‘guix publish’ now compresses archives

[ludo@gnu.org (Ludovic Courtès)]

Hi!

‘guix publish --compression’ can now compress archives in gzip format,
using zlib, which makes it much more usable in real life.  See commit
4a1fc562ae5eedf40f6ae4eabe30580b0983b8f6.

‘guix publish’ is cache-friendly: it uses /nar/gzip URLs for gzipped
archives, and /nar for uncompressed archives.  That way, even if ‘guix
publish’ is restarted with different compression parameters, previously
announced /nar/* URLs remain valid.

Please try it and report back.

To recap, the difficulty with all this is that we couldn’t just call out
the ‘gzip’ command because ‘guix publish’ uses threads, and threads and
fork(2) don’t go together well.  So we needed zlib bindings, and when
discussing it earlier with Dave, I thought we’d have to wrap zlib’s
low-level API, which is painful, so we were sad and all.  Next we had
this hydra.gnu.org outage, which entailed more sadness.

Then I realized that zlib has this high-level ‘gz’ API, which is easy
and does what we need; hence, (guix zlib) provides bindings to that.
The only limitation is that it needs a file descriptor as its source or
sink and cannot compress into memory.  That’s enough for ‘guix publish’
though.

Comments welcome!

Ludo’.