search_plan_tree(): handling of non-leaf CustomScanState nodes causes segfault

Hi hackers,

While working with cursors that reference plans with CustomScanStates
nodes, I encountered a segfault which originates from
search_plan_tree(). The query plan is the result of a simple SELECT
statement into which I inject a Custom Scan node at the root to do some
post-processing before returning rows. This plan is referenced by a
second plan with a Tid Scan which originates from a query of the form
DELETE FROM foo WHERE CURRENT OF my_cursor;

search_plan_tree() assumes that
CustomScanState::ScanState::ss_currentRelation is never NULL. In my
understanding that only holds for CustomScanState nodes which are at the
bottom of the plan and actually read from a relation. CustomScanState
nodes which are not at the bottom don't have ss_currentRelation set. I
believe for such nodes, instead search_plan_tree() should recurse into
CustomScanState::custom_ps.

I attached a patch. Any thoughts?

Best regards,
David
Swarm64