Tweak and test for NotFound handler

Apply NotFound whenever the pattern doesn't match; don't apply the
Allowed header convenience behavior in that case.

mux.go

@@ -89,8 +89,14 @@ import (
 // convenience, PatternServeMux will add the Allow header for requests that
 // match a pattern for a method other than the method requested and set the
 // Status to "405 Method Not Allowed".
+//
+// If the NotFound handler is set, then it is used whenever the pattern doesn't
+// match the request path for the current method (and the Allow header is not
+// altered).
 type PatternServeMux struct {
-	// NotFound allows you to register a custom not found handler
+	// NotFound, if set, is used whenever the request doesn't match any
+	// pattern for its method. NotFound should be set before serving any
+	// requests.
 	NotFound http.Handler
 	handlers map[string][]*patHandler
 }
@@ -113,6 +119,11 @@ func (p *PatternServeMux) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
+	if p.NotFound != nil {
+		p.NotFound.ServeHTTP(w, r)
+		return
+	}
+
 	allowed := make([]string, 0, len(p.handlers))
 	for meth, handlers := range p.handlers {
 		if meth == r.Method {
@@ -127,11 +138,7 @@ func (p *PatternServeMux) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 
 	if len(allowed) == 0 {
-		if p.NotFound != nil {
-			p.NotFound.ServeHTTP(w, r)
-		} else {
-			http.NotFound(w, r)
-		}
+		http.NotFound(w, r)
 		return
 	}
 

mux_test.go

@@ -200,6 +200,22 @@ func TestTail(t *testing.T) {
 	}
 }
 
+func TestNotFound(t *testing.T) {
+	p := New()
+	p.NotFound = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(123)
+	})
+	p.Post("/bar", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {}))
+
+	for _, path := range []string{"/foo", "/bar"} {
+		res := httptest.NewRecorder()
+		p.ServeHTTP(res, newRequest("GET", path, nil))
+		if res.Code != 123 {
+			t.Errorf("for path %q: got code %d; want 123", path, res.Code)
+		}
+	}
+}
+
 func newRequest(method, urlStr string, body io.Reader) *http.Request {
 	req, err := http.NewRequest(method, urlStr, body)
 	if err != nil {