handlers.go

// Copyright 2013 The Gorilla Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package handlers

import (
	"bufio"
	"fmt"
	"net"
	"net/http"
	"sort"
	"strings"
)

// MethodHandler is an http.Handler that dispatches to a handler whose key in the
// MethodHandler's map matches the name of the HTTP request's method, eg: GET
//
// If the request's method is OPTIONS and OPTIONS is not a key in the map then
// the handler responds with a status of 200 and sets the Allow header to a
// comma-separated list of available methods.
//
// If the request's method doesn't match any of its keys the handler responds
// with a status of HTTP 405 "Method Not Allowed" and sets the Allow header to a
// comma-separated list of available methods.
type MethodHandler map[string]http.Handler

func (h MethodHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	if handler, ok := h[req.Method]; ok {
		handler.ServeHTTP(w, req)
	} else {
		allow := []string{}
		for k := range h {
			allow = append(allow, k)
		}
		sort.Strings(allow)
		w.Header().Set("Allow", strings.Join(allow, ", "))
		if req.Method == http.MethodOptions {
			w.WriteHeader(http.StatusOK)
		} else {
			http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
		}
	}
}

// responseLogger is wrapper of http.ResponseWriter that keeps track of its HTTP
// status code and body size.
type responseLogger struct {
	w      http.ResponseWriter
	status int
	size   int
}

func (l *responseLogger) Write(b []byte) (int, error) {
	size, err := l.w.Write(b)
	l.size += size
	return size, err
}

func (l *responseLogger) WriteHeader(s int) {
	l.w.WriteHeader(s)
	l.status = s
}

func (l *responseLogger) Status() int {
	return l.status
}

func (l *responseLogger) Size() int {
	return l.size
}

func (l *responseLogger) Hijack() (net.Conn, *bufio.ReadWriter, error) {
	conn, rw, err := l.w.(http.Hijacker).Hijack()
	if err == nil && l.status == 0 {
		// The status will be StatusSwitchingProtocols if there was no error and
		// WriteHeader has not been called yet
		l.status = http.StatusSwitchingProtocols
	}
	return conn, rw, err
}

// isContentType validates the Content-Type header matches the supplied
// contentType. That is, its type and subtype match.
func isContentType(h http.Header, contentType string) bool {
	ct := h.Get("Content-Type")
	if i := strings.IndexRune(ct, ';'); i != -1 {
		ct = ct[0:i]
	}
	return ct == contentType
}

// ContentTypeHandler wraps and returns a http.Handler, validating the request
// content type is compatible with the contentTypes list. It writes a HTTP 415
// error if that fails.
//
// Only PUT, POST, and PATCH requests are considered.
func ContentTypeHandler(h http.Handler, contentTypes ...string) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if !(r.Method == http.MethodPut || r.Method == http.MethodPost || r.Method == http.MethodPatch) {
			h.ServeHTTP(w, r)
			return
		}

		for _, ct := range contentTypes {
			if isContentType(r.Header, ct) {
				h.ServeHTTP(w, r)
				return
			}
		}
		http.Error(w, fmt.Sprintf("Unsupported content type %q; expected one of %q",
			r.Header.Get("Content-Type"),
			contentTypes),
			http.StatusUnsupportedMediaType)
	})
}

const (
	// HTTPMethodOverrideHeader is a commonly used
	// http header to override a request method.
	HTTPMethodOverrideHeader = "X-HTTP-Method-Override"
	// HTTPMethodOverrideFormKey is a commonly used
	// HTML form key to override a request method.
	HTTPMethodOverrideFormKey = "_method"
)

// HTTPMethodOverrideHandler wraps and returns a http.Handler which checks for
// the X-HTTP-Method-Override header or the _method form key, and overrides (if
// valid) request.Method with its value.
//
// This is especially useful for HTTP clients that don't support many http verbs.
// It isn't secure to override e.g a GET to a POST, so only POST requests are
// considered.  Likewise, the override method can only be a "write" method: PUT,
// PATCH or DELETE.
//
// Form method takes precedence over header method.
func HTTPMethodOverrideHandler(h http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if r.Method == http.MethodPost {
			om := r.FormValue(HTTPMethodOverrideFormKey)
			if om == "" {
				om = r.Header.Get(HTTPMethodOverrideHeader)
			}
			if om == http.MethodPut || om == http.MethodPatch || om == http.MethodDelete {
				r.Method = om
			}
		}
		h.ServeHTTP(w, r)
	})
}