If we're going to have a non-panic check for held_lwlocks[] overrun,

it must occur *before* we get into the critical state of holding a
lock we have no place to record.  Per discussion with Qingqing Zhou.

src/backend/storage/lmgr/lwlock.c

@@ -15,7 +15,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/lwlock.c,v 1.26 2005/04/08 03:43:54 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/lwlock.c,v 1.27 2005/04/08 14:18:35 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -213,6 +213,10 @@ LWLockAcquire(LWLockId lockid, LWLockMode mode)
 	 */
 	Assert(!(proc == NULL && IsUnderPostmaster));
 
+	/* Ensure we will have room to remember the lock */
+	if (num_held_lwlocks >= MAX_SIMUL_LWLOCKS)
+		elog(ERROR, "too many LWLocks taken");
+
 	/*
 	 * Lock out cancel/die interrupts until we exit the code section
 	 * protected by the LWLock.  This ensures that interrupts will not
@@ -328,8 +332,6 @@ LWLockAcquire(LWLockId lockid, LWLockMode mode)
 	SpinLockRelease_NoHoldoff(&lock->mutex);
 
 	/* Add lock to list of locks held by this backend */
-	if (num_held_lwlocks >= MAX_SIMUL_LWLOCKS)
-		elog(ERROR, "too many LWLocks taken");
 	held_lwlocks[num_held_lwlocks++] = lockid;
 
 	/*
@@ -354,6 +356,10 @@ LWLockConditionalAcquire(LWLockId lockid, LWLockMode mode)
 
 	PRINT_LWDEBUG("LWLockConditionalAcquire", lockid, lock);
 
+	/* Ensure we will have room to remember the lock */
+	if (num_held_lwlocks >= MAX_SIMUL_LWLOCKS)
+		elog(ERROR, "too many LWLocks taken");
+
 	/*
 	 * Lock out cancel/die interrupts until we exit the code section
 	 * protected by the LWLock.  This ensures that interrupts will not
@@ -398,8 +404,6 @@ LWLockConditionalAcquire(LWLockId lockid, LWLockMode mode)
 	else
 	{
 		/* Add lock to list of locks held by this backend */
-		if (num_held_lwlocks >= MAX_SIMUL_LWLOCKS)
-			elog(ERROR, "too many LWLocks taken");
 		held_lwlocks[num_held_lwlocks++] = lockid;
 	}