@@ -4677,17 +4676,14 @@ XLOGShmemInit(void)
 		(WALInsertLockPadded *) allocptr;
 	allocptr += sizeof(WALInsertLockPadded) * NUM_XLOGINSERT_LOCKS;
 
-	XLogCtl->Insert.WALInsertLockTrancheId = LWLockNewTrancheId();
-
 	XLogCtl->Insert.WALInsertLockTranche.name = "WALInsertLocks";
 	XLogCtl->Insert.WALInsertLockTranche.array_base = WALInsertLocks;
 	XLogCtl->Insert.WALInsertLockTranche.array_stride = sizeof(WALInsertLockPadded);
 
-	LWLockRegisterTranche(XLogCtl->Insert.WALInsertLockTrancheId, &XLogCtl->Insert.WALInsertLockTranche);
+	LWLockRegisterTranche(LWTRANCHE_WAL_INSERT, &XLogCtl->Insert.WALInsertLockTranche);
 	for (i = 0; i < NUM_XLOGINSERT_LOCKS; i++)
 	{
-		LWLockInitialize(&WALInsertLocks[i].l.lock,
-						 XLogCtl->Insert.WALInsertLockTrancheId);
+		LWLockInitialize(&WALInsertLocks[i].l.lock, LWTRANCHE_WAL_INSERT);
 		WALInsertLocks[i].l.insertingAt = InvalidXLogRecPtr;
 	}
 

src/backend/storage/lmgr/lwlock.c

@@ -445,7 +445,7 @@ CreateLWLocks(void)
 
 		/* Initialize all LWLocks in main array */
 		for (id = 0, lock = MainLWLockArray; id < numLocks; id++, lock++)
-			LWLockInitialize(&lock->lock, 0);
+			LWLockInitialize(&lock->lock, LWTRANCHE_MAIN);
 
 		/*
 		 * Initialize the dynamic-allocation counters, which are stored just
@@ -457,7 +457,7 @@ CreateLWLocks(void)
 		LWLockCounter = (int *) ((char *) MainLWLockArray - 3 * sizeof(int));
 		LWLockCounter[0] = NUM_FIXED_LWLOCKS;
 		LWLockCounter[1] = numLocks;
-		LWLockCounter[2] = 1;	/* 0 is the main array */
+		LWLockCounter[2] = LWTRANCHE_FIRST_USER_DEFINED;
 	}
 
 	if (LWLockTrancheArray == NULL)
@@ -466,12 +466,13 @@ CreateLWLocks(void)
 		LWLockTrancheArray = (LWLockTranche **)
 			MemoryContextAlloc(TopMemoryContext,
 						  LWLockTranchesAllocated * sizeof(LWLockTranche *));
+		Assert(LWLockTranchesAllocated >= LWTRANCHE_FIRST_USER_DEFINED);
 	}
 
 	MainLWLockTranche.name = "main";
 	MainLWLockTranche.array_base = MainLWLockArray;
 	MainLWLockTranche.array_stride = sizeof(LWLockPadded);
-	LWLockRegisterTranche(0, &MainLWLockTranche);
+	LWLockRegisterTranche(LWTRANCHE_MAIN, &MainLWLockTranche);
 }
 
 /*

src/include/storage/lwlock.h

@@ -176,6 +176,17 @@ extern int	LWLockNewTrancheId(void);
 extern void LWLockRegisterTranche(int tranche_id, LWLockTranche *tranche);
 extern void LWLockInitialize(LWLock *lock, int tranche_id);
 
+/*
+ * We reserve a few predefined tranche IDs.  A call to LWLockNewTrancheId
+ * will never return a value less than LWTRANCHE_FIRST_USER_DEFINED.
+ */
+typedef enum BuiltinTrancheIds
+{
+	LWTRANCHE_MAIN,
+	LWTRANCHE_WAL_INSERT,
+	LWTRANCHE_FIRST_USER_DEFINED
+}	BuiltinTrancheIds;
+
 /*
  * Prior to PostgreSQL 9.4, we used an enum type called LWLockId to refer
  * to LWLocks.  New code should instead use LWLock *.  However, for the