Fix ALTER COLUMN TYPE to not open a relation without any lock.

If the column being modified is referenced by a foreign key constraint
of another table, ALTER TABLE would open the other table (to re-parse
the constraint's definition) without having first obtained a lock on it.
This was evidently intentional, but that doesn't mean it's really safe.
It's especially not safe in 9.3, which pre-dates use of MVCC scans for
catalog reads, but even in current releases it doesn't seem like a good
idea.

We know we'll need AccessExclusiveLock shortly to drop the obsoleted
constraint, so just get that a little sooner to close the hole.

Per testing with a patch that complains if we open a relation without
holding any lock on it.  I don't plan to back-patch that patch, but we
should close the holes it identifies in all supported branches.

Discussion: https://postgr.es/m/2038.1538335244@sss.pgh.pa.us

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 9e60bb37a7948a12ff183e039e87da98e5f96a4a..c145385f8435010d9a93d29ef92ce9a6e9050f47 100644 (file)
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -9888,8 +9888,7 @@ ATPostAlterTypeCleanup(List **wqueue, AlteredTableInfo *tab, LOCKMODE lockmode)
     * appropriate work queue entries.  We do this before dropping because in
     * the case of a FOREIGN KEY constraint, we might not yet have exclusive
     * lock on the table the constraint is attached to, and we need to get
-    * that before dropping.  It's safe because the parser won't actually look
-    * at the catalogs to detect the existing entry.
+    * that before reparsing/dropping.
     *
     * We can't rely on the output of deparsing to tell us which relation to
     * operate on, because concurrent activity might have made the name
@@ -9905,6 +9904,7 @@ ATPostAlterTypeCleanup(List **wqueue, AlteredTableInfo *tab, LOCKMODE lockmode)
        Form_pg_constraint con;
        Oid         relid;
        Oid         confrelid;
+       char        contype;
        bool        conislocal;
 
        tup = SearchSysCache1(CONSTROID, ObjectIdGetDatum(oldId));
@@ -9921,10 +9921,11 @@ ATPostAlterTypeCleanup(List **wqueue, AlteredTableInfo *tab, LOCKMODE lockmode)
                elog(ERROR, "could not identify relation associated with constraint %u", oldId);
        }
        confrelid = con->confrelid;
+       contype = con->contype;
        conislocal = con->conislocal;
        ReleaseSysCache(tup);
 
-       ObjectAddressSet(obj, ConstraintRelationId, lfirst_oid(oid_item));
+       ObjectAddressSet(obj, ConstraintRelationId, oldId);
        add_exact_object_address(&obj, objects);
 
        /*
@@ -9936,6 +9937,15 @@ ATPostAlterTypeCleanup(List **wqueue, AlteredTableInfo *tab, LOCKMODE lockmode)
        if (!conislocal)
            continue;
 
+       /*
+        * When rebuilding an FK constraint that references the table we're
+        * modifying, we might not yet have any lock on the FK's table, so get
+        * one now.  We'll need AccessExclusiveLock for the DROP CONSTRAINT
+        * step, so there's no value in asking for anything weaker.
+        */
+       if (relid != tab->relid && contype == CONSTRAINT_FOREIGN)
+           LockRelationOid(relid, AccessExclusiveLock);
+
        ATPostAlterTypeParse(oldId, relid, confrelid,
                             (char *) lfirst(def_item),
                             wqueue, lockmode, tab->rewrite);
@@ -9951,7 +9961,7 @@ ATPostAlterTypeCleanup(List **wqueue, AlteredTableInfo *tab, LOCKMODE lockmode)
                             (char *) lfirst(def_item),
                             wqueue, lockmode, tab->rewrite);
 
-       ObjectAddressSet(obj, RelationRelationId, lfirst_oid(oid_item));
+       ObjectAddressSet(obj, RelationRelationId, oldId);
        add_exact_object_address(&obj, objects);
    }