Subject: [bug#56045] [PATCH] Back up and restore PostgreSQL databases with Shepherd

From: Ludovic Courtès <[email-masked]>
Date: Wed, 22 Jun 2022 22:46:36 +0200

Hello!

Marius Bakke <[email-masked]> skribis:

> The attached patch adds backup and restore mechanisms to the PostgreSQL
> Shepherd service.  It looks like this (here with a db named 'mreg'):
>
> $ sudo herd backup postgres mreg
> $ sudo -u postgres psql -c 'drop database mreg' # whoops ...
> DROP DATABASE
> $ sudo herd list-backups postgres mreg
> mreg@2022-06-16_21-55-07
> mreg@2022-06-16_22-48-59
> $ sudo herd restore postgres mreg@2022-06-16_22-48-59
> $ sudo -u postgres psql mreg
> mreg=#
>
> Pretty cool, no?  :-)

Indeed!  :-)

> With this patch you can 'herd backup' each database, stop postgres,
> _delete_ /var/lib/postgresql/data, reconfigure with a newer version, and
> 'herd restore' them again -- but you'll lose any role passwords (and
> roles not declared by postgresql-role-service-type).
>
> Not sure what to about roles, maybe a backup-roles command?

No idea, we need input from PG practitioners!

> From edc8a2e5ae3c89b78fb837d4351f0ddfab8fe474 Mon Sep 17 00:00:00 2001
> From: Marius Bakke <[email-masked]>
> Date: Thu, 16 Jun 2022 22:46:01 +0200
> Subject: [PATCH] services: Shepherd can backup and restore PostgreSQL
>  databases.
>
> * gnu/services/databases.scm (<postgresql-configuration>)[backup-directory]:
> New field.
> (postgresql-activation): Create it.
> (postgresql-backup-action, postgresql-list-backups-action,
> postgresql-restore-action): New variables.
> (postgresql-shepherd-service)[actions]: Register them.
> * gnu/tests/databases.scm (%postgresql-backup-directory): New variable.
> (run-postgresql-test): Trim unused module imports from existing tests.  Add
> "insert test data", "backup database", "list backups", "drop database",
> "restore database", "update test data", "restore again", and "verify restore"
> tests.

Not being a database person, I’ll comment on the code:

>    (match-lambda
>      (($ <postgresql-configuration> postgresql port locale config-file
> -                                   log-directory data-directory
> +                                   log-directory data-directory backup-directory
>                                     extension-packages)

Time to use ‘match-record’!

> +(define (postgresql-backup-action postgresql backup-directory)

Please add a docstring (and on other top-level procedures).

> +   (procedure
> +    #~(lambda* (pid #:optional database #:rest rest)
> +        (use-modules (guix build utils)
> +                     (ice-9 match)
> +                     (srfi srfi-19))

Non-top-level ‘use-modules’ should be avoided; it’s not really supposed
to work.  If you have these three modules in the ‘modules’ field of the
parent <shepherd-service> record, that’s enough (I know, it’s not pretty).

> +              ;; Fork so we can drop privileges.
> +              (match (primitive-fork)
> +                (0
> +                 ;; Exit with a non-zero status code if an exception is thrown.
> +                 (dynamic-wind
> +                   (const #t)
> +                   (lambda ()
> +                     (setgid (passwd:gid user))
> +                     (setuid (passwd:uid user))
> +                     (umask #o027)
> +                     (format (current-output-port)
> +                             "postgres: creating backup ~a.~%"
> +                             (basename file-name))
> +                     (mkdir-p (dirname file-name))
> +                     (let* ((result (apply system* pg_dump database
> +                                           "-f" file-name
> +                                           options))
> +                            (exit-value (status:exit-val result)))

Would it work to use ‘fork+exec-command’ to do all this?  It’d be great
if we could avoid the boilerplate.

> +(define (postgresql-list-backups-action backup-directory)

Docstring.  :-)

> +              (match (primitive-fork)
> +                (0
> +                 (dynamic-wind
> +                   (const #t)
> +                   (lambda ()
> +                     (setgid (passwd:gid user))
> +                     (setuid (passwd:uid user))
> +                     (let* ((backup-file (string-append #$backup-directory
> +                                                        "/" file))
> +                            (database (match (string-split file #\@)
> +                                        ((name date) name)))
> +                            (create? (not (database-exists? database)))
> +                            (options (list "--clean" "--if-exists"
> +                                           (if create?
> +                                               "--create"
> +                                               "--single-transaction"))))
> +                       (format (current-output-port)
> +                               "postgres: restoring ~a.~%" file)
> +                       (let* ((result (apply system* pg_restore backup-file
> +                                             "-d" (if create? "postgres" database)
> +                                             options))

Same here: ‘fork+exec-command’?

Overall I find it nice and convenient, but I wonder how far we should go
with our services.  After all, it’s just one way to make backups, there
are probably other ways, so should we have this particular method
hardwired?

Thanks,
Ludo’.