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// Copyright 2016 The LUCI Authors.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
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
// the Context has been written to.
// - Middleware may modify the embedded context before calling next.
type Middleware func(c *Context, next Handler)
// MiddlewareChain is an ordered collection of Middleware.
// MiddlewareChain's zero value is a middleware chain with no Middleware.
// Allocating a MiddlewareChain with Middleware can be done with
// NewMiddlewareChain.
type MiddlewareChain struct {
middleware []Middleware
// RunMiddleware executes the middleware chain and handlers with the given
// initial context. Useful to execute a chain of functions in tests.
func RunMiddleware(c *Context, mc MiddlewareChain, h Handler) {
runChains(c, mc, MiddlewareChain{}, h)
// NewMiddlewareChain creates a new MiddlewareChain with the supplied Middleware
// entries.
func NewMiddlewareChain(mw ...Middleware) (mc MiddlewareChain) {
if len(mw) > 0 {
mc = mc.Extend(mw...)
// Extend returns a new MiddlewareChain with the supplied Middleware appended to
// the end.
func (mc MiddlewareChain) Extend(mw ...Middleware) MiddlewareChain {
if len(mw) == 0 {
return mc
ext := make([]Middleware, 0, len(mc.middleware)+len(mw))
return MiddlewareChain{append(append(ext, mc.middleware...), mw...)}
// ExtendFrom returns a new MiddlewareChain with the supplied MiddlewareChain
// appended to the end.
func (mc MiddlewareChain) ExtendFrom(other MiddlewareChain) MiddlewareChain {
return mc.Extend(other.middleware...)
func runChains(c *Context, mc, nc MiddlewareChain, h Handler) {
run(c, mc.middleware, nc.middleware, h)
// run executes the middleware chains m and n and the handler h using
// c as the initial context. If a middleware or handler is nil, run
// simply advances to the next middleware or handler.
func run(c *Context, m, n []Middleware, h Handler) {
switch {
case len(m) > 0:
if m[0] != nil {
m[0](c, func(ctx *Context) { run(ctx, m[1:], n, h) })
} else {
run(c, m[1:], n, h)
case len(n) > 0:
if n[0] != nil {
n[0](c, func(ctx *Context) { run(ctx, nil, n[1:], h) })
} else {
run(c, nil, n[1:], h)
case h != nil: