ロギングとリクエストIDのミドルウェア

func RequestID(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        id := r.Header.Get("X-Request-ID")
        if id == "" { id = uuid.New().String() }
        w.Header().Set("X-Request-ID", id)
        ctx := context.WithValue(r.Context(), reqIDKey{}, id)
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

func StructuredLogger(logger *slog.Logger) func(http.Handler) http.Handler {
    return func(next http.Handler) http.Handler {
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            start := time.Now()
            cw := &captureWriter{ResponseWriter: w}
            next.ServeHTTP(cw, r)
            logger.Info("request",
                "method", r.Method,
                "path", r.URL.Path,
                "status", cw.status,
                "latency_ms", time.Since(start).Milliseconds(),
                "request_id", r.Context().Value(reqIDKey{}),
            )
        })
    }
}

type captureWriter struct {
    http.ResponseWriter
    status int
}
func (cw *captureWriter) WriteHeader(code int) {
    cw.status = code
    cw.ResponseWriter.WriteHeader(code)
}
func (cw *captureWriter) Status() int {
    if cw.status == 0 { return http.StatusOK }
    return cw.status
}

reqLogger := logger.With("request_id", id)
ctx := context.WithValue(r.Context(), loggerKey{}, reqLogger)

func LoggerFromContext(ctx context.Context) *slog.Logger {
    l, ok := ctx.Value(loggerKey{}).(*slog.Logger)
    if !ok { return slog.Default() }
    return l
}