/*
Copyright 2014 Google Inc. All rights reserved.

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

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package apiserver

import (
	"bytes"
	"compress/gzip"
	"fmt"
	"io"
	"io/ioutil"
	"net/http"
	"net/http/httputil"
	"net/url"
	"path"
	"strings"
	"time"

	"github.com/GoogleCloudPlatform/kubernetes/pkg/api"
	"github.com/GoogleCloudPlatform/kubernetes/pkg/api/errors"
	"github.com/GoogleCloudPlatform/kubernetes/pkg/httplog"
	"github.com/GoogleCloudPlatform/kubernetes/pkg/runtime"
	"github.com/GoogleCloudPlatform/kubernetes/pkg/util"

	"github.com/golang/glog"
	"golang.org/x/net/html"
)

// tagsToAttrs states which attributes of which tags require URL substitution.
// Sources: http://www.w3.org/TR/REC-html40/index/attributes.html
//          http://www.w3.org/html/wg/drafts/html/master/index.html#attributes-1
var tagsToAttrs = map[string]util.StringSet{
	"a":          util.NewStringSet("href"),
	"applet":     util.NewStringSet("codebase"),
	"area":       util.NewStringSet("href"),
	"audio":      util.NewStringSet("src"),
	"base":       util.NewStringSet("href"),
	"blockquote": util.NewStringSet("cite"),
	"body":       util.NewStringSet("background"),
	"button":     util.NewStringSet("formaction"),
	"command":    util.NewStringSet("icon"),
	"del":        util.NewStringSet("cite"),
	"embed":      util.NewStringSet("src"),
	"form":       util.NewStringSet("action"),
	"frame":      util.NewStringSet("longdesc", "src"),
	"head":       util.NewStringSet("profile"),
	"html":       util.NewStringSet("manifest"),
	"iframe":     util.NewStringSet("longdesc", "src"),
	"img":        util.NewStringSet("longdesc", "src", "usemap"),
	"input":      util.NewStringSet("src", "usemap", "formaction"),
	"ins":        util.NewStringSet("cite"),
	"link":       util.NewStringSet("href"),
	"object":     util.NewStringSet("classid", "codebase", "data", "usemap"),
	"q":          util.NewStringSet("cite"),
	"script":     util.NewStringSet("src"),
	"source":     util.NewStringSet("src"),
	"video":      util.NewStringSet("poster", "src"),

	// TODO: css URLs hidden in style elements.
}

// ProxyHandler provides a http.Handler which will proxy traffic to locations
// specified by items implementing Redirector.
type ProxyHandler struct {
	prefix  string
	storage map[string]RESTStorage
	codec   runtime.Codec
}

func (r *ProxyHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	namespace, kind, parts, err := KindAndNamespace(req)
	if err != nil {
		notFound(w, req)
		return
	}
	ctx := api.WithNamespace(api.NewContext(), namespace)
	if len(parts) < 2 {
		notFound(w, req)
		return
	}
	id := parts[1]
	rest := ""
	if len(parts) > 2 {
		proxyParts := parts[2:]
		rest = strings.Join(proxyParts, "/")
		if strings.HasSuffix(req.URL.Path, "/") {
			// The original path had a trailing slash, which has been stripped
			// by KindAndNamespace(). We should add it back because some
			// servers (like etcd) require it.
			rest = rest + "/"
		}
	}
	storage, ok := r.storage[kind]
	if !ok {
		httplog.LogOf(req, w).Addf("'%v' has no storage object", kind)
		notFound(w, req)
		return
	}

	redirector, ok := storage.(Redirector)
	if !ok {
		httplog.LogOf(req, w).Addf("'%v' is not a redirector", kind)
		errorJSON(errors.NewMethodNotSupported(kind, "proxy"), r.codec, w)
		return
	}

	location, err := redirector.ResourceLocation(ctx, id)
	if err != nil {
		httplog.LogOf(req, w).Addf("Error getting ResourceLocation: %v", err)
		status := errToAPIStatus(err)
		writeJSON(status.Code, r.codec, status, w)
		return
	}
	if location == "" {
		httplog.LogOf(req, w).Addf("ResourceLocation for %v returned ''", id)
		notFound(w, req)
		return
	}

	destURL, err := url.Parse(location)
	if err != nil {
		status := errToAPIStatus(err)
		writeJSON(status.Code, r.codec, status, w)
		return
	}
	if destURL.Scheme == "" {
		// If no scheme was present in location, url.Parse sometimes mistakes
		// hosts for paths.
		destURL.Host = location
	}
	destURL.Path = rest
	destURL.RawQuery = req.URL.RawQuery
	newReq, err := http.NewRequest(req.Method, destURL.String(), req.Body)
	if err != nil {
		status := errToAPIStatus(err)
		writeJSON(status.Code, r.codec, status, w)
		notFound(w, req)
		return
	}
	newReq.Header = req.Header

	proxy := httputil.NewSingleHostReverseProxy(&url.URL{Scheme: "http", Host: destURL.Host})
	proxy.Transport = &proxyTransport{
		proxyScheme:      req.URL.Scheme,
		proxyHost:        req.URL.Host,
		proxyPathPrepend: path.Join(r.prefix, "ns", namespace, kind, id),
	}
	proxy.FlushInterval = 200 * time.Millisecond
	proxy.ServeHTTP(w, newReq)
}

type proxyTransport struct {
	proxyScheme      string
	proxyHost        string
	proxyPathPrepend string
}

func (t *proxyTransport) RoundTrip(req *http.Request) (*http.Response, error) {
	// Add reverse proxy headers.
	req.Header.Set("X-Forwarded-Uri", t.proxyPathPrepend+req.URL.Path)
	req.Header.Set("X-Forwarded-Host", t.proxyHost)
	req.Header.Set("X-Forwarded-Proto", t.proxyScheme)

	resp, err := http.DefaultTransport.RoundTrip(req)

	if err != nil {
		message := fmt.Sprintf("Error: '%s'\nTrying to reach: '%v'", err.Error(), req.URL.String())
		resp = &http.Response{
			StatusCode: http.StatusServiceUnavailable,
			Body:       ioutil.NopCloser(strings.NewReader(message)),
		}
		return resp, nil
	}

	cType := resp.Header.Get("Content-Type")
	cType = strings.TrimSpace(strings.SplitN(cType, ";", 2)[0])
	if cType != "text/html" {
		// Do nothing, simply pass through
		return resp, nil
	}

	return t.fixLinks(req, resp)
}

// updateURLs checks and updates any of n's attributes that are listed in tagsToAttrs.
// Any URLs found are, if they're relative, updated with the necessary changes to make
// a visit to that URL also go through the proxy.
// sourceURL is the URL of the page which we're currently on; it's required to make
// relative links work.
func (t *proxyTransport) updateURLs(n *html.Node, sourceURL *url.URL) {
	if n.Type != html.ElementNode {
		return
	}
	attrs, ok := tagsToAttrs[n.Data]
	if !ok {
		return
	}
	for i, attr := range n.Attr {
		if !attrs.Has(attr.Key) {
			continue
		}
		url, err := url.Parse(attr.Val)
		if err != nil {
			continue
		}

		// Is this URL referring to the same host as sourceURL?
		if url.Host == "" || url.Host == sourceURL.Host {
			url.Scheme = t.proxyScheme
			url.Host = t.proxyHost
			origPath := url.Path

			if strings.HasPrefix(url.Path, "/") {
				// The path is rooted at the host. Just add proxy prepend.
				url.Path = path.Join(t.proxyPathPrepend, url.Path)
			} else {
				// The path is relative to sourceURL.
				url.Path = path.Join(t.proxyPathPrepend, path.Dir(sourceURL.Path), url.Path)
			}

			if strings.HasSuffix(origPath, "/") {
				// Add back the trailing slash, which was stripped by path.Join().
				url.Path += "/"
			}

			n.Attr[i].Val = url.String()
		}
	}
}

// scan recursively calls f for every n and every subnode of n.
func (t *proxyTransport) scan(n *html.Node, f func(*html.Node)) {
	f(n)
	for c := n.FirstChild; c != nil; c = c.NextSibling {
		t.scan(c, f)
	}
}

// fixLinks modifies links in an HTML file such that they will be redirected through the proxy if needed.
func (t *proxyTransport) fixLinks(req *http.Request, resp *http.Response) (*http.Response, error) {
	origBody := resp.Body
	defer origBody.Close()

	newContent := &bytes.Buffer{}
	var reader io.Reader = origBody
	var writer io.Writer = newContent
	encoding := resp.Header.Get("Content-Encoding")
	switch encoding {
	case "gzip":
		var err error
		reader, err = gzip.NewReader(reader)
		if err != nil {
			return nil, fmt.Errorf("errorf making gzip reader: %v", err)
		}
		gzw := gzip.NewWriter(writer)
		defer gzw.Close()
		writer = gzw
	// TODO: support flate, other encodings.
	case "":
		// This is fine
	default:
		// Some encoding we don't understand-- don't try to parse this
		glog.Errorf("Proxy encountered encoding %v for text/html; can't understand this so not fixing links.", encoding)
		return resp, nil
	}

	doc, err := html.Parse(reader)
	if err != nil {
		glog.Errorf("Parse failed: %v", err)
		return resp, err
	}

	t.scan(doc, func(n *html.Node) { t.updateURLs(n, req.URL) })
	if err := html.Render(writer, doc); err != nil {
		glog.Errorf("Failed to render: %v", err)
	}

	resp.Body = ioutil.NopCloser(newContent)
	// Update header node with new content-length
	// TODO: Remove any hash/signature headers here?
	resp.Header.Del("Content-Length")
	resp.ContentLength = int64(newContent.Len())

	return resp, err
}