Fix two race conditions between the pending unlink mechanism that was put in

place to prevent reusing relation OIDs before next checkpoint, and DROP
DATABASE. First, if a database was dropped, bgwriter would still try to unlink
the files that the rmtree() call by the DROP DATABASE command has already
deleted, or is just about to delete. Second, if a database is dropped, and
another database is created with the same OID, bgwriter would in the worst
case delete a relation in the new database that happened to get the same OID
as a dropped relation in the old database.

To fix these race conditions:
- make rmtree() ignore ENOENT errors. This fixes the 1st race condition.
- make ForgetDatabaseFsyncRequests forget unlink requests as well.
- force checkpoint on in dropdb on all platforms

Since ForgetDatabaseFsyncRequests() is asynchronous, the 2nd change isn't
enough on its own to fix the problem of dropping and creating a database with
same OID, but forcing a checkpoint on DROP DATABASE makes it sufficient.

Per Tom Lane's bug report and proposal. Backpatch to 8.3.

1 files changed, 28 insertions, 5 deletions

diff --git a/src/backend/storage/smgr/md.c b/src/backend/storage/smgr/md.c
index 6ea4a00b017..fdc7c7d0726 100644
--- a/src/backend/storage/smgr/md.c
+++ b/src/backend/storage/smgr/md.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/smgr/md.c,v 1.136 2008/03/10 20:06:27 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/smgr/md.c,v 1.137 2008/04/18 06:48:38 heikki Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1196,8 +1196,11 @@ mdpostckpt(void)
 		if (unlink(path) < 0)
 		{
 			/*
-			 * ENOENT shouldn't happen either, but it doesn't really matter
-			 * because we would've deleted it now anyway.
+			 * There's a race condition, when the database is dropped at the
+			 * same time that we process the pending unlink requests. If the
+			 * DROP DATABASE deletes the file before we do, we will get ENOENT
+			 * here. rmtree() also has to ignore ENOENT errors, to deal with
+			 * the possibility that we delete the file first.
 			 */
 			if (errno != ENOENT)
 				ereport(WARNING,
@@ -1321,7 +1324,11 @@ RememberFsyncRequest(RelFileNode rnode, BlockNumber segno)
 		/* Remove any pending requests for the entire database */
 		HASH_SEQ_STATUS hstat;
 		PendingOperationEntry *entry;
+		ListCell   *cell, 
+				   *prev,
+				   *next;
 
+		/* Remove fsync requests */
 		hash_seq_init(&hstat, pendingOpsTable);
 		while ((entry = (PendingOperationEntry *) hash_seq_search(&hstat)) != NULL)
 		{
@@ -1331,6 +1338,22 @@ RememberFsyncRequest(RelFileNode rnode, BlockNumber segno)
 				entry->canceled = true;
 			}
 		}
+	
+		/* Remove unlink requests */
+		prev = NULL;
+		for (cell = list_head(pendingUnlinks); cell; cell = next)
+		{
+			PendingUnlinkEntry *entry = (PendingUnlinkEntry *) lfirst(cell);
+
+			next = lnext(cell);
+			if (entry->rnode.dbNode == rnode.dbNode) 
+			{
+				pendingUnlinks = list_delete_cell(pendingUnlinks, cell, prev);
+				pfree(entry);
+			}
+			else
+				prev = cell;
+		}
 	}
 	else if (segno == UNLINK_RELATION_REQUEST)
 	{
@@ -1386,7 +1409,7 @@ RememberFsyncRequest(RelFileNode rnode, BlockNumber segno)
 }
 
 /*
- * ForgetRelationFsyncRequests -- ensure any fsyncs for a rel are forgotten
+ * ForgetRelationFsyncRequests -- forget any fsyncs for a rel
  */
 void
 ForgetRelationFsyncRequests(RelFileNode rnode)
@@ -1419,7 +1442,7 @@ ForgetRelationFsyncRequests(RelFileNode rnode)
 }
 
 /*
- * ForgetDatabaseFsyncRequests -- ensure any fsyncs for a DB are forgotten
+ * ForgetDatabaseFsyncRequests -- forget any fsyncs and unlinks for a DB
  */
 void
 ForgetDatabaseFsyncRequests(Oid dbid)