// Copyright 2017 The Go Authors. All rights reserved.
//
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file or at
// https://developers.google.com/open-source/licenses/bsd.

// Command gddo-server is the GoPkgDoc server.
package main

import (
	"bytes"
	"context"
	"crypto/md5"
	"encoding/json"
	"errors"
	"fmt"
	"go/build"
	"html/template"
	"io"
	"log"
	"net/http"
	"net/url"
	"os"
	"path"
	"regexp"
	"runtime/debug"
	"sort"
	"strconv"
	"strings"
	"time"

	"cloud.google.com/go/logging"
	"cloud.google.com/go/pubsub"
	"cloud.google.com/go/trace"
	"github.com/spf13/viper"

	"github.com/golang/gddo/database"
	"github.com/golang/gddo/doc"
	"github.com/golang/gddo/gosrc"
	"github.com/golang/gddo/httputil"
	"github.com/golang/gddo/internal/health"
)

const (
	jsonMIMEType = "application/json; charset=utf-8"
	textMIMEType = "text/plain; charset=utf-8"
	htmlMIMEType = "text/html; charset=utf-8"
)

var errUpdateTimeout = errors.New("refresh timeout")

type httpError struct {
	status int   // HTTP status code.
	err    error // Optional reason for the HTTP error.
}

func (err *httpError) Error() string {
	if err.err != nil {
		return fmt.Sprintf("status %d, reason %s", err.status, err.err.Error())
	}
	return fmt.Sprintf("Status %d", err.status)
}

const (
	humanRequest = iota
	robotRequest
	queryRequest
	refreshRequest
	apiRequest
)

type crawlResult struct {
	pdoc *doc.Package
	err  error
}

// getDoc gets the package documentation from the database or from the version
// control system as needed.
func (s *server) getDoc(ctx context.Context, path string, requestType int) (*doc.Package, []database.Package, error) {
	if path == "-" {
		// A hack in the database package uses the path "-" to represent the
		// next document to crawl. Block "-" here so that requests to /- always
		// return not found.
		return nil, nil, &httpError{status: http.StatusNotFound}
	}

	pdoc, pkgs, nextCrawl, err := s.db.Get(ctx, path)
	if err != nil {
		return nil, nil, err
	}

	needsCrawl := false
	switch requestType {
	case queryRequest, apiRequest:
		needsCrawl = nextCrawl.IsZero() && len(pkgs) == 0
	case humanRequest:
		needsCrawl = nextCrawl.Before(time.Now())
	case robotRequest:
		needsCrawl = nextCrawl.IsZero() && len(pkgs) > 0
	}

	if !needsCrawl {
		return pdoc, pkgs, nil
	}

	c := make(chan crawlResult, 1)
	go func() {
		pdoc, err := s.crawlDoc(ctx, "web  ", path, pdoc, len(pkgs) > 0, nextCrawl)
		c <- crawlResult{pdoc, err}
	}()

	timeout := s.v.GetDuration(ConfigGetTimeout)
	if pdoc == nil {
		timeout = s.v.GetDuration(ConfigFirstGetTimeout)
	}

	select {
	case cr := <-c:
		err = cr.err
		if err == nil {
			pdoc = cr.pdoc
		}
	case <-time.After(timeout):
		err = errUpdateTimeout
	}

	switch {
	case err == nil:
		return pdoc, pkgs, nil
	case gosrc.IsNotFound(err):
		return nil, nil, err
	case pdoc != nil:
		log.Printf("Serving %q from database after error getting doc: %v", path, err)
		return pdoc, pkgs, nil
	case err == errUpdateTimeout:
		log.Printf("Serving %q as not found after timeout getting doc", path)
		return nil, nil, &httpError{status: http.StatusNotFound}
	default:
		return nil, nil, err
	}
}

func templateExt(req *http.Request) string {
	if httputil.NegotiateContentType(req, []string{"text/html", "text/plain"}, "text/html") == "text/plain" {
		return ".txt"
	}
	return ".html"
}

var robotPat = regexp.MustCompile(`(:?\+https?://)|(?:\Wbot\W)|(?:^Python-urllib)|(?:^Go )|(?:^Java/)`)

func (s *server) isRobot(req *http.Request) bool {
	if robotPat.MatchString(req.Header.Get("User-Agent")) {
		return true
	}
	host := httputil.StripPort(req.RemoteAddr)
	n, err := s.db.IncrementCounter(host, 1)
	if err != nil {
		log.Printf("error incrementing counter for %s, %v", host, err)
		return false
	}
	if n > s.v.GetFloat64(ConfigRobotThreshold) {
		log.Printf("robot %.2f %s %s", n, host, req.Header.Get("User-Agent"))
		return true
	}
	return false
}

func popularLinkReferral(req *http.Request) bool {
	return strings.HasSuffix(req.Header.Get("Referer"), "//"+req.Host+"/")
}

func isView(req *http.Request, key string) bool {
	rq := req.URL.RawQuery
	return strings.HasPrefix(rq, key) &&
		(len(rq) == len(key) || rq[len(key)] == '=' || rq[len(key)] == '&')
}

// httpEtag returns the package entity tag used in HTTP transactions.
func (s *server) httpEtag(pdoc *doc.Package, pkgs []database.Package, importerCount int, flashMessages []flashMessage) string {
	b := make([]byte, 0, 128)
	b = strconv.AppendInt(b, pdoc.Updated.Unix(), 16)
	b = append(b, 0)
	b = append(b, pdoc.Etag...)
	if importerCount >= 8 {
		importerCount = 8
	}
	b = append(b, 0)
	b = strconv.AppendInt(b, int64(importerCount), 16)
	for _, pkg := range pkgs {
		b = append(b, 0)
		b = append(b, pkg.Path...)
		b = append(b, 0)
		b = append(b, pkg.Synopsis...)
	}
	if s.v.GetBool(ConfigSidebar) {
		b = append(b, "\000xsb"...)
	}
	for _, m := range flashMessages {
		b = append(b, 0)
		b = append(b, m.ID...)
		for _, a := range m.Args {
			b = append(b, 1)
			b = append(b, a...)
		}
	}
	h := md5.New()
	h.Write(b)
	b = h.Sum(b[:0])
	return fmt.Sprintf("\"%x\"", b)
}

func (s *server) servePackage(resp http.ResponseWriter, req *http.Request) error {
	p := path.Clean(req.URL.Path)
	if strings.HasPrefix(p, "/pkg/") {
		p = p[len("/pkg"):]
	}
	if p != req.URL.Path {
		http.Redirect(resp, req, p, http.StatusMovedPermanently)
		return nil
	}

	if isView(req, "status.svg") {
		s.statusSVG.ServeHTTP(resp, req)
		return nil
	}

	if isView(req, "status.png") {
		s.statusPNG.ServeHTTP(resp, req)
		return nil
	}

	requestType := humanRequest
	if s.isRobot(req) {
		requestType = robotRequest
	}

	importPath := strings.TrimPrefix(req.URL.Path, "/")
	pdoc, pkgs, err := s.getDoc(req.Context(), importPath, requestType)

	if e, ok := err.(gosrc.NotFoundError); ok && e.Redirect != "" {
		// To prevent dumb clients from following redirect loops, respond with
		// status 404 if the target document is not found.
		if _, _, err := s.getDoc(req.Context(), e.Redirect, requestType); gosrc.IsNotFound(err) {
			return &httpError{status: http.StatusNotFound}
		}
		u := "/" + e.Redirect
		if req.URL.RawQuery != "" {
			u += "?" + req.URL.RawQuery
		}
		setFlashMessages(resp, []flashMessage{{ID: "redir", Args: []string{importPath}}})
		http.Redirect(resp, req, u, http.StatusFound)
		return nil
	}
	if err != nil {
		return err
	}

	flashMessages := getFlashMessages(resp, req)

	if pdoc == nil {
		if len(pkgs) == 0 {
			return &httpError{status: http.StatusNotFound}
		}
		pdocChild, _, _, err := s.db.Get(req.Context(), pkgs[0].Path)
		if err != nil {
			return err
		}
		pdoc = &doc.Package{
			ProjectName: pdocChild.ProjectName,
			ProjectRoot: pdocChild.ProjectRoot,
			ProjectURL:  pdocChild.ProjectURL,
			ImportPath:  importPath,
		}
	}

	showPkgGoDevRedirectToast := userReturningFromPkgGoDev(req)

	switch {
	case isView(req, "imports"):
		if pdoc.Name == "" {
			return &httpError{status: http.StatusNotFound}
		}
		pkgs, err = s.db.Packages(pdoc.Imports)
		if err != nil {
			return err
		}
		return s.templates.execute(resp, "imports.html", http.StatusOK, nil, map[string]interface{}{
			"flashMessages":             flashMessages,
			"pkgs":                      pkgs,
			"pdoc":                      newTDoc(s.v, pdoc),
			"showPkgGoDevRedirectToast": showPkgGoDevRedirectToast,
		})
	case isView(req, "tools"):
		proto := "http"
		if req.Host == "godoc.org" {
			proto = "https"
		}
		return s.templates.execute(resp, "tools.html", http.StatusOK, nil, map[string]interface{}{
			"flashMessages":             flashMessages,
			"uri":                       fmt.Sprintf("%s://%s/%s", proto, req.Host, importPath),
			"pdoc":                      newTDoc(s.v, pdoc),
			"showPkgGoDevRedirectToast": showPkgGoDevRedirectToast,
		})
	case isView(req, "importers"):
		if pdoc.Name == "" {
			return &httpError{status: http.StatusNotFound}
		}
		pkgs, err = s.db.Importers(importPath)
		if err != nil {
			return err
		}
		template := "importers.html"
		if requestType == robotRequest {
			// Hide back links from robots.
			template = "importers_robot.html"
		}
		return s.templates.execute(resp, template, http.StatusOK, nil, map[string]interface{}{
			"flashMessages":             flashMessages,
			"pkgs":                      pkgs,
			"pdoc":                      newTDoc(s.v, pdoc),
			"showPkgGoDevRedirectToast": showPkgGoDevRedirectToast,
		})
	case isView(req, "import-graph"):
		if requestType == robotRequest {
			return &httpError{status: http.StatusForbidden}
		}
		if pdoc.Name == "" {
			return &httpError{status: http.StatusNotFound}
		}

		// Throttle ?import-graph requests.
		select {
		case s.importGraphSem <- struct{}{}:
		default:
			return &httpError{status: http.StatusTooManyRequests}
		}
		defer func() { <-s.importGraphSem }()

		hide := database.ShowAllDeps
		switch req.Form.Get("hide") {
		case "1":
			hide = database.HideStandardDeps
		case "2":
			hide = database.HideStandardAll
		}
		pkgs, edges, err := s.db.ImportGraph(pdoc, hide)
		if err != nil {
			return err
		}
		b, err := renderGraph(pdoc, pkgs, edges)
		if err != nil {
			return err
		}
		return s.templates.execute(resp, "graph.html", http.StatusOK, nil, map[string]interface{}{
			"flashMessages":             flashMessages,
			"svg":                       template.HTML(b),
			"pdoc":                      newTDoc(s.v, pdoc),
			"hide":                      hide,
			"showPkgGoDevRedirectToast": showPkgGoDevRedirectToast,
		})
	case isView(req, "play"):
		u, err := s.playURL(pdoc, req.Form.Get("play"), req.Header.Get("X-AppEngine-Country"))
		if err != nil {
			return err
		}
		http.Redirect(resp, req, u, http.StatusMovedPermanently)
		return nil
	case req.Form.Get("view") != "":
		// Redirect deprecated view= queries.
		var q string
		switch view := req.Form.Get("view"); view {
		case "imports", "importers":
			q = view
		case "import-graph":
			if req.Form.Get("hide") == "1" {
				q = "import-graph&hide=1"
			} else {
				q = "import-graph"
			}
		}
		if q != "" {
			u := *req.URL
			u.RawQuery = q
			http.Redirect(resp, req, u.String(), http.StatusMovedPermanently)
			return nil
		}
		return &httpError{status: http.StatusNotFound}
	default:
		importerCount := 0
		if pdoc.Name != "" {
			importerCount, err = s.db.ImporterCount(importPath)
			if err != nil {
				return err
			}
		}

		etag := s.httpEtag(pdoc, pkgs, importerCount, flashMessages)
		status := http.StatusOK
		if req.Header.Get("If-None-Match") == etag {
			status = http.StatusNotModified
		}

		if requestType == humanRequest &&
			pdoc.Name != "" && // not a directory
			pdoc.ProjectRoot != "" && // not a standard package
			!pdoc.IsCmd &&
			len(pdoc.Errors) == 0 &&
			!popularLinkReferral(req) {
			if err := s.db.IncrementPopularScore(pdoc.ImportPath); err != nil {
				log.Printf("ERROR db.IncrementPopularScore(%s): %v", pdoc.ImportPath, err)
			}
		}
		if s.gceLogger != nil {
			s.gceLogger.LogEvent(resp, req, nil)
		}

		template := "dir"
		switch {
		case pdoc.IsCmd:
			template = "cmd"
		case pdoc.Name != "":
			template = "pkg"
		}
		template += templateExt(req)

		return s.templates.execute(resp, template, status, http.Header{"Etag": {etag}}, map[string]interface{}{
			"flashMessages":             flashMessages,
			"pkgs":                      pkgs,
			"pdoc":                      newTDoc(s.v, pdoc),
			"importerCount":             importerCount,
			"showPkgGoDevRedirectToast": showPkgGoDevRedirectToast,
		})
	}
}

func (s *server) serveRefresh(resp http.ResponseWriter, req *http.Request) error {
	importPath := req.Form.Get("path")
	_, pkgs, _, err := s.db.Get(req.Context(), importPath)
	if err != nil {
		return err
	}
	c := make(chan error, 1)
	go func() {
		_, err := s.crawlDoc(req.Context(), "rfrsh", importPath, nil, len(pkgs) > 0, time.Time{})
		c <- err
	}()
	select {
	case err = <-c:
	case <-time.After(s.v.GetDuration(ConfigGetTimeout)):
		err = errUpdateTimeout
	}
	if e, ok := err.(gosrc.NotFoundError); ok && e.Redirect != "" {
		setFlashMessages(resp, []flashMessage{{ID: "redir", Args: []string{importPath}}})
		importPath = e.Redirect
		err = nil
	} else if err != nil {
		setFlashMessages(resp, []flashMessage{{ID: "refresh", Args: []string{errorText(err)}}})
	}
	http.Redirect(resp, req, "/"+importPath, http.StatusFound)
	return nil
}

func (s *server) serveGoIndex(resp http.ResponseWriter, req *http.Request) error {
	pkgs, err := s.db.GoIndex()
	if err != nil {
		return err
	}
	return s.templates.execute(resp, "std.html", http.StatusOK, nil, map[string]interface{}{
		"pkgs": pkgs,
	})
}

func (s *server) serveGoSubrepoIndex(resp http.ResponseWriter, req *http.Request) error {
	pkgs, err := s.db.GoSubrepoIndex()
	if err != nil {
		return err
	}
	return s.templates.execute(resp, "subrepo.html", http.StatusOK, nil, map[string]interface{}{
		"pkgs": pkgs,
	})
}

type byPath struct {
	pkgs []database.Package
	rank []int
}

func (bp *byPath) Len() int           { return len(bp.pkgs) }
func (bp *byPath) Less(i, j int) bool { return bp.pkgs[i].Path < bp.pkgs[j].Path }
func (bp *byPath) Swap(i, j int) {
	bp.pkgs[i], bp.pkgs[j] = bp.pkgs[j], bp.pkgs[i]
	bp.rank[i], bp.rank[j] = bp.rank[j], bp.rank[i]
}

type byRank struct {
	pkgs []database.Package
	rank []int
}

func (br *byRank) Len() int           { return len(br.pkgs) }
func (br *byRank) Less(i, j int) bool { return br.rank[i] < br.rank[j] }
func (br *byRank) Swap(i, j int) {
	br.pkgs[i], br.pkgs[j] = br.pkgs[j], br.pkgs[i]
	br.rank[i], br.rank[j] = br.rank[j], br.rank[i]
}

func (s *server) popular() ([]database.Package, error) {
	const n = 25

	pkgs, err := s.db.Popular(2 * n)
	if err != nil {
		return nil, err
	}

	rank := make([]int, len(pkgs))
	for i := range pkgs {
		rank[i] = i
	}

	sort.Sort(&byPath{pkgs, rank})

	j := 0
	prev := "."
	for i, pkg := range pkgs {
		if strings.HasPrefix(pkg.Path, prev) {
			if rank[j-1] < rank[i] {
				rank[j-1] = rank[i]
			}
			continue
		}
		prev = pkg.Path + "/"
		pkgs[j] = pkg
		rank[j] = rank[i]
		j++
	}
	pkgs = pkgs[:j]

	sort.Sort(&byRank{pkgs, rank})

	if len(pkgs) > n {
		pkgs = pkgs[:n]
	}

	sort.Sort(&byPath{pkgs, rank})

	return pkgs, nil
}

func (s *server) serveHome(resp http.ResponseWriter, req *http.Request) error {
	if req.URL.Path != "/" {
		return s.servePackage(resp, req)
	}

	q := strings.TrimSpace(req.Form.Get("q"))
	if q == "" {
		pkgs, err := s.popular()
		if err != nil {
			return err
		}

		return s.templates.execute(resp, "home"+templateExt(req), http.StatusOK, nil,
			map[string]interface{}{
				"Popular": pkgs,

				"showPkgGoDevRedirectToast": userReturningFromPkgGoDev(req),
			})
	}

	if path, ok := isBrowseURL(q); ok {
		q = path
	}

	if gosrc.IsValidRemotePath(q) || (strings.Contains(q, "/") && gosrc.IsGoRepoPath(q)) {
		pdoc, pkgs, err := s.getDoc(req.Context(), q, queryRequest)
		if e, ok := err.(gosrc.NotFoundError); ok && e.Redirect != "" {
			http.Redirect(resp, req, "/"+e.Redirect, http.StatusFound)
			return nil
		}
		if err == nil && (pdoc != nil || len(pkgs) > 0) {
			http.Redirect(resp, req, "/"+q, http.StatusFound)
			return nil
		}
	}

	pkgs, err := s.db.Search(req.Context(), q)
	if err != nil {
		return err
	}
	if s.gceLogger != nil {
		// Log up to top 10 packages we served upon a search.
		logPkgs := pkgs
		if len(pkgs) > 10 {
			logPkgs = pkgs[:10]
		}
		s.gceLogger.LogEvent(resp, req, logPkgs)
	}

	return s.templates.execute(resp, "results"+templateExt(req), http.StatusOK, nil,
		map[string]interface{}{
			"q":    q,
			"pkgs": pkgs,

			"showPkgGoDevRedirectToast": userReturningFromPkgGoDev(req),
		})
}

func (s *server) serveAbout(resp http.ResponseWriter, req *http.Request) error {
	return s.templates.execute(resp, "about.html", http.StatusOK, nil,
		map[string]interface{}{
			"Host": req.Host,

			"showPkgGoDevRedirectToast": userReturningFromPkgGoDev(req),
		})
}

func (s *server) serveBot(resp http.ResponseWriter, req *http.Request) error {
	return s.templates.execute(resp, "bot.html", http.StatusOK, nil, nil)
}

func logError(req *http.Request, err error, rv interface{}) {
	if err != nil {
		var buf bytes.Buffer
		fmt.Fprintf(&buf, "Error serving %s: %v\n", req.URL, err)
		if rv != nil {
			fmt.Fprintln(&buf, rv)
			buf.Write(debug.Stack())
		}
		log.Print(buf.String())
	}
}

func (s *server) serveAPISearch(resp http.ResponseWriter, req *http.Request) error {
	q := strings.TrimSpace(req.Form.Get("q"))

	var pkgs []database.Package

	if gosrc.IsValidRemotePath(q) || (strings.Contains(q, "/") && gosrc.IsGoRepoPath(q)) {
		pdoc, _, err := s.getDoc(req.Context(), q, apiRequest)
		if e, ok := err.(gosrc.NotFoundError); ok && e.Redirect != "" {
			pdoc, _, err = s.getDoc(req.Context(), e.Redirect, robotRequest)
		}
		if err == nil && pdoc != nil {
			pkgs = []database.Package{{Path: pdoc.ImportPath, Synopsis: pdoc.Synopsis}}
		}
	}

	if pkgs == nil {
		var err error
		pkgs, err = s.db.Search(req.Context(), q)
		if err != nil {
			return err
		}
	}

	var data = struct {
		Results []database.Package `json:"results"`
	}{
		pkgs,
	}
	resp.Header().Set("Content-Type", jsonMIMEType)
	return json.NewEncoder(resp).Encode(&data)
}

func (s *server) serveAPIPackages(resp http.ResponseWriter, req *http.Request) error {
	pkgs, err := s.db.AllPackages()
	if err != nil {
		return err
	}
	data := struct {
		Results []database.Package `json:"results"`
	}{
		pkgs,
	}
	resp.Header().Set("Content-Type", jsonMIMEType)
	return json.NewEncoder(resp).Encode(&data)
}

func (s *server) serveAPIImporters(resp http.ResponseWriter, req *http.Request) error {
	importPath := strings.TrimPrefix(req.URL.Path, "/importers/")
	pkgs, err := s.db.Importers(importPath)
	if err != nil {
		return err
	}
	data := struct {
		Results []database.Package `json:"results"`
	}{
		pkgs,
	}
	resp.Header().Set("Content-Type", jsonMIMEType)
	return json.NewEncoder(resp).Encode(&data)
}

func (s *server) serveAPIImports(resp http.ResponseWriter, req *http.Request) error {
	importPath := strings.TrimPrefix(req.URL.Path, "/imports/")
	pdoc, _, err := s.getDoc(req.Context(), importPath, robotRequest)
	if err != nil {
		return err
	}
	if pdoc == nil || pdoc.Name == "" {
		return &httpError{status: http.StatusNotFound}
	}
	imports, err := s.db.Packages(pdoc.Imports)
	if err != nil {
		return err
	}
	testImports, err := s.db.Packages(pdoc.TestImports)
	if err != nil {
		return err
	}
	data := struct {
		Imports     []database.Package `json:"imports"`
		TestImports []database.Package `json:"testImports"`
	}{
		imports,
		testImports,
	}
	resp.Header().Set("Content-Type", jsonMIMEType)
	return json.NewEncoder(resp).Encode(&data)
}

func serveAPIHome(resp http.ResponseWriter, req *http.Request) error {
	return &httpError{status: http.StatusNotFound}
}

type requestCleaner struct {
	h                 http.Handler
	trustProxyHeaders bool
}

func (rc requestCleaner) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	req2 := new(http.Request)
	*req2 = *req
	if rc.trustProxyHeaders {
		if s := req.Header.Get("X-Forwarded-For"); s != "" {
			req2.RemoteAddr = s
		}
	}
	req2.Body = http.MaxBytesReader(w, req.Body, 2048)
	req2.ParseForm()
	rc.h.ServeHTTP(w, req2)
}

type errorHandler struct {
	fn    func(resp http.ResponseWriter, req *http.Request) error
	errFn httputil.Error
}

func (eh errorHandler) ServeHTTP(resp http.ResponseWriter, req *http.Request) {
	defer func() {
		if rv := recover(); rv != nil {
			err := errors.New("handler panic")
			logError(req, err, rv)
			eh.errFn(resp, req, http.StatusInternalServerError, err)
		}
	}()

	rb := new(httputil.ResponseBuffer)
	err := eh.fn(rb, req)
	if err == nil {
		rb.WriteTo(resp)
	} else if e, ok := err.(*httpError); ok {
		if e.status >= 500 {
			logError(req, err, nil)
		}
		eh.errFn(resp, req, e.status, e.err)
	} else if gosrc.IsNotFound(err) {
		eh.errFn(resp, req, http.StatusNotFound, nil)
	} else {
		logError(req, err, nil)
		eh.errFn(resp, req, http.StatusInternalServerError, err)
	}
}

func errorText(err error) string {
	if err == errUpdateTimeout {
		return "Timeout getting package files from the version control system."
	}
	if e, ok := err.(*gosrc.RemoteError); ok {
		return "Error getting package files from " + e.Host + "."
	}
	return "Internal server error."
}

func (s *server) handleError(resp http.ResponseWriter, req *http.Request, status int, err error) {
	switch status {
	case http.StatusNotFound:
		s.templates.execute(resp, "notfound"+templateExt(req), status, nil, map[string]interface{}{
			"flashMessages": getFlashMessages(resp, req),
		})
	default:
		resp.Header().Set("Content-Type", textMIMEType)
		resp.WriteHeader(http.StatusInternalServerError)
		io.WriteString(resp, errorText(err))
	}
}

func handleAPIError(resp http.ResponseWriter, req *http.Request, status int, err error) {
	var data struct {
		Error struct {
			Message string `json:"message"`
		} `json:"error"`
	}
	data.Error.Message = http.StatusText(status)
	resp.Header().Set("Content-Type", jsonMIMEType)
	resp.WriteHeader(status)
	json.NewEncoder(resp).Encode(&data)
}

// httpsRedirectHandler redirects all requests with an X-Forwarded-Proto: http
// handler to their https equivalent.
type httpsRedirectHandler struct {
	h http.Handler
}

func (h httpsRedirectHandler) ServeHTTP(resp http.ResponseWriter, req *http.Request) {
	if req.Header.Get("X-Forwarded-Proto") == "http" {
		u := *req.URL
		u.Scheme = "https"
		u.Host = req.Host
		http.Redirect(resp, req, u.String(), http.StatusFound)
		return
	}
	h.h.ServeHTTP(resp, req)
}

type rootHandler []struct {
	prefix string
	h      http.Handler
}

func (m rootHandler) ServeHTTP(resp http.ResponseWriter, req *http.Request) {
	var h http.Handler
	for _, ph := range m {
		if strings.HasPrefix(req.Host, ph.prefix) {
			h = ph.h
			break
		}
	}

	h.ServeHTTP(resp, req)
}

// otherDomainHandler redirects to another domain keeping the rest of the URL.
type otherDomainHandler struct {
	scheme       string
	targetDomain string
}

func (h otherDomainHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	u := *req.URL
	u.Scheme = h.scheme
	u.Host = h.targetDomain
	http.Redirect(w, req, u.String(), http.StatusFound)
}

func defaultBase(path string) string {
	p, err := build.Default.Import(path, "", build.FindOnly)
	if err != nil {
		return "."
	}
	return p.Dir
}

type server struct {
	v           *viper.Viper
	db          *database.Database
	httpClient  *http.Client
	gceLogger   *GCELogger
	templates   templateMap
	traceClient *trace.Client
	crawlTopic  *pubsub.Topic

	statusPNG http.Handler
	statusSVG http.Handler

	root rootHandler

	// A semaphore to limit concurrent ?import-graph requests.
	importGraphSem chan struct{}
}

func newServer(ctx context.Context, v *viper.Viper) (*server, error) {
	s := &server{
		v:              v,
		httpClient:     newHTTPClient(v),
		importGraphSem: make(chan struct{}, 10),
	}

	var err error
	if proj := s.v.GetString(ConfigProject); proj != "" {
		if s.traceClient, err = trace.NewClient(ctx, proj); err != nil {
			return nil, err
		}
		sp, err := trace.NewLimitedSampler(s.v.GetFloat64(ConfigTraceSamplerFraction), s.v.GetFloat64(ConfigTraceSamplerMaxQPS))
		if err != nil {
			return nil, err
		}
		s.traceClient.SetSamplingPolicy(sp)

		// This topic should be created in the cloud console.
		ps, err := pubsub.NewClient(ctx, proj)
		if err != nil {
			return nil, err
		}
		s.crawlTopic = ps.Topic(ConfigCrawlPubSubTopic)
	}

	assets := v.GetString(ConfigAssetsDir)
	staticServer := httputil.StaticServer{
		Dir:    assets,
		MaxAge: time.Hour,
		MIMETypes: map[string]string{
			".css": "text/css; charset=utf-8",
			".js":  "text/javascript; charset=utf-8",
		},
	}
	s.statusPNG = staticServer.FileHandler("status.png")
	s.statusSVG = staticServer.FileHandler("status.svg")

	apiHandler := func(f func(http.ResponseWriter, *http.Request) error) http.Handler {
		return requestCleaner{
			h: errorHandler{
				fn:    f,
				errFn: handleAPIError,
			},
			trustProxyHeaders: v.GetBool(ConfigTrustProxyHeaders),
		}
	}
	apiMux := http.NewServeMux()
	apiMux.Handle("/favicon.ico", staticServer.FileHandler("favicon.ico"))
	apiMux.Handle("/google3d2f3cd4cc2bb44b.html", staticServer.FileHandler("google3d2f3cd4cc2bb44b.html"))
	apiMux.Handle("/humans.txt", staticServer.FileHandler("humans.txt"))
	apiMux.Handle("/robots.txt", staticServer.FileHandler("apiRobots.txt"))
	apiMux.Handle("/search", apiHandler(s.serveAPISearch))
	apiMux.Handle("/packages", apiHandler(s.serveAPIPackages))
	apiMux.Handle("/importers/", apiHandler(s.serveAPIImporters))
	apiMux.Handle("/imports/", apiHandler(s.serveAPIImports))
	apiMux.Handle("/", apiHandler(serveAPIHome))

	mux := http.NewServeMux()
	mux.Handle("/-/site.js", staticServer.FilesHandler(
		"third_party/jquery.timeago.js",
		"site.js"))
	mux.Handle("/-/site.css", staticServer.FilesHandler("site.css"))
	mux.Handle("/-/bootstrap.min.css", staticServer.FilesHandler("bootstrap.min.css"))
	mux.Handle("/-/bootstrap.min.js", staticServer.FilesHandler("bootstrap.min.js"))
	mux.Handle("/-/jquery-2.0.3.min.js", staticServer.FilesHandler("jquery-2.0.3.min.js"))
	if s.v.GetBool(ConfigSidebar) {
		mux.Handle("/-/sidebar.css", staticServer.FilesHandler("sidebar.css"))
	}
	mux.Handle("/-/", http.NotFoundHandler())

	handler := func(f func(http.ResponseWriter, *http.Request) error) http.Handler {
		return requestCleaner{
			h: errorHandler{
				fn:    f,
				errFn: s.handleError,
			},
			trustProxyHeaders: v.GetBool(ConfigTrustProxyHeaders),
		}
	}

	mux.Handle("/-/about", handler(pkgGoDevRedirectHandler(s.serveAbout)))
	mux.Handle("/-/bot", handler(s.serveBot))
	mux.Handle("/-/go", handler(pkgGoDevRedirectHandler(s.serveGoIndex)))
	mux.Handle("/-/subrepo", handler(s.serveGoSubrepoIndex))
	mux.Handle("/-/refresh", handler(s.serveRefresh))
	mux.Handle("/about", http.RedirectHandler("/-/about", http.StatusMovedPermanently))
	mux.Handle("/favicon.ico", staticServer.FileHandler("favicon.ico"))
	mux.Handle("/google3d2f3cd4cc2bb44b.html", staticServer.FileHandler("google3d2f3cd4cc2bb44b.html"))
	mux.Handle("/humans.txt", staticServer.FileHandler("humans.txt"))
	mux.Handle("/robots.txt", staticServer.FileHandler("robots.txt"))
	mux.Handle("/BingSiteAuth.xml", staticServer.FileHandler("BingSiteAuth.xml"))
	mux.Handle("/C", http.RedirectHandler("http://golang.org/doc/articles/c_go_cgo.html", http.StatusMovedPermanently))
	mux.Handle("/code.jquery.com/", http.NotFoundHandler())
	mux.Handle("/", handler(pkgGoDevRedirectHandler(s.serveHome)))

	ahMux := http.NewServeMux()
	ready := new(health.Handler)
	ahMux.HandleFunc("/_ah/health", health.HandleLive)
	ahMux.Handle("/_ah/ready", ready)

	mainMux := http.NewServeMux()
	mainMux.Handle("/_ah/", ahMux)
	mainMux.Handle("/", s.traceClient.HTTPHandler(mux))

	s.root = rootHandler{
		{"api.", httpsRedirectHandler{s.traceClient.HTTPHandler(apiMux)}},
		{"talks.godoc.org", otherDomainHandler{"https", "go-talks.appspot.com"}},
		{"", httpsRedirectHandler{mainMux}},
	}

	cacheBusters := &httputil.CacheBusters{Handler: mux}
	s.templates, err = parseTemplates(assets, cacheBusters, v)
	if err != nil {
		return nil, err
	}
	s.db, err = database.New(
		v.GetString(ConfigDBServer),
		v.GetDuration(ConfigDBIdleTimeout),
		v.GetBool(ConfigDBLog),
		v.GetString(ConfigGAERemoteAPI),
	)
	if err != nil {
		return nil, fmt.Errorf("open database: %v", err)
	}
	ready.Add(s.db)
	if gceLogName := v.GetString(ConfigGCELogName); gceLogName != "" {
		logc, err := logging.NewClient(ctx, v.GetString(ConfigProject))
		if err != nil {
			return nil, fmt.Errorf("create cloud logging client: %v", err)
		}
		logger := logc.Logger(gceLogName)
		if err := logc.Ping(ctx); err != nil {
			return nil, fmt.Errorf("pinging cloud logging: %v", err)
		}
		s.gceLogger = newGCELogger(logger)
	}
	return s, nil
}

const (
	pkgGoDevRedirectCookie = "pkggodev-redirect"
	pkgGoDevRedirectParam  = "redirect"
	pkgGoDevRedirectOn     = "on"
	pkgGoDevRedirectOff    = "off"
	pkgGoDevHost           = "pkg.go.dev"
	teeproxyHost           = "teeproxy-dot-go-discovery.appspot.com"
)

func (s *server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	start := time.Now()
	s.logRequestStart(r)
	s.root.ServeHTTP(w, r)
	latency := time.Since(start)
	s.logRequestEnd(r, latency)
	if f, ok := w.(http.Flusher); ok {
		f.Flush()
	}
	// Don't tee App Engine requests to pkg.go.dev.
	if strings.HasPrefix(r.URL.Path, "/_ah/") {
		return
	}
	if err := makePkgGoDevRequest(r, latency, s.isRobot(r)); err != nil {
		log.Printf("makePkgGoDevRequest(%q, %d) error: %v", r.URL, latency, err)
	}
}

func makePkgGoDevRequest(r *http.Request, latency time.Duration, isRobot bool) error {
	event := newGDDOEvent(r, latency, isRobot)
	b, err := json.Marshal(event)
	if err != nil {
		return fmt.Errorf("json.Marshal(%v): %v", event, err)
	}

	teeproxyURL := url.URL{Scheme: "https", Host: teeproxyHost}
	if _, err := http.Post(teeproxyURL.String(), jsonMIMEType, bytes.NewReader(b)); err != nil {
		return fmt.Errorf("http.Post(%q, %q, %v): %v", teeproxyURL.String(), jsonMIMEType, event, err)
	}
	log.Printf("makePkgGoDevRequest: request made to %q for %+v", teeproxyURL.String(), event)
	return nil
}

type gddoEvent struct {
	Host         string
	Path         string
	URL          string
	Header       http.Header
	RedirectHost string
	Latency      time.Duration
	IsRobot      bool
	UsePkgGoDev  bool
}

func newGDDOEvent(r *http.Request, latency time.Duration, isRobot bool) *gddoEvent {
	targetURL := url.URL{
		Scheme:   "https",
		Host:     r.URL.Host,
		Path:     r.URL.Path,
		RawQuery: r.URL.RawQuery,
	}
	if targetURL.Host == "" && r.Host != "" {
		targetURL.Host = r.Host
	}
	pkgGoDevURL := url.URL{Scheme: "https", Host: pkgGoDevHost}
	return &gddoEvent{
		Host:         targetURL.Host,
		Path:         r.URL.Path,
		URL:          targetURL.String(),
		Header:       r.Header,
		RedirectHost: pkgGoDevURL.String(),
		Latency:      latency,
		IsRobot:      isRobot,
		UsePkgGoDev:  shouldRedirectToPkgGoDev(r),
	}
}

func (s *server) logRequestStart(req *http.Request) {
	if s.gceLogger == nil {
		return
	}
	s.gceLogger.Log(logging.Entry{
		HTTPRequest: &logging.HTTPRequest{Request: req},
		Payload:     fmt.Sprintf("%s request start", req.Host),
		Severity:    logging.Info,
	})
}

func (s *server) logRequestEnd(req *http.Request, latency time.Duration) {
	if s.gceLogger == nil {
		return
	}
	s.gceLogger.Log(logging.Entry{
		HTTPRequest: &logging.HTTPRequest{
			Request: req,
			Latency: latency,
		},
		Payload:  fmt.Sprintf("%s request end", req.Host),
		Severity: logging.Info,
	})
}

func userReturningFromPkgGoDev(req *http.Request) bool {
	return req.FormValue("utm_source") == "backtogodoc"
}

func shouldRedirectToPkgGoDev(req *http.Request) bool {
	// API requests are not redirected.
	if strings.HasPrefix(req.URL.Host, "api") {
		return false
	}
	redirectParam := req.FormValue(pkgGoDevRedirectParam)
	if redirectParam == pkgGoDevRedirectOn || redirectParam == pkgGoDevRedirectOff {
		return redirectParam == pkgGoDevRedirectOn
	}
	cookie, err := req.Cookie(pkgGoDevRedirectCookie)
	return (err == nil && cookie.Value == pkgGoDevRedirectOn)
}

// pkgGoDevRedirectHandler redirects requests from godoc.org to pkg.go.dev,
// based on whether a cookie is set for pkggodev-redirect. The cookie
// can be turned on/off using a query param.
func pkgGoDevRedirectHandler(f func(http.ResponseWriter, *http.Request) error) func(http.ResponseWriter, *http.Request) error {
	return func(w http.ResponseWriter, r *http.Request) error {
		if userReturningFromPkgGoDev(r) {
			return f(w, r)
		}

		redirectParam := r.FormValue(pkgGoDevRedirectParam)

		if redirectParam == pkgGoDevRedirectOn {
			cookie := &http.Cookie{Name: pkgGoDevRedirectCookie, Value: redirectParam, Path: "/"}
			http.SetCookie(w, cookie)
		}
		if redirectParam == pkgGoDevRedirectOff {
			cookie := &http.Cookie{Name: pkgGoDevRedirectCookie, Value: "", MaxAge: -1, Path: "/"}
			http.SetCookie(w, cookie)
		}

		if !shouldRedirectToPkgGoDev(r) {
			return f(w, r)
		}

		http.Redirect(w, r, pkgGoDevURL(r.URL).String(), http.StatusFound)
		return nil
	}
}

func pkgGoDevURL(godocURL *url.URL) *url.URL {
	u := &url.URL{Scheme: "https", Host: pkgGoDevHost}
	q := url.Values{"utm_source": []string{"godoc"}}

	if strings.Contains(godocURL.Path, "/vendor/") || strings.HasSuffix(godocURL.Path, "/vendor") {
		u.Path = "/"
		u.RawQuery = q.Encode()
		return u
	}

	switch godocURL.Path {
	case "/-/go":
		u.Path = "/std"
		q.Add("tab", "packages")
	case "/-/about":
		u.Path = "/about"
	case "/":
		if qparam := godocURL.Query().Get("q"); qparam != "" {
			u.Path = "/search"
			q.Set("q", qparam)
		} else {
			u.Path = "/"
		}
	default:
		{
			u.Path = godocURL.Path
			if _, ok := godocURL.Query()["imports"]; ok {
				q.Set("tab", "imports")
			} else if _, ok := godocURL.Query()["importers"]; ok {
				q.Set("tab", "importedby")
			} else {
				q.Set("tab", "doc")
			}
		}
	}

	u.RawQuery = q.Encode()
	return u
}

func main() {
	ctx := context.Background()
	v, err := loadConfig(ctx, os.Args)
	if err != nil {
		log.Fatal(ctx, "load config", "error", err.Error())
	}
	doc.SetDefaultGOOS(v.GetString(ConfigDefaultGOOS))

	s, err := newServer(ctx, v)
	if err != nil {
		log.Fatal("error creating server:", err)
	}

	go func() {
		for range time.Tick(s.v.GetDuration(ConfigCrawlInterval)) {
			if err := s.doCrawl(ctx); err != nil {
				log.Printf("Task Crawl: %v", err)
			}
		}
	}()
	go func() {
		for range time.Tick(s.v.GetDuration(ConfigGithubInterval)) {
			if err := s.readGitHubUpdates(ctx); err != nil {
				log.Printf("Task GitHub updates: %v", err)
			}
		}
	}()
	http.Handle("/", s)
	log.Fatal(http.ListenAndServe(s.v.GetString(ConfigBindAddress), s))
}