Hello Guix! I have pushed a ‘wip-fibers’ branch of the Shepherd:
https://git.savannah.gnu.org/cgit/shepherd.git/log/?h=wip-fibers The goal is to make shepherd (the daemon) use Fibers¹ for concurrency. Right now, actions in shepherd are all serialized: starting services, waiting for their PID files, accepting client connections and serving them (when running the ‘herd’ command), etc. This slows down startup, prevents things like running two clients at the same time, or serving a client while waiting for a PID file. In other words, it sucks. Using Fibers, we can introduce concurrency with few changes to the code; we can even do new things writing code that looks sequential. This is what the branch does. To illustrate that, it introduces a new, incredible feature: logging! As you know, shepherd had (euphemism ahead) limited support for logging, essentially in the form of #:log-file, which would redirect a daemon’s stdout/stderr to a file, and also in the form of “let’s hope the daemon talks to syslogd”. Here I was able to trivially add logging, such that anything spawned by ‘fork+exec-command’ is logged, with timestamps and all (the logger is a fiber that calls ‘read-line’ in a loop, which is automagically non-blocking—delightful!). The next fun step (besides logging) will be adding inetd-style and/or systemd-style “socket activation”. Fibers is used in a single-threaded fashion, which is the main constraint for shepherd since it forks. That also means that fibers cannot be preempted, so it’s fully cooperative scheduling. There’s one catch: Fibers is currently Linux-only. The good news is that work has been done to port it to other kernels via libevent². Until it is merged, we could keep using the Shepherd 0.8 on GNU/Hurd. I’ve done some Guix System testing in VMs and didn’t notice any major issues. I’d like to merge that branch in ‘master’ and to eventually release it as 0.9.0 (with or without socket activation, we’ll see.) Hopefully, we could be running it within a couple of weeks. Thoughts? Ludo’. ¹ https://github.com/wingo/fibers ² https://github.com/wingo/fibers/pull/53
