bug#64317: [Cuirass] Download products can disappear, leading to HTTP 500

[Ludovic Courtès]

Ludovic Courtès <ludovic.courtes@inria.fr> skribis:

> The /download endpoint refers directly to store items.  However, they
> can be GC’d in the meantime, in which case the HTTP handler fails
> gracelessly (HTTP 500 or so) as shown here:
>
> 2023-06-27 16:02:24 GET /download/718
> 2023-06-27 16:02:24 Uncaught exception in task:
> 2023-06-27 16:02:24 In fibers.scm:
> 2023-06-27 16:02:24     172:8  1 (_)
> 2023-06-27 16:02:24 In ice-9/boot-9.scm:
> 2023-06-27 16:02:24   1685:16  0 (raise-exception _ #:continuable? _)
> 2023-06-27 16:02:24 ice-9/boot-9.scm:1685:16: In procedure raise-exception:
> 2023-06-27 16:02:24 In procedure open-file: No such file or directory: 
> "/gnu/store/bnsh1yk4l913af8swqi3x7xp1xsa8gq7-guix-binary.tar.xz"
>
> The solution IMO is to (1) register GC roots for these, (2) periodically
> remove old GC roots, and (3) return 404 when the file has vanished.

Fixed with these Cuirass commits:

  9e897d8 'create-build-outputs' registers a GC root on build products.
  103a6ec http: Gracefully handle missing files in "/download".
  5e3e49c http: "/download" returns 404 rather than 500 for "not found".

Ludo’.