Ensure that foreign scans with lateral refs are planned correctly.

As reported in bug #15613 from Srinivasan S A, file_fdw and postgres_fdw
neglected to mark plain baserel foreign paths as parameterized when the
relation has lateral_relids.  Other FDWs have surely copied this mistake,
so rather than just patching those two modules, install a band-aid fix
in create_foreignscan_path to rectify the mistake centrally.

Although the band-aid is enough to fix the visible symptom, correct
the calls in file_fdw and postgres_fdw anyway, so that they are valid
examples for external FDWs.

Also, since the band-aid isn't enough to make this work for parameterized
foreign joins, throw an elog(ERROR) if such a case is passed to
create_foreignscan_path.  This shouldn't pose much of a problem for
existing external FDWs, since it's likely they aren't trying to make such
paths anyway (though some of them may need a defense against joins with
lateral_relids, similar to the one this patch installs into postgres_fdw).

Add some assertions in relnode.c to catch future occurrences of the same
error --- in particular, as backstop against core-code mistakes like the
one fixed by commit bdd9a99aa.

Discussion: https://postgr.es/m/15613-092be1be9576c728@postgresql.org

1 files changed, 23 insertions, 0 deletions

diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index e74aa094c02..af0c35876dc 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -1467,6 +1467,29 @@ create_foreignscan_path(PlannerInfo *root, RelOptInfo *rel,
 {
 	ForeignPath *pathnode = makeNode(ForeignPath);
 
+	/*
+	 * Since the path's required_outer should always include all the rel's
+	 * lateral_relids, forcibly add those if necessary.  This is a bit of a
+	 * hack, but up till early 2019 the contrib FDWs failed to ensure that,
+	 * and it's likely that the same error has propagated into many external
+	 * FDWs.  Don't risk modifying the passed-in relid set here.
+	 */
+	if (rel->lateral_relids && !bms_is_subset(rel->lateral_relids,
+											  required_outer))
+		required_outer = bms_union(required_outer, rel->lateral_relids);
+
+	/*
+	 * Although this function is only designed to be used for scans of
+	 * baserels, before v12 postgres_fdw abused it to make paths for join and
+	 * upper rels.  It will work for such cases as long as required_outer is
+	 * empty (otherwise get_baserel_parampathinfo does the wrong thing), which
+	 * fortunately is the expected case for now.
+	 */
+	if (!bms_is_empty(required_outer) &&
+		!(rel->reloptkind == RELOPT_BASEREL ||
+		  rel->reloptkind == RELOPT_OTHER_MEMBER_REL))
+		elog(ERROR, "parameterized foreign joins are not supported yet");
+
 	pathnode->path.pathtype = T_ForeignScan;
 	pathnode->path.parent = rel;
 	pathnode->path.param_info = get_baserel_parampathinfo(root, rel,