'with' regression tests fails rarely (and spuriously)

Hi,

I've twice seen the below failure when running tests in a loop (to
verify another rare issue in a patch is fixed):

diff -du10 /home/andres/src/postgresql/src/test/regress/expected/with.out /home/andres/build/postgres/dev-assert/vpath/src/test/regress/results/with.out
--- /home/andres/src/postgresql/src/test/regress/expected/with.out      2020-07-21 15:03:11.239754712 -0700
+++ /home/andres/build/postgres/dev-assert/vpath/src/test/regress/results/with.out      2020-07-23 04:25:07.955839299 -0700
@@ -2207,28 +2207,30 @@
                Output: a_1.ctid, a_1.aa
          ->  CTE Scan on wcte
                Output: wcte.*, wcte.q2
    ->  Nested Loop
          Output: a_2.ctid, wcte.*
          Join Filter: (a_2.aa = wcte.q2)
          ->  Seq Scan on public.c a_2
                Output: a_2.ctid, a_2.aa
          ->  CTE Scan on wcte
                Output: wcte.*, wcte.q2
-   ->  Nested Loop
+   ->  Hash Join
          Output: a_3.ctid, wcte.*
-         Join Filter: (a_3.aa = wcte.q2)
+         Hash Cond: (a_3.aa = wcte.q2)
          ->  Seq Scan on public.d a_3
                Output: a_3.ctid, a_3.aa
-         ->  CTE Scan on wcte
+         ->  Hash
                Output: wcte.*, wcte.q2
-(38 rows)
+               ->  CTE Scan on wcte
+                     Output: wcte.*, wcte.q2
+(40 rows)

-- error cases
-- data-modifying WITH tries to use its own output
WITH RECURSIVE t AS (
INSERT INTO y
SELECT * FROM t
)
VALUES(FALSE);
ERROR: recursive query "t" must not contain data-modifying statements
LINE 1: WITH RECURSIVE t AS (

Searching the archives didn't unearth other reports of the same.

This was the first failure after 404 iterations of installcheck, so it's
clearly not a common occurance on my machine.

Greetings,

Andres Freund