Support for systemd socket activation

[atheriel]

On Linux systems it seems sensible to run the daemon as a systemd user service for local reviews, but right now there is an awkward choice between starting the service at login (which is almost certainly too early) or making it completely manual.

I'd suggest adding support for systemd socket activation in the daemon so that it can be configured to start up automatically when there is review work to do. I believe there is fairly good support in the Go ecosystem for detecting when the right systemd-related environment variables are set and converting the passed file descriptor to something you can Listen() on already.

I'm not sure you'd even need a new flag; it would make sense to me that socket activation takes precedence over --server and --addr when detected.

[wesm]

What do you mean by “completely manual”? The daemon does not have to be manually started — it is started the first time a repository fires the post commit hook or the TUI is launched. The daemon has to be running for the cli to talk to (it is responsible for the review database).

EDIT: I see, you want the daemon to be running always as a systemd service (that’s starts on demand). It seems like this should be easily configurable in config.toml. I’ll take a look