Fix corner case where SELECT FOR UPDATE could return a row twice.

In READ COMMITTED mode, if a SELECT FOR UPDATE discovers it has to redo
WHERE-clause checking on rows that have been updated since the SELECT's
snapshot, it invokes EvalPlanQual processing to do that.  If this first
occurs within a non-first child table of an inheritance tree, the previous
coding could accidentally re-return a matching row from an earlier,
already-scanned child table.  (And, to add insult to injury, I think this
could make it miss returning a row that should have been returned, if the
updated row that this happens on should still have passed the WHERE qual.)
Per report from Kyotaro Horiguchi; the added isolation test is based on his
test case.

This has been broken for quite awhile, so back-patch to all supported
branches.

diff --git a/src/backend/executor/nodeLockRows.c b/src/backend/executor/nodeLockRows.c
index 6f5c2d2290d44ce4d2ae109dc53e5efecda1e6c0..6fac96197b7ff6238f4cd9280ad479ea94370f27 100644 (file)
--- a/src/backend/executor/nodeLockRows.c
+++ b/src/backend/executor/nodeLockRows.c
@@ -160,7 +160,29 @@ lnext:
                                 */
                                if (!epq_started)
                                {
+                                       ListCell   *lc2;
+
                                        EvalPlanQualBegin(&node->lr_epqstate, estate);
+
+                                       /*
+                                        * Ensure that rels with already-visited rowmarks are told
+                                        * not to return tuples during the first EPQ test.  We can
+                                        * exit this loop once it reaches the current rowmark;
+                                        * rels appearing later in the list will be set up
+                                        * correctly by the EvalPlanQualSetTuple call at the top
+                                        * of the loop.
+                                        */
+                                       foreach(lc2, node->lr_arowMarks)
+                                       {
+                                               ExecAuxRowMark *aerm2 = (ExecAuxRowMark *) lfirst(lc2);
+
+                                               if (lc2 == lc)
+                                                       break;
+                                               EvalPlanQualSetTuple(&node->lr_epqstate,
+                                                                                        aerm2->rowmark->rti,
+                                                                                        NULL);
+                                       }
+
                                        epq_started = true;
                                }