Re: Assertion failure in HEAD and 13 after calling COMMIT in a stored proc

I wrote:
> Hmm.  I think the real issue here is that commit 84f5c2908 did
> not cover the "simple expression" code path in plpgsql.  We
> need to re-establish an outer snapshot when the next thing
> that happens after COMMIT is a simple expression, too.

The attached seems to be enough to resolve Jim's example.  I'd like
to invent a test case that involves a detoast of the simple
expression's result, too, to show that transiently pushing a
snapshot for the duration of the expression is not the right fix.

            regards, tom lane

diff --git a/src/pl/plpgsql/src/pl_exec.c b/src/pl/plpgsql/src/pl_exec.c
index 0ce382e123..96bb77e0b1 100644
--- a/src/pl/plpgsql/src/pl_exec.c
+++ b/src/pl/plpgsql/src/pl_exec.c
@@ -38,6 +38,7 @@
 #include "plpgsql.h"
 #include "storage/proc.h"
 #include "tcop/cmdtag.h"
+#include "tcop/pquery.h"
 #include "tcop/tcopprot.h"
 #include "tcop/utility.h"
 #include "utils/array.h"
@@ -5958,6 +5959,15 @@ exec_eval_simple_expr(PLpgSQL_execstate *estate,
         expr->expr_simple_lxid == curlxid)
         return false;
 
+    /*
+     * Ensure that there's a portal-level snapshot, in case this simple
+     * expression is the first thing evaluated after a COMMIT or ROLLBACK.
+     * We'd have to do this anyway before executing the expression, so we
+     * might as well do it now to ensure that any possible replanning doesn't
+     * need to take a new snapshot.
+     */
+    EnsurePortalSnapshotExists();
+
     /*
      * Check to see if the cached plan has been invalidated.  If not, and this
      * is the first use in the current transaction, save a plan refcount in