common/logger/journald.go

package logger

import (
	"bytes"
	"context"
	"log/slog"
	"slices"

	slogjournal "github.com/systemd/slog-journal"
)

// newJournalHandler returns a slog.Handler that delivers records to the
// systemd journal using the native protocol. It returns nil when stderr
// is not connected to journald, in which case the caller falls back to
// the stderr text handler.
func newJournalHandler(opts *slog.HandlerOptions) slog.Handler {
	if !stderrIsJournal() {
		return nil
	}
	h, err := slogjournal.NewHandler(&slogjournal.Options{
		Level:        opts.Level,
		ReplaceAttr:  journalReplaceAttr,
		ReplaceGroup: sanitizeJournalKey,
	})
	if err != nil {
		return nil
	}
	return &journalMessageHandler{inner: h, level: opts.Level}
}

// journalMessageHandler wraps a slog-journal handler so each record's
// MESSAGE field contains the full logfmt line (level, msg, and all
// attributes) — matching the pre-journal stderr TextHandler output so
// plain `journalctl` remains readable. Attributes are still delivered as
// structured journal fields, so `journalctl -o verbose` or
// `--output-fields=` keep working for filtering.
type journalMessageHandler struct {
	inner slog.Handler
	level slog.Leveler
	ops   []handlerOp
}

type handlerOp struct {
	attrs []slog.Attr // non-nil for WithAttrs
	group string      // non-empty for WithGroup
}

func (h *journalMessageHandler) Enabled(ctx context.Context, level slog.Level) bool {
	return h.inner.Enabled(ctx, level)
}

func (h *journalMessageHandler) Handle(ctx context.Context, r slog.Record) error {
	var buf bytes.Buffer
	var th slog.Handler = slog.NewTextHandler(&buf, &slog.HandlerOptions{
		Level:       h.level,
		ReplaceAttr: logRemoveTime,
	})
	for _, op := range h.ops {
		if op.group != "" {
			th = th.WithGroup(op.group)
		} else {
			th = th.WithAttrs(op.attrs)
		}
	}
	if err := th.Handle(ctx, r); err != nil {
		return err
	}
	msg := bytes.TrimRight(buf.Bytes(), "\n")

	nr := r.Clone()
	nr.Message = string(msg)
	return h.inner.Handle(ctx, nr)
}

func (h *journalMessageHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
	if len(attrs) == 0 {
		return h
	}
	h2 := *h
	h2.ops = append(slices.Clip(h.ops), handlerOp{attrs: attrs})
	h2.inner = h.inner.WithAttrs(attrs)
	return &h2
}

func (h *journalMessageHandler) WithGroup(name string) slog.Handler {
	if name == "" {
		return h
	}
	h2 := *h
	h2.ops = append(slices.Clip(h.ops), handlerOp{group: name})
	h2.inner = h.inner.WithGroup(name)
	return &h2
}

// journalReplaceAttr sanitizes slog attribute keys so they satisfy the
// journal's ^[A-Z_][A-Z0-9_]*$ constraint. Without this, otherwise
// valid attributes from third-party handlers (e.g. the trace_id/span_id
// injected by slogtraceid) are silently dropped by the journal.
func journalReplaceAttr(groups []string, a slog.Attr) slog.Attr {
	a.Key = sanitizeJournalKey(a.Key)
	return a
}

// sanitizeJournalKey maps an arbitrary string to a journal-legal key:
// upper-cases ASCII letters, replaces everything outside [A-Z0-9_] with
// '_', and prefixes '_' if the first byte is a digit.
func sanitizeJournalKey(k string) string {
	if k == "" {
		return k
	}
	b := make([]byte, 0, len(k)+1)
	for i := 0; i < len(k); i++ {
		c := k[i]
		switch {
		case c >= 'A' && c <= 'Z', c == '_':
			b = append(b, c)
		case c >= 'a' && c <= 'z':
			b = append(b, c-'a'+'A')
		case c >= '0' && c <= '9':
			if i == 0 {
				b = append(b, '_')
			}
			b = append(b, c)
		default:
			b = append(b, '_')
		}
	}
	return string(b)
}