Several changes here, not very related but touching some of the same files.

* Buffer refcount cleanup (per my "progress report" to pghackers, 9/22).
* Add links to backend PROC structs to sinval's array of per-backend info,
and use these links for routines that need to check the state of all
backends (rather than the slow, complicated search of the ShmemIndex
hashtable that was used before).  Add databaseOID to PROC structs.
* Use this to implement an interlock that prevents DESTROY DATABASE of
a database containing running backends.  (It's a little tricky to prevent
a concurrently-starting backend from getting in there, since the new
backend is not able to lock anything at the time it tries to look up
its database in pg_database.  My solution is to recheck that the DB is
OK at the end of InitPostgres.  It may not be a 100% solution, but it's
a lot better than no interlock at all...)
* In ALTER TABLE RENAME, flush buffers for the relation before doing the
rename of the physical files, to ensure we don't get failures later from
mdblindwrt().
* Update TRUNCATE patch so that it actually compiles against current
sources :-(.
You should do "make clean all" after pulling these changes.

src/include/storage/bufmgr.h

@@ -6,7 +6,7 @@
  *
  * Copyright (c) 1994, Regents of the University of California
  *
- * $Id: bufmgr.h,v 1.30 1999/09/23 17:03:27 momjian Exp $
+ * $Id: bufmgr.h,v 1.31 1999/09/24 00:25:27 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -71,12 +71,35 @@ extern int	ShowPinTrace;
 
 /*
  * BufferIsValid
- *		True iff the refcnt of the local buffer is > 0
+ *		True iff the given buffer number is valid (either as a shared
+ *		or local buffer).
+ *
  * Note:
  *		BufferIsValid(InvalidBuffer) is False.
  *		BufferIsValid(UnknownBuffer) is False.
+ *
+ * Note: For a long time this was defined the same as BufferIsPinned,
+ * that is it would say False if you didn't hold a pin on the buffer.
+ * I believe this was bogus and served only to mask logic errors.
+ * Code should always know whether it has a buffer reference,
+ * independently of the pin state.
  */
 #define BufferIsValid(bufnum) \
+( \
+	BufferIsLocal(bufnum) ? \
+		((bufnum) >= -NLocBuffer) \
+	: \
+		(! BAD_BUFFER_ID(bufnum)) \
+)
+
+/*
+ * BufferIsPinned
+ *		True iff the buffer is pinned (also checks for valid buffer number).
+ *
+ *		NOTE: what we check here is that *this* backend holds a pin on
+ *		the buffer.  We do not care whether some other backend does.
+ */
+#define BufferIsPinned(bufnum) \
 ( \
 	BufferIsLocal(bufnum) ? \
 		((bufnum) >= -NLocBuffer && LocalRefCount[-(bufnum) - 1] > 0) \
@@ -90,28 +113,27 @@ extern int	ShowPinTrace;
 )
 
 /*
- * BufferIsPinned
- *		True iff the buffer is pinned (and therefore valid)
+ * IncrBufferRefCount
+ *		Increment the pin count on a buffer that we have *already* pinned
+ *		at least once.
  *
- * Note:
- *		Smenatics are identical to BufferIsValid
- *		XXX - need to remove either one eventually.
+ *		This macro cannot be used on a buffer we do not have pinned,
+ *		because it doesn't change the shared buffer state.  Therefore the
+ *		Assert checks are for refcount > 0.  Someone got this wrong once...
  */
-#define BufferIsPinned BufferIsValid
-
-
 #define IncrBufferRefCount(buffer) \
 ( \
 	BufferIsLocal(buffer) ? \
 	( \
-		(void)AssertMacro(LocalRefCount[-(buffer) - 1] >= 0), \
-		(void)LocalRefCount[-(buffer) - 1]++ \
+		(void) AssertMacro((buffer) >= -NLocBuffer), \
+		(void) AssertMacro(LocalRefCount[-(buffer) - 1] > 0), \
+		(void) LocalRefCount[-(buffer) - 1]++ \
 	) \
 	: \
 	( \
-		(void)AssertMacro(!BAD_BUFFER_ID(buffer)), \
-		(void)AssertMacro(PrivateRefCount[(buffer) - 1] >= 0), \
-		(void)PrivateRefCount[(buffer) - 1]++ \
+		(void) AssertMacro(!BAD_BUFFER_ID(buffer)), \
+		(void) AssertMacro(PrivateRefCount[(buffer) - 1] > 0), \
+		(void) PrivateRefCount[(buffer) - 1]++ \
 	) \
 )
 
@@ -151,19 +173,18 @@ extern int	BufferPoolCheckLeak(void);
 extern void FlushBufferPool(int StableMainMemoryFlag);
 extern BlockNumber BufferGetBlockNumber(Buffer buffer);
 extern BlockNumber RelationGetNumberOfBlocks(Relation relation);
+extern int	FlushRelationBuffers(Relation rel, BlockNumber block,
+								 bool doFlush);
 extern void ReleaseRelationBuffers(Relation rel);
 extern void DropBuffers(Oid dbid);
 extern void PrintPinnedBufs(void);
 extern int	BufferShmemSize(void);
 extern int	ReleaseBuffer(Buffer buffer);
 
-extern void BufferRefCountReset(int *refcountsave);
-extern void BufferRefCountRestore(int *refcountsave);
 extern int	SetBufferWriteMode(int mode);
 extern void SetBufferCommitInfoNeedsSave(Buffer buffer);
-extern int BlowawayRelationBuffers(Relation rel, BlockNumber block);
 
 extern void UnlockBuffers(void);
 extern void LockBuffer(Buffer buffer, int mode);
 
-#endif	 /* !defined(BufMgrIncluded) */
+#endif