Fix orphan on cancel of drop index concurrently.

Canceling DROP INDEX CONCURRENTLY during
wait could allow an orphaned index to be
left behind which could not be dropped.

Backpatch to 9.2

Andres Freund, tested by Abhijit Menon-Sen

src/backend/catalog/dependency.c

@@ -355,11 +355,6 @@ performMultipleDeletions(const ObjectAddresses *objects,
 	/* And clean up */
 	free_object_addresses(targetObjects);
 
-	/*
-	 * We closed depRel earlier in deleteOneObject if doing a drop
-	 * concurrently
-	 */
-	if ((flags & PERFORM_DELETION_CONCURRENTLY) != PERFORM_DELETION_CONCURRENTLY)
 	heap_close(depRel, RowExclusiveLock);
 }
 
@@ -1000,6 +995,7 @@ deleteOneObject(const ObjectAddress *object, Relation depRel, int flags)
 	int			nkeys;
 	SysScanDesc scan;
 	HeapTuple	tup;
+	Oid			depRelOid = depRel->rd_id;
 
 	/* DROP hook of the objects being removed */
 	if (object_access_hook)
@@ -1012,7 +1008,33 @@ deleteOneObject(const ObjectAddress *object, Relation depRel, int flags)
 	}
 
 	/*
-	 * First remove any pg_depend records that link from this object to
+	 * Close depRel if we are doing a drop concurrently. The individual
+	 * deletion has to commit the transaction and we don't want dangling
+	 * references.
+	 */
+	if (flags & PERFORM_DELETION_CONCURRENTLY)
+		heap_close(depRel, RowExclusiveLock);
+
+	/*
+	 * Delete the object itself, in an object-type-dependent way.
+	 *
+	 * Do this before removing outgoing dependencies as deletions can be
+	 * happening in concurrent mode. That will only happen for a single object
+	 * at once and if so the object will be invalidated inside a separate
+	 * transaction and only dropped inside a transaction thats in-progress when
+	 * doDeletion returns. This way no observer can see dangling dependency
+	 * entries.
+	 */
+	doDeletion(object, flags);
+
+	/*
+	 * Reopen depRel if we closed it before
+	 */
+	if (flags & PERFORM_DELETION_CONCURRENTLY)
+		depRel = heap_open(depRelOid, RowExclusiveLock);
+
+	/*
+	 * Then remove any pg_depend records that link from this object to
 	 * others.	(Any records linking to this object should be gone already.)
 	 *
 	 * When dropping a whole object (subId = 0), remove all pg_depend records
@@ -1054,17 +1076,6 @@ deleteOneObject(const ObjectAddress *object, Relation depRel, int flags)
 	deleteSharedDependencyRecordsFor(object->classId, object->objectId,
 									 object->objectSubId);
 
-	/*
-	 * Close depRel if we are doing a drop concurrently because it commits the
-	 * transaction, so we don't want dangling references.
-	 */
-	if ((flags & PERFORM_DELETION_CONCURRENTLY) == PERFORM_DELETION_CONCURRENTLY)
-		heap_close(depRel, RowExclusiveLock);
-
-	/*
-	 * Now delete the object itself, in an object-type-dependent way.
-	 */
-	doDeletion(object, flags);
 
 	/*
 	 * Delete any comments or security labels associated with this object.
@@ -1247,7 +1258,7 @@ AcquireDeletionLock(const ObjectAddress *object, int flags)
 {
 	if (object->classId == RelationRelationId)
 	{
-		if ((flags & PERFORM_DELETION_CONCURRENTLY) == PERFORM_DELETION_CONCURRENTLY)
+		if (flags & PERFORM_DELETION_CONCURRENTLY)
 			LockRelationOid(object->objectId, ShareUpdateExclusiveLock);
 		else
 			LockRelationOid(object->objectId, AccessExclusiveLock);