main: Log to journal if possible

If `luci` is running as a systemd service and the systemd-journal socket
is available, logs will now be sent there instead of to the standard
error stream.  This has the advantage of logging individual fields,
including level/severity, code location, span variables, etc., rather
than a single string.

Cargo.lock

@@ -662,6 +662,7 @@ dependencies = [
  "tokio",
  "toml",
  "tracing",
+ "tracing-journald",
  "tracing-subscriber",
  "url",
 ]
@@ -1540,6 +1541,17 @@ dependencies = [
  "valuable",
 ]
 
+[[package]]
+name = "tracing-journald"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ba316a74e8fc3c3896a850dba2375928a9fa171b085ecddfc7c054d39970f3fd"
+dependencies = [
+ "libc",
+ "tracing-core",
+ "tracing-subscriber",
+]
+
 [[package]]
 name = "tracing-log"
 version = "0.2.0"

Cargo.toml

@@ -19,5 +19,6 @@ thiserror = "1.0.63"
 tokio = { version = "1.39.1", default-features = false, features = ["rt", "macros", "rt-multi-thread", "signal", "process"] }
 toml = "0.8.15"
 tracing = "0.1.40"
+tracing-journald = "0.3.0"
 tracing-subscriber = { version = "0.3.18", features = ["env-filter"] }
 url = { version = "2.5.2", features = ["serde"] }

src/main.rs

@@ -11,16 +11,34 @@ use std::time::Duration;
 use tokio::signal::unix::SignalKind;
 use tokio::sync::Notify;
 use tracing::{debug, error, info};
+use tracing_subscriber::prelude::*;
 
 use config::Configuration;
 
 #[tokio::main]
 async fn main() {
-    tracing_subscriber::fmt()
-        .without_time()
-        .with_env_filter(tracing_subscriber::EnvFilter::from_default_env())
-        .with_writer(std::io::stderr)
-        .init();
+    match tracing_journald::layer() {
+        Ok(l) => {
+            let subscriber = tracing_subscriber::Registry::default()
+                .with(l)
+                .with(tracing_subscriber::EnvFilter::from_default_env());
+            if let Err(e) = tracing::subscriber::set_global_default(subscriber)
+            {
+                eprintln!("Failed to set global tracing subscriber: {}", e);
+            }
+        }
+        Err(e) => {
+            if e.kind() != std::io::ErrorKind::NotFound {
+                eprintln!("Unable to write to journald socket: {}", e);
+            }
+            tracing_subscriber::fmt()
+                .with_env_filter(
+                    tracing_subscriber::EnvFilter::from_default_env(),
+                )
+                .with_writer(std::io::stderr)
+                .init();
+        }
+    };
 
     let mut args = std::env::args_os();
     let config_path = args.nth(1).map(PathBuf::from);