use sd-notify to signal readyness

Provide a way to signal nsncd has successfully started up, and is ready
to accept connections.

A more reliable way would be to only signal this once at least one
worker has started up, but this is good enough (tm) for now.

This can be used in a systemd.service file with Type=notify.

Cargo.toml

@@ -23,6 +23,7 @@ crossbeam-channel = "^0.5"
 nix = "^0.21.2"
 num-derive = "^0.3"
 num-traits = "^0.2"
+sd-notify = "0.4.1"
 
 [dev-dependencies]
 criterion = "^0.3"

nsncd.service

@@ -18,6 +18,7 @@ Description=name-service non-caching daemon
 [Service]
 ExecStart=/usr/lib/nsncd
 Restart=always
+Type=notify
 
 [Install]
 WantedBy=multi-user.target

src/main.rs

@@ -54,6 +54,7 @@ use std::time::Duration;
 
 use anyhow::{Context, Result};
 use crossbeam_channel as channel;
+use sd_notify::NotifyState;
 use slog::{debug, error, o, Drain};
 
 mod ffi;
@@ -83,7 +84,6 @@ fn main() -> Result<()> {
         "worker_count" => worker_count,
         "handoff_timeout" => ?handoff_timeout,
     );
-
     let mut wg = WorkGroup::new();
     let tx = spawn_workers(&mut wg, &logger, worker_count);
 
@@ -93,6 +93,8 @@ fn main() -> Result<()> {
     std::fs::set_permissions(path, std::fs::Permissions::from_mode(0o777))?;
     spawn_acceptor(&mut wg, &logger, listener, tx, handoff_timeout);
 
+    let _ = sd_notify::notify(true, &[NotifyState::Ready]);
+
     let (result, handles) = wg.run();
     if let Err(e) = result {
         // if a thread unwound with a panic, just start panicing here. the nss