Server: Improves systemd support

Closes #499

server/README.md

@@ -220,6 +220,25 @@ brew install oxigraph
 It installs the `oxigraph_server` binary. [See the usage documentation to know how to use it](#usage).
 
 
+## Systemd
+
+It is possible to run Oxigraph in the background using systemd.
+
+For that, you can use the following `oxigraph_server.service` file (it might be inserted into `/etc/systemd/system/` or `$HOME/.config/systemd/user`):
+```ini
+[Unit]
+Description=Oxigraph database server
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=notify
+ExecStart=/PATH/TO/oxigraph_server serve --location /PATH/TO/OXIGRAPH/DATA
+
+[Install]
+WantedBy=multi-user.target
+```
+
 ## Migration guide
 
 ### From 0.2 to 0.3

server/src/main.rs

@@ -17,9 +17,13 @@ use sparesults::{QueryResultsFormat, QueryResultsSerializer};
 use std::borrow::Cow;
 use std::cell::RefCell;
 use std::cmp::{max, min};
+#[cfg(target_os = "linux")]
+use std::env;
 use std::ffi::OsStr;
 use std::fs::File;
 use std::io::{self, stdin, stdout, BufRead, BufReader, BufWriter, Read, Write};
+#[cfg(target_os = "linux")]
+use std::os::unix::net::UnixDatagram;
 use std::path::{Path, PathBuf};
 use std::rc::Rc;
 use std::str::FromStr;
@@ -776,6 +780,8 @@ fn serve(store: Store, bind: String, read_only: bool, cors: bool) -> anyhow::Res
     };
     server.set_global_timeout(HTTP_TIMEOUT);
     server.set_server_name(concat!("Oxigraph/", env!("CARGO_PKG_VERSION")))?;
+    #[cfg(target_os = "linux")]
+    systemd_notify_ready()?;
     eprintln!("Listening for requests at http://{}", &bind);
     server.listen(bind)?;
     Ok(())
@@ -1698,6 +1704,14 @@ impl Write for ReadForWriteWriter {
     }
 }
 
+#[cfg(target_os = "linux")]
+fn systemd_notify_ready() -> io::Result<()> {
+    if let Some(path) = env::var_os("NOTIFY_SOCKET") {
+        UnixDatagram::unbound()?.send_to(b"READY=1", path)?;
+    }
+    Ok(())
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;