An assertion failure (server crash in assert-enabled builds) occurs when WAIT FOR LSN ... INTO is used inside PL/pgSQL DO blocks or within void procedures.

Repro:

-- Run this on a standby

CREATE PROCEDURE test_wait()
  LANGUAGE plpgsql AS $$
  DECLARE
    result text;
  BEGIN
    WAIT FOR LSN '0/1234' INTO result;
    RAISE NOTICE '%', result;
  END;
  $$;
  CALL test_wait();

The WAIT FOR itself succeeds, but the very next PL/pgSQL statement that requires a snapshot crashes the backend with:

  TRAP: failed Assert("portal->portalSnapshot == NULL"),
  File: "pquery.c", Line: 1776

Attached patches for both the test case and a potential fix. Please review.

Thanks,

Satya