Mega-commit to make heap_open/heap_openr/heap_close take an

additional argument specifying the kind of lock to acquire/release (or
'NoLock' to do no lock processing).  Ensure that all relations are locked
with some appropriate lock level before being examined --- this ensures
that relevant shared-inval messages have been processed and should prevent
problems caused by concurrent VACUUM.  Fix several bugs having to do with
mismatched increment/decrement of relation ref count and mismatched
heap_open/close (which amounts to the same thing).  A bogus ref count on
a relation doesn't matter much *unless* a SI Inval message happens to
arrive at the wrong time, which is probably why we got away with this
sloppiness for so long.  Repair missing grab of AccessExclusiveLock in
DROP TABLE, ALTER/RENAME TABLE, etc, as noted by Hiroshi.
Recommend 'make clean all' after pulling this update; I modified the
Relation struct layout slightly.
Will post further discussion to pghackers list shortly.

1 files changed, 11 insertions, 16 deletions

diff --git a/src/backend/commands/dbcommands.c b/src/backend/commands/dbcommands.c
index b779715fe8e..24eb5b531d3 100644
--- a/src/backend/commands/dbcommands.c
+++ b/src/backend/commands/dbcommands.c
@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/commands/dbcommands.c,v 1.39 1999/07/17 20:16:52 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/commands/dbcommands.c,v 1.40 1999/09/18 19:06:40 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -103,6 +103,8 @@ destroydb(char *dbname, CommandDest dest)
 	/* stop the vacuum daemon */
 	stop_vacuum(dbpath, dbname);
 
+	/* XXX what about stopping backends connected to the target database? */
+
 	path = ExpandDatabasePath(dbpath);
 	if (path == NULL)
 		elog(ERROR, "Unable to locate path '%s'"
@@ -189,6 +191,7 @@ check_permissions(char *command,
 	utup = SearchSysCacheTuple(USENAME,
 							   PointerGetDatum(userName),
 							   0, 0, 0);
+	Assert(utup);
 	*userIdP = ((Form_pg_shadow) GETSTRUCT(utup))->usesysid;
 	use_super = ((Form_pg_shadow) GETSTRUCT(utup))->usesuper;
 	use_createdb = ((Form_pg_shadow) GETSTRUCT(utup))->usecreatedb;
@@ -211,22 +214,13 @@ check_permissions(char *command,
 	/* Check to make sure database is owned by this user */
 
 	/*
-	 * need the reldesc to get the database owner out of dbtup and to set
-	 * a write lock on it.
+	 * Acquire exclusive lock on pg_database from the beginning, even though
+	 * we only need read access right here, to avoid potential deadlocks
+	 * from upgrading our lock later.  (Is this still necessary?  Could we
+	 * use something weaker than exclusive lock?)
 	 */
-	dbrel = heap_openr(DatabaseRelationName);
-
-	if (!RelationIsValid(dbrel))
-		elog(FATAL, "%s: cannot open relation \"%-.*s\"",
-			 command, DatabaseRelationName);
+	dbrel = heap_openr(DatabaseRelationName, AccessExclusiveLock);
 
-	/*
-	 * Acquire a write lock on pg_database from the beginning to avoid
-	 * upgrading a read lock to a write lock.  Upgrading causes long
-	 * delays when multiple 'createdb's or 'destroydb's are run simult.
-	 * -mer 7/3/91
-	 */
-	LockRelation(dbrel, AccessExclusiveLock);
 	dbtup = get_pg_dbtup(command, dbname, dbrel);
 	dbfound = HeapTupleIsValid(dbtup);
 
@@ -248,7 +242,8 @@ check_permissions(char *command,
 	else
 		*dbIdP = InvalidOid;
 
-	heap_close(dbrel);
+	/* We will keep the lock on dbrel until end of transaction. */
+	heap_close(dbrel, NoLock);
 
 	/*
 	 * Now be sure that the user is allowed to do this.