Re: recovery test failures on hoverfly

I wrote:
> Michael Paquier <michael@paquier.xyz> writes:
>> This is the same problem as c757a3da and 6d41dd0, where we write a
>> query to a pipe but the kill, causing a failure, makes the test fail
>> with a SIGPIPE in IPC::Run as a query is sent down to a pipe.

> The precedent of the previous fixes would seem to suggest seeing if
> we can replace 'SELECT 1' with "undef".  Not sure if that'll work
> without annoying changes to poll_query_until, though.

I noticed that elver failed this same way today, so that got me
annoyed enough to pursue a fix.  Using "undef" as poll_query_until's
input almost works, except it turns out that it fails to notice psql
connection failures in that case!  It is *only* looking at psql's
stdout, not at either stderr or the exit status, which seems seriously
bogus in its own right; not least because poll_query_until's own
documentation claims it will continue waiting after an error, which
is exactly what it's not doing.  So I propose the attached.

(I first tried to make it check $result == 0, but it seems there are a
lot of cases where psql returns status 1 in these tests.  That seems
pretty bogus too, but probably beta is no time to change that
behavior.)

			regards, tom lane