log: Customizable default error function

config/setup/log.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/mholt/caddy/middleware"
 	caddylog "github.com/mholt/caddy/middleware/log"
+	"github.com/mholt/caddy/server"
 )
 
 // Log sets up the logging middleware.
@@ -39,7 +40,7 @@ func Log(c *Controller) (middleware.Middleware, error) {
 	})
 
 	return func(next middleware.Handler) middleware.Handler {
-		return caddylog.Logger{Next: next, Rules: rules}
+		return caddylog.Logger{Next: next, Rules: rules, ErrorFunc: server.DefaultErrorFunc}
 	}, nil
 }
 

middleware/log/log.go

@@ -13,6 +13,7 @@ import (
 type Logger struct {
 	Next      middleware.Handler
 	Rules     []Rule
+	ErrorFunc func(http.ResponseWriter, *http.Request, int) // failover error handler
 }
 
 func (l Logger) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error) {
@@ -21,8 +22,15 @@ func (l Logger) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error) {
 			responseRecorder := middleware.NewResponseRecorder(w)
 			status, err := l.Next.ServeHTTP(responseRecorder, r)
 			if status >= 400 {
+				// There was an error up the chain, but no response has been written yet.
+				// The error must be handled here so the log entry will record the response size.
+				if l.ErrorFunc != nil {
+					l.ErrorFunc(responseRecorder, r, status)
+				} else {
+					// Default failover error handler
 					responseRecorder.WriteHeader(status)
 					fmt.Fprintf(responseRecorder, "%d %s", status, http.StatusText(status))
+				}
 				status = 0
 			}
 			rep := middleware.NewReplacer(r, responseRecorder)

server/server.go

@@ -221,11 +221,15 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 		// Fallback error response in case error handling wasn't chained in
 		if status >= 400 {
-			w.WriteHeader(status)
-			fmt.Fprintf(w, "%d %s", status, http.StatusText(status))
+			DefaultErrorFunc(w, r, status)
 		}
 	} else {
 		w.WriteHeader(http.StatusNotFound)
 		fmt.Fprintf(w, "No such host at %s", s.address)
 	}
 }
+
+func DefaultErrorFunc(w http.ResponseWriter, r *http.Request, status int) {
+	w.WriteHeader(status)
+	fmt.Fprintf(w, "%d %s", status, http.StatusText(status))
+}