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 aa9079136fed60d085168cdc275d414b969ffe40..6bbd906c0b71e3b94be268006d5580072b7d104b 100644 (file)
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -8236,8 +8236,7 @@ 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
@@ -8253,6 +8252,7 @@ ATPostAlterTypeCleanup(List **wqueue, AlteredTableInfo *tab, LOCKMODE lockmode)
        Form_pg_constraint con;
        Oid         relid;
        Oid         confrelid;
+       char        contype;
        bool        conislocal;
 
        tup = SearchSysCache1(CONSTROID, ObjectIdGetDatum(oldId));
@@ -8261,6 +8261,7 @@ ATPostAlterTypeCleanup(List **wqueue, AlteredTableInfo *tab, LOCKMODE lockmode)
        con = (Form_pg_constraint) GETSTRUCT(tup);
        relid = con->conrelid;
        confrelid = con->confrelid;
+       contype = con->contype;
        conislocal = con->conislocal;
        ReleaseSysCache(tup);
 
@@ -8273,6 +8274,15 @@ 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);