Add a way to set a NotFound HTTP handler.

server/router/router.go

 // Copyright 2016 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 // Package router provides an HTTP router with support for middleware and
 // subrouters. It wraps around julienschmidt/httprouter.
 package router
 
 import (
         "net/http"
@@ skipping 100 matching lines @@
 // GetParams parases the httprouter.Params from the supplied method and path.
 //
 // If nothing is registered for method/path, GetParams will return false.
 func (r *Router) GetParams(method, path string) (httprouter.Params, bool) {
         if h, p, _ := r.hrouter.Lookup(method, path); h != nil {
                 return p, true
         }
         return nil, false
 }
 
+// NotFound sets the handler to be called when no matching route is found.
+func (r *Router) NotFound(mc MiddlewareChain, h Handler) {
+        handle := r.adapt(mc, h)
+        r.hrouter.NotFound = http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
+                handle(rw, req, nil)
+        })
+}
+
 // adapt adapts given middleware chain and handler into a httprouter-style handle.
 func (r *Router) adapt(mc MiddlewareChain, h Handler) httprouter.Handle {
         return httprouter.Handle(func(rw http.ResponseWriter, req *http.Request, p httprouter.Params) {
                 runChains(&Context{
                         Context: context.Background(),
                         Writer:  rw,
                         Request: req,
                         Params:  p,
                 }, r.middleware, mc, h)
         })
 }
 
 // makeBasePath combines the given base and relative path using "/".
 // The result is: "/"+base+"/"+relative. Consecutive "/" are collapsed
 // into a single "/". In addition, the following rules apply:
 //         - The "/" between base and relative exists only if either base has a
 //           trailing "/" or relative is not the empty string.
 //         - A trailing "/" is added to the result if relative has a trailing
 //           "/".
 func makeBasePath(base, relative string) string {
         if !strings.HasSuffix(base, "/") && relative != "" {
                 base += "/"
         }
         return httprouter.CleanPath(base + relative)
 }