Re: dblink query interruptibility

On Thu, Mar 06, 2025 at 02:57:01PM +0100, Andreas Karlsson wrote:
> 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:

> 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.

> 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

The CREATE DATABASE hang is indeed new in v15. The general dblink missed
interrupt processing (e.g. pg_cancel_backend response delay) is an old bug.

> And now that
> we actually have a user complaint what do you think about backporting the
> fix?

Yes, that seems fine to do. No PGXN module refers to libpq-be-fe-helpers.h so
I'm unconcerned about a compatibility risk from adding it. In the context of
https://github.com/2ndQuadrant/pglogical/pull/454, I did test the functions
against all versions v9.4+.

Commit d3c5f37 used the new functions for postgres_fdw, not just dblink. That
caused message changes detailed in
postgr.es/m/CAHGQGwGpDTXeg8K1oTmDv9nankaKTrCD-XW-tnkzo6%3DE9p5%3Duw%40mail.gmail.com
so I'm inclined to omit postgres_fdw changes in back branches. postgres_fdw
was already interruptible, so the point of making postgres_fdw adopt the
functions was to reduce code duplication.

Overall, in the absence of objections, I will queue a task to back-patch the
non-postgres_fdw portion of commit d3c5f37 to v13-v16.