Make HTTP middleware easier to use

server/router/router.go

Index: server/router/router.go
diff --git a/server/router/router.go b/server/router/router.go
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+// 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"
+	"strings"
+
+	"github.com/julienschmidt/httprouter"
+	"golang.org/x/net/context"
+)
+
+type (
+	// Router is the main type for the package. To create a Router, use New.
+	Router struct {
+		hrouter    *httprouter.Router
+		middleware MiddlewareChain
+		BasePath   string
+	}
+
+	// Context contains the context, response writer, request, and params shared
+	// across Middleware and Handler functions.
+	Context struct {
+		context.Context
+		writer  http.ResponseWriter
+		Request *http.Request
+		Params  httprouter.Params
+		status  int
+	}
+)
+
+var (
+	_ http.Handler        = (*Router)(nil)
+	_ http.ResponseWriter = (*Context)(nil)
+	_ context.Context     = (*Context)(nil)
+)
+
+// New creates a Router.
+func New() *Router {
+	return &Router{
+		hrouter:  httprouter.New(),
+		BasePath: "/",
+	}
+}
+
+// Use adds middleware chains to the group. The added middleware applies to
+// all handlers registered on the router and to all handlers registered on
+// routers that may be derived from the router (using Subrouter).
+func (r *Router) Use(mc MiddlewareChain) {
+	r.middleware = append(r.middleware, mc...)
+}
+
+// Subrouter creates a new router with an updated base path.
+// The new router copies middleware and configuration from the
+// router it derives from.
+func (r *Router) Subrouter(relativePath string) *Router {
+	newRouter := &Router{
+		hrouter:  r.hrouter,
+		BasePath: makeBasePath(r.BasePath, relativePath),
+	}
+	if len(r.middleware) > 0 {
+		newRouter.middleware = make(MiddlewareChain, len(r.middleware))
+		copy(newRouter.middleware, r.middleware)
+	}
+	return newRouter
+}
+
+// GET is a shortcut for router.Handle("GET", path, mc, h)
+func (r *Router) GET(path string, mc MiddlewareChain, h Handler) {
+	r.Handle("GET", path, mc, h)
+}
+
+// HEAD is a shortcut for router.Handle("HEAD", path, mc, h)
+func (r *Router) HEAD(path string, mc MiddlewareChain, h Handler) {
+	r.Handle("HEAD", path, mc, h)
+}
+
+// OPTIONS is a shortcut for router.Handle("OPTIONS", path, mc, h)
+func (r *Router) OPTIONS(path string, mc MiddlewareChain, h Handler) {
+	r.Handle("OPTIONS", path, mc, h)
+}
+
+// POST is a shortcut for router.Handle("POST", path, mc, h)
+func (r *Router) POST(path string, mc MiddlewareChain, h Handler) {
+	r.Handle("POST", path, mc, h)
+}
+
+// PUT is a shortcut for router.Handle("PUT", path, mc, h)
+func (r *Router) PUT(path string, mc MiddlewareChain, h Handler) {
+	r.Handle("PUT", path, mc, h)
+}
+
+// PATCH is a shortcut for router.Handle("PATCH", path, mc, h)
+func (r *Router) PATCH(path string, mc MiddlewareChain, h Handler) {
+	r.Handle("PATCH", path, mc, h)
+}
+
+// DELETE is a shortcut for router.Handle("DELETE", path, mc, h)
+func (r *Router) DELETE(path string, mc MiddlewareChain, h Handler) {
+	r.Handle("DELETE", path, mc, h)
+}
+
+// Handle registers a middleware chain and a handler for the given method and
+// path. len(mc)==0 is allowed. See https://godoc.org/github.com/julienschmidt/httprouter
+// for documentation on how the path may be formatted.
+func (r *Router) Handle(method, path string, mc MiddlewareChain, h Handler) {
+	handle := r.adapt(mc, h)
+	r.hrouter.Handle(method, httprouter.CleanPath(r.BasePath+path), handle)
+}
+
+// ServeHTTP makes Router implement the http.Handler interface.
+func (r *Router) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
+	r.hrouter.ServeHTTP(rw, req)
+}
+
+// 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) {
+		// TODO(maybe): Use a free list for making Contexts.
+		run(&Context{
+			Context: context.Background(),
+			writer:  rw,
+			Request: req,
+			Params:  p,
+		}, r.middleware, mc, h)
+	})
+}
+
+func makeBasePath(base, relative string) string {
+	path := httprouter.CleanPath(base + relative)
+	if !strings.HasSuffix(path, "/") {
+		path += "/"
+	}
+	return path
+}
+
+// Header is used to implement http.ResponseWriter interface.
+func (c *Context) Header() http.Header {
+	return c.writer.Header()
+}
+
+// Write is used to implement http.ResponseWriter interface.
+// If called from a Middleware function, the rules mentioned in the
+// documentation for Middleware must be followed.
+func (c *Context) Write(p []byte) (int, error) {
+	if !c.Written() {
+		c.status = http.StatusOK
+	}
+	return c.writer.Write(p)
+}
+
+// WriteHeader is used to implement http.ResponseWriter interface.
+// If called from a Middleware function, the rules mentioned in the
+// documentation for Middleware must be followed.
+func (c *Context) WriteHeader(code int) {
+	if !c.Written() {
+		c.status = code
+	}
+	c.writer.WriteHeader(code)
+}
+
+// StatusCode returns the written HTTP status code or 0 if it was not written.
+func (c *Context) StatusCode() int {
+	return c.status
+}
+
+// Written indicates whether a HTTP response has been written.
+func (c *Context) Written() bool {
+	return c.status != 0
+}