Shepherd log rotation service

[Ludovic Courtès]

Hello Guix!

I’ve just pushed a simple log rotation service for Shepherd:

  
https://git.savannah.gnu.org/cgit/shepherd.git/commit/?h=devel&id=0484726801c2b5c1b7deecb9a96054c2c510ac71

It rotates log files roughly the same way we’ve been doing since the
70’s, but there’s a couple of advantages compared to what we’re
currently doing in Guix System:

  • No need to repeat the name of log files since shepherd already knows
    them via #:log-file (that’s another reason to avoid
    non-shepherd-managed log files).

  • Rotation is race-free: it’s impossible to lose a line of log while
    the file is being rotated.  This is guaranteed by the “logger”,
    which offers a method to atomically close its log file, rename it,
    and open a new empty log file.

  • The log rotation service is a timer so one can inspect it with ‘herd
    status log-rotation’, trigger it with ‘herd trigger log-rotation’,
    and so on.

At this point the log file of shepherd itself, for instance
/var/log/messages, is not handled; this will have to be fixed.

There aren’t many options to specify how to rotate logs (very few
compared to rottlog!), but I figured we’d rather have something simple
that works well and without surprises; we can always add knobs later if
necessary.

Feedback welcome!

Ludo’.