Fix lock upgrade hazard in ATExecAttachPartition.

Amit Langote

Discussion: http://postgr.es/m/CAFjFpReT_kq_uwU_B8aWDxR7jNGE=P0iELycdq5oupi=xSQTOw@mail.gmail.com

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 3c42e160b2dbe57f454b798c6bef99f798974fbe..7859ef13ac4aafaf600a1923ddf3aa94b69f967b 100644 (file)
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -13489,9 +13489,20 @@ ATExecAttachPartition(List **wqueue, Relation rel, PartitionCmd *cmd)
    /*
     * Prevent circularity by seeing if rel is a partition of attachrel. (In
     * particular, this disallows making a rel a partition of itself.)
+    *
+    * We do that by checking if rel is a member of the list of attachRel's
+    * partitions provided the latter is partitioned at all.  We want to avoid
+    * having to construct this list again, so we request the strongest lock
+    * on all partitions.  We need the strongest lock, because we may decide
+    * to scan them if we find out that the table being attached (or its leaf
+    * partitions) may contain rows that violate the partition constraint.
+    * If the table has a constraint that would prevent such rows, which by
+    * definition is present in all the partitions, we need not scan the
+    * table, nor its partitions.  But we cannot risk a deadlock by taking a
+    * weaker lock now and the stronger one only when needed.
     */
    attachrel_children = find_all_inheritors(RelationGetRelid(attachrel),
-                                            AccessShareLock, NULL);
+                                            AccessExclusiveLock, NULL);
    if (list_member_oid(attachrel_children, RelationGetRelid(rel)))
        ereport(ERROR,
                (errcode(ERRCODE_DUPLICATE_TABLE),
@@ -13694,17 +13705,9 @@ ATExecAttachPartition(List **wqueue, Relation rel, PartitionCmd *cmd)
    else
    {
        /* Constraints proved insufficient, so we need to scan the table. */
-       List       *all_parts;
        ListCell   *lc;
 
-       /* Take an exclusive lock on the partitions to be checked */
-       if (attachrel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
-           all_parts = find_all_inheritors(RelationGetRelid(attachrel),
-                                           AccessExclusiveLock, NULL);
-       else
-           all_parts = list_make1_oid(RelationGetRelid(attachrel));
-
-       foreach(lc, all_parts)
+       foreach(lc, attachrel_children)
        {
            AlteredTableInfo *tab;
            Oid         part_relid = lfirst_oid(lc);