feat: add log to journald feature

The systemd-journald is used in most major Linux distros that use systemd.
This enables logging using the systemd-journald native protocol, instead
of just writing to stderr.

src/garage/Cargo.toml

@@ -57,6 +57,7 @@ opentelemetry.workspace = true
 opentelemetry-prometheus = { workspace = true, optional = true }
 opentelemetry-otlp = { workspace = true, optional = true }
 syslog-tracing = { workspace = true, optional = true }
+tracing-journald = { workspace = true, optional = true }
 
 [dev-dependencies]
 garage_api_common.workspace = true
@@ -101,6 +102,8 @@ metrics = [ "garage_api_admin/metrics", "opentelemetry-prometheus" ]
 telemetry-otlp = [ "opentelemetry-otlp" ]
 # Logging to syslog
 syslog = [ "syslog-tracing" ]
+# Logging to journald
+journald = [ "tracing-journald" ]
 
 # NOTE: bundled-libs and system-libs should be treat as mutually exclusive;
 # exactly one of them should be enabled.

src/garage/main.rs

@@ -208,6 +208,43 @@ fn init_logging(opt: &Opt) {
 		}
 	}
 
+	if std::env::var("GARAGE_LOG_TO_JOURNALD")
+		.map(|x| x == "1" || x == "true")
+		.unwrap_or(false)
+	{
+		#[cfg(feature = "journald")]
+		{
+			use tracing_journald::{Priority, PriorityMappings};
+			use tracing_subscriber::layer::SubscriberExt;
+			use tracing_subscriber::util::SubscriberInitExt;
+
+			let registry = tracing_subscriber::registry()
+				.with(tracing_subscriber::fmt::layer().with_writer(std::io::sink))
+				.with(env_filter);
+			match tracing_journald::layer() {
+				Ok(layer) => {
+					registry
+						.with(layer.with_priority_mappings(PriorityMappings {
+							info: Priority::Informational,
+							debug: Priority::Debug,
+							..PriorityMappings::new()
+						}))
+						.init();
+				}
+				Err(e) => {
+					eprintln!("Couldn't connect to journald: {}.", e);
+					std::process::exit(1);
+				}
+			}
+			return;
+		}
+		#[cfg(not(feature = "journald"))]
+		{
+			eprintln!("Journald support is not enabled in this build.");
+			std::process::exit(1);
+		}
+	}
+
 	tracing_subscriber::fmt()
 		.with_writer(std::io::stderr)
 		.with_env_filter(env_filter)