feat(server): add syslog priority prefixes for systemd-journald (#5192)

* fix: add syslog priority prefixes for systemd-journald

When running under systemd, all log messages were assigned priority 3
(error) by journald because navidrome wrote plain text to stderr without
syslog priority prefixes.

Add a journalFormatter that wraps the existing logrus formatter and
prepends <N> syslog priority prefixes (RFC 5424) to each log line.
The formatter is automatically enabled when the JOURNAL_STREAM
environment variable is set (indicating the process is managed by
systemd).

Priority mapping:
- Fatal/Panic → <2>/<0> (crit/emerg)
- Error → <3> (err)
- Warn → <4> (warning)
- Info → <6> (info)
- Debug/Trace → <7> (debug)

Fixes #5142

* test: refactor journalFormatter tests to use Ginkgo and DescribeTable

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>
Co-authored-by: Deluan <deluan@navidrome.org>

conf/configuration.go

@@ -340,6 +340,10 @@ func Load(noConfigDump bool) {
 			os.Exit(1)
 		}
 		log.SetOutput(out)
+	} else if os.Getenv("JOURNAL_STREAM") != "" {
+		// When running under systemd, prepend syslog priority prefixes so
+		// journald assigns the correct severity to each log line.
+		log.EnableJournalFormat()
 	}
 
 	log.SetLevelString(Server.LogLevel)

log/journal.go

@@ -0,0 +1,41 @@
+package log
+
+import (
+	"fmt"
+
+	"github.com/sirupsen/logrus"
+)
+
+// journalFormatter wraps a logrus.Formatter and prepends a syslog priority
+// prefix (<N>) to each log line. When stderr is captured by systemd-journald,
+// this prefix tells journald the correct severity for each message.
+//
+// See https://www.freedesktop.org/software/systemd/man/sd-daemon.html
+type journalFormatter struct {
+	inner logrus.Formatter
+}
+
+// levelToPriority maps logrus levels to syslog priority values.
+// The mapping follows RFC 5424 severity levels.
+var levelToPriority = map[logrus.Level]int{
+	logrus.PanicLevel: 0, // emerg
+	logrus.FatalLevel: 2, // crit
+	logrus.ErrorLevel: 3, // err
+	logrus.WarnLevel:  4, // warning
+	logrus.InfoLevel:  6, // info
+	logrus.DebugLevel: 7, // debug
+	logrus.TraceLevel: 7, // debug
+}
+
+func (f *journalFormatter) Format(entry *logrus.Entry) ([]byte, error) {
+	formatted, err := f.inner.Format(entry)
+	if err != nil {
+		return formatted, err
+	}
+	priority, ok := levelToPriority[entry.Level]
+	if !ok {
+		priority = 6 // default to info for unknown levels
+	}
+	prefix := []byte(fmt.Sprintf("<%d>", priority))
+	return append(prefix, formatted...), nil
+}

log/journal_test.go

@@ -0,0 +1,41 @@
+package log
+
+import (
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+	"github.com/sirupsen/logrus"
+)
+
+var _ = Describe("journalFormatter", func() {
+	var formatter *journalFormatter
+
+	BeforeEach(func() {
+		inner := &logrus.TextFormatter{
+			DisableTimestamp: true,
+			DisableColors:    true,
+		}
+		formatter = &journalFormatter{inner: inner}
+	})
+
+	DescribeTable("prefixes log lines with syslog priority",
+		func(level logrus.Level, expectedPrefix string) {
+			entry := &logrus.Entry{
+				Logger:  logrus.New(),
+				Level:   level,
+				Message: "test message",
+				Data:    logrus.Fields{},
+			}
+			out, err := formatter.Format(entry)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(string(out)).To(HavePrefix(expectedPrefix))
+		},
+		Entry("error", logrus.ErrorLevel, "<3>"),
+		Entry("warning", logrus.WarnLevel, "<4>"),
+		Entry("info", logrus.InfoLevel, "<6>"),
+		Entry("debug", logrus.DebugLevel, "<7>"),
+		Entry("trace", logrus.TraceLevel, "<7>"),
+		Entry("fatal", logrus.FatalLevel, "<2>"),
+		Entry("panic", logrus.PanicLevel, "<0>"),
+		Entry("unknown level defaults to info", logrus.Level(99), "<6>"),
+	)
+})

log/log.go

@@ -145,6 +145,15 @@ func SetOutput(w io.Writer) {
 	defaultLogger.SetOutput(w)
 }
 
+// EnableJournalFormat wraps the current logger formatter with syslog
+// priority prefixes for systemd-journald. Only call this when output
+// goes to stderr and JOURNAL_STREAM is set.
+func EnableJournalFormat() {
+	loggerMu.Lock()
+	defer loggerMu.Unlock()
+	defaultLogger.Formatter = &journalFormatter{inner: defaultLogger.Formatter}
+}
+
 // Redact applies redaction to a single string
 func Redact(msg string) string {
 	r, _ := redacted.redact(msg)