Fix failure in WHERE CURRENT OF after rewinding the referenced cursor.

In a case where we have multiple relation-scan nodes in a cursor plan,
such as a scan of an inheritance tree, it's possible to fetch from a
given scan node, then rewind the cursor and fetch some row from an
earlier scan node.  In such a case, execCurrent.c mistakenly thought
that the later scan node was still active, because ExecReScan hadn't
done anything to make it look not-active.  We'd get some sort of
failure in the case of a SeqScan node, because the node's scan tuple
slot would be pointing at a HeapTuple whose t_self gets reset to
invalid by heapam.c.  But it seems possible that for other relation
scan node types we'd actually return a valid tuple TID to the caller,
resulting in updating or deleting a tuple that shouldn't have been
considered current.  To fix, forcibly clear the ScanTupleSlot in
ExecScanReScan.

Another issue here, which seems only latent at the moment but could
easily become a live bug in future, is that rewinding a cursor does
not necessarily lead to *immediately* applying ExecReScan to every
scan-level node in the plan tree.  Upper-level nodes will think that
they can postpone that call if their child node is already marked
with chgParam flags.  I don't see a way for that to happen today in
a plan tree that's simple enough for execCurrent.c's search_plan_tree
to understand, but that's one heck of a fragile assumption.  So, add
some logic in search_plan_tree to detect chgParam flags being set on
nodes that it descended to/through, and assume that that means we
should consider lower scan nodes to be logically reset even if their
ReScan call hasn't actually happened yet.

Per bug #15395 from Matvey Arye.  This has been broken for a long time,
so back-patch to all supported branches.

Discussion: https://postgr.es/m/153764171023.14986.280404050547008575@wrigleys.postgresql.org

1 files changed, 24 insertions, 0 deletions

diff --git a/src/test/regress/sql/portals.sql b/src/test/regress/sql/portals.sql
index d1a589094ea..52560ac0275 100644
--- a/src/test/regress/sql/portals.sql
+++ b/src/test/regress/sql/portals.sql
@@ -472,6 +472,30 @@ DELETE FROM onek WHERE CURRENT OF c1;
 SELECT stringu1 FROM onek WHERE stringu1 = 'DZAAAA';
 ROLLBACK;
 
+-- Check behavior with rewinding to a previous child scan node,
+-- as per bug #15395
+BEGIN;
+CREATE TABLE current_check (currentid int, payload text);
+CREATE TABLE current_check_1 () INHERITS (current_check);
+CREATE TABLE current_check_2 () INHERITS (current_check);
+INSERT INTO current_check_1 SELECT i, 'p' || i FROM generate_series(1,9) i;
+INSERT INTO current_check_2 SELECT i, 'P' || i FROM generate_series(10,19) i;
+
+DECLARE c1 SCROLL CURSOR FOR SELECT * FROM current_check;
+
+-- This tests the fetch-backwards code path
+FETCH ABSOLUTE 12 FROM c1;
+FETCH ABSOLUTE 8 FROM c1;
+DELETE FROM current_check WHERE CURRENT OF c1 RETURNING *;
+
+-- This tests the ExecutorRewind code path
+FETCH ABSOLUTE 13 FROM c1;
+FETCH ABSOLUTE 1 FROM c1;
+DELETE FROM current_check WHERE CURRENT OF c1 RETURNING *;
+
+SELECT * FROM current_check;
+ROLLBACK;
+
 -- Make sure snapshot management works okay, per bug report in
 -- 235395b90909301035v7228ce63q392931f15aa74b31@mail.gmail.com
 BEGIN;