Re: dblink query interruptibility

On 11/22/23 2:29 AM, Noah Misch wrote:
> Something as simple as the following doesn't respond to cancellation.  In
> v15+, any DROP DATABASE will hang as long as it's running:

Hi,

One of our customers ran into this bug when upgrading from PostgreSQL 14 
to PostgreSQL 16. Your commit[1] fixed this issue in PostgreSQL 17 but 
the bugfix was not backported with the explanation below.

 > Code inspection identified the bug at least thirteen years ago, but 
user complaints have not appeared. Hence, no back-patch for now.

But that is as far as I can tell not the case because at least for 
CREATE DATABASE the bug was introduced in a commit[2] in PostgeSQL 15. 
And now that we actually have a user complaint what do you think about 
backporting the fix?

The patch seems small and relatively safe to backport and the functions 
have had no bugfixes as far as I could see. And I did a quick git 
cherry-pick myself on top of PG 16 (see attached patch) and the only 
conflict was related to the introduction of custom wait events which was 
easy to fix.

Here is a small snippet which reproduces the bug. It hangs reliably on 
PostgreSQL 15 and 16, but not 14, 17 or HEAD.

CREATE EXTENSION dblink;

CREATE FUNCTION f(text, int, text) RETURNS VOID LANGUAGE plpgsql AS $$
BEGIN
     PERFORM dblink_connect(format('host=%s port=%s user=%s 
dbname=postgres', $1, $2, $3));
     RAISE NOTICE 'dblink connected!';
     PERFORM dblink_exec('DROP DATABASE test');
     RAISE NOTICE 'Database dropped!';
     PERFORM dblink_disconnect();
     RAISE NOTICE 'dblink disconnected!';
END
$$;

CREATE DATABASE test;
SELECT f(:'HOST', :PORT, :'USER');


Andreas

1. 
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=d3c5f37dd543498cc7c678815d3921823beec9e9
2. 
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=e2f65f42555ff531c6d7c8f151526b4ef7c016f8