nbtree VACUUM: Cope with buggy opclasses.

Teach nbtree VACUUM to press on with vacuuming in the event of a page
deletion attempt that fails to "re-find" a downlink for its child/target
page.

There is no good reason to treat this as an irrecoverable error.  But
there is a good reason not to: pressing on at this point removes any
question of VACUUM not making progress solely due to misbehavior from
user-defined operator class code.

Discussion: https://postgr.es/m/CAH2-Wzma5G9CTtMjbrXTwOym+U=aWg-R7=-htySuztgoJLvZXg@mail.gmail.com

diff --git a/src/backend/access/nbtree/nbtpage.c b/src/backend/access/nbtree/nbtpage.c
index 530d924bff85359caa8fe681fbdbd250a6b63d3c..ef48679cc2eb03b093be3e22625262f55a742985 100644 (file)
--- a/src/backend/access/nbtree/nbtpage.c
+++ b/src/backend/access/nbtree/nbtpage.c
@@ -2791,10 +2791,26 @@ _bt_lock_subtree_parent(Relation rel, BlockNumber child, BTStack stack,
     */
    pbuf = _bt_getstackbuf(rel, stack, child);
    if (pbuf == InvalidBuffer)
-       ereport(ERROR,
+   {
+       /*
+        * Failed to "re-find" a pivot tuple whose downlink matched our child
+        * block number on the parent level -- the index must be corrupt.
+        * Don't even try to delete the leafbuf subtree.  Just report the
+        * issue and press on with vacuuming the index.
+        *
+        * Note: _bt_getstackbuf() recovers from concurrent page splits that
+        * take place on the parent level.  Its approach is a near-exhaustive
+        * linear search.  This also gives it a surprisingly good chance of
+        * recovering in the event of a buggy or inconsistent opclass.  But we
+        * don't rely on that here.
+        */
+       ereport(LOG,
                (errcode(ERRCODE_INDEX_CORRUPTED),
                 errmsg_internal("failed to re-find parent key in index \"%s\" for deletion target page %u",
                                 RelationGetRelationName(rel), child)));
+       return false;
+   }
+
    parent = stack->bts_blkno;
    parentoffset = stack->bts_offset;