[syslog] Add support to logging to syslog

Original patch by Jakub Jirutka for Alpine Linux port.

src/garage/Cargo.toml

@@ -58,6 +58,7 @@ opentelemetry.workspace = true
 opentelemetry-prometheus = { workspace = true, optional = true }
 opentelemetry-otlp = { workspace = true, optional = true }
 prometheus = { workspace = true, optional = true }
+syslog-tracing = { workspace = true, optional = true }
 
 [dev-dependencies]
 aws-config.workspace = true
@@ -97,6 +98,8 @@ kubernetes-discovery = [ "garage_rpc/kubernetes-discovery" ]
 metrics = [ "garage_api/metrics", "opentelemetry-prometheus", "prometheus" ]
 # Exporter for the OpenTelemetry Collector.
 telemetry-otlp = [ "opentelemetry-otlp" ]
+# Logging to syslog
+syslog = [ "syslog-tracing" ]
 
 # NOTE: bundled-libs and system-libs should be treat as mutually exclusive;
 # exactly one of them should be enabled.

src/garage/main.rs

@@ -140,17 +140,8 @@ async fn main() {
 	let opt = Opt::from_clap(&Opt::clap().version(version.as_str()).get_matches());
 
 	// Initialize logging as well as other libraries used in Garage
-	if std::env::var("RUST_LOG").is_err() {
-		let default_log = match &opt.cmd {
-			Command::Server => "netapp=info,garage=info",
-			_ => "netapp=warn,garage=warn",
-		};
-		std::env::set_var("RUST_LOG", default_log)
-	}
-	tracing_subscriber::fmt()
-		.with_writer(std::io::stderr)
-		.with_env_filter(tracing_subscriber::filter::EnvFilter::from_default_env())
-		.init();
+	init_logging(&opt);
+
 	sodiumoxide::init().expect("Unable to init sodiumoxide");
 
 	let res = match opt.cmd {
@@ -173,6 +164,51 @@ async fn main() {
 	}
 }
 
+fn init_logging(opt: &Opt) {
+	if std::env::var("RUST_LOG").is_err() {
+		let default_log = match &opt.cmd {
+			Command::Server => "netapp=info,garage=info",
+			_ => "netapp=warn,garage=warn",
+		};
+		std::env::set_var("RUST_LOG", default_log)
+	}
+
+	let env_filter = tracing_subscriber::filter::EnvFilter::from_default_env();
+
+	#[cfg(feature = "syslog")]
+	if std::env::var("GARAGE_LOG_TO_SYSLOG")
+		.map(|x| x == "1" || x == "true")
+		.unwrap_or(false)
+	{
+		use std::ffi::CStr;
+		use syslog_tracing::{Facility, Options, Syslog};
+
+		let syslog = Syslog::new(
+			CStr::from_bytes_with_nul(b"garage\0").unwrap(),
+			Options::LOG_PID | Options::LOG_PERROR,
+			Facility::Daemon,
+		)
+		.expect("Unable to init syslog");
+
+		tracing_subscriber::fmt()
+			.with_writer(syslog)
+			.with_env_filter(env_filter)
+			.with_ansi(false) // disable ANSI escape sequences (colours)
+			.with_file(false)
+			.with_level(false)
+			.without_time()
+			.compact()
+			.init();
+
+		return;
+	}
+
+	tracing_subscriber::fmt()
+		.with_writer(std::io::stderr)
+		.with_env_filter(env_filter)
+		.init();
+}
+
 async fn cli_command(opt: Opt) -> Result<(), Error> {
 	let config = if (opt.secrets.rpc_secret.is_none() && opt.secrets.rpc_secret_file.is_none())
 		|| opt.rpc_host.is_none()