Make HTTP middleware easier to use

server/router/handler.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
+
+// Handler is the type for all request handlers.
+type Handler func(*Context)
+
+// Middleware is a function that accepts a shared context and the next
+// function. Since Middleware is typically part of a chain of functions
+// that handles an HTTP request, it must obey the following rules.
+//
+//     - Middleware must call next if it has not written to the Context
+//       by the end of the function.
+//     - Middleware must not call next if it has written to the Context.
+//     - Middleware must not write to the Context after next is called and
+//       c.Written()==true.
+//     - Middleware may modify the embedded context before calling next.
+type Middleware func(c *Context, next Handler)
+
+// MiddlewareChain is a list of Middleware.
+type MiddlewareChain []Middleware
+
+// Run executes the middleware chain and final handler with the given context as
+// the initial context.
+func (mc MiddlewareChain) Run(c *Context, h Handler) {
+        run(c, mc, nil, h)
+}
+
+// run executes the middleware chains m and n, and the handler h using the
+// Context c as the initial context.
+func run(c *Context, m, n MiddlewareChain, h Handler) {
+        switch {
+        case len(m) > 0:
+                m[0](c, func(ctx *Context) {
+                        run(ctx, m[1:], n, h)
+                })
+        case len(n) > 0:
+                n[0](c, func(ctx *Context) {
+                        run(ctx, nil, n[1:], h)
+                })
+        default:
+                h(c)
+        }
+}