package main

import (
	"log"
	"net/http"
	"net/http/cgi"
	"os"
	"path/filepath"
	"regexp"
	"strconv"
	"strings"
	"sync"
)

// gitBackend is git's own CGI, located by asking git. On first use, not at
// startup: the filter subcommands must not fork git.
var gitBackend = sync.OnceValue(func() string {
	out, err := git("", nil, "--exec-path")
	if err != nil {
		log.Printf("git: no exec path, so no push or pull over http: %v", err)
		return ""
	}
	return filepath.Join(strings.TrimSpace(string(out)), "git-http-backend")
})

var gitProtocolRE = regexp.MustCompile(`^[a-zA-Z0-9=:._-]+$`)

// transferring bounds the git-http-backend children in flight: an upload-pack
// packs with a thread per core.
var transferring = make(chan struct{}, 8)

// What one push may weigh; git has no bound of its own. The wiki is held to
// far less, because its knob can be set to any account at all.
const (
	maxPushCode = 1 << 30
	maxPushWiki = 16 << 20
)

// handleGitHTTP authorises before anything is exec'd. No form token and no
// session cookie: a push authenticated by a cookie could be driven from
// another site.
func (f *Forge) handleGitHTTP(w http.ResponseWriter, r *http.Request, path, kind, rest string) {
	backend := gitBackend()
	if backend == "" {
		http.Error(w, "git-http-backend is not installed\n", http.StatusInternalServerError)
		return
	}
	v, ok := f.gitViewer(w, r, path)
	if !ok {
		return
	}
	// What the viewer may not read does not exist, as on the web. Anonymous is
	// challenged instead, so git sends credentials, and challenged whether or
	// not the repository is there: a 401 beside a 404 would name the private
	// ones.
	repo, exists := f.store.Repo(path)
	if !exists || !v.CanRead(repo) {
		if v.SignedIn() {
			http.NotFound(w, r)
		} else {
			f.gitChallenge(w, "authentication required")
		}
		return
	}
	// A feature turned off keeps its repository on disk.
	if (kind == "issues" && repo.Issues == featDisabled) || (kind == "wiki" && repo.Wiki == featDisabled) {
		http.NotFound(w, r)
		return
	}
	dir := path + gitSuffix(kind)
	if _, err := os.Stat(filepath.Join(f.store.reposDir, dir)); err != nil {
		http.NotFound(w, r)
		return
	}

	write := isPush(r, rest)
	if write && kind == "issues" {
		// The tracker is well formed because the server is its only writer.
		http.Error(w, "the issue tracker is written by the forge, not by push. clone and read it, but file issues through the web\n",
			http.StatusForbidden)
		return
	}
	if write {
		// The wiki knob, not the code write grant: documentation fixes and
		// code are separate permissions.
		allowed := v.CanWrite(repo)
		if kind == "wiki" {
			allowed = v.CanWiki(repo)
		}
		switch {
		case allowed:
		case v.SignedIn():
			http.Error(w, "you do not have that access to this repository\n", http.StatusForbidden)
			return
		default:
			f.gitChallenge(w, "authentication required")
			return
		}
	}

	weight := maxPushCode
	if kind == "wiki" {
		weight = maxPushWiki
	}
	// PATH_INFO comes from the resolved repository, not the URL. The receive
	// settings come through the environment: nothing the forge writes goes
	// inside a repository.
	env := []string{
		"GIT_PROJECT_ROOT=" + f.store.reposDir,
		"GIT_HTTP_EXPORT_ALL=1",
		"REMOTE_USER=" + v.Name,
		"PATH_INFO=/" + dir + "/" + rest,
		"SCRIPT_NAME=",
		"GIT_CONFIG_NOSYSTEM=1",
		"GIT_CONFIG_GLOBAL=/dev/null",
		"GIT_CONFIG_COUNT=2",
		"GIT_CONFIG_KEY_0=receive.unpackLimit",
		"GIT_CONFIG_VALUE_0=1",
		"GIT_CONFIG_KEY_1=receive.maxInputSize",
		"GIT_CONFIG_VALUE_1=" + strconv.Itoa(weight),
	}
	// As HTTP_GIT_PROTOCOL git-http-backend would not read it, and every
	// client would fall back to protocol v0.
	if p := r.Header.Get("Git-Protocol"); p != "" && gitProtocolRE.MatchString(p) {
		env = append(env, "GIT_PROTOCOL="+p)
	}
	// git sends a push past http.postBuffer chunked, which net/http/cgi
	// refuses; the framing is already decoded here.
	r.TransferEncoding = nil
	release, ok := childSlot(transferring, r)
	if !ok {
		return
	}
	defer release()
	(&cgi.Handler{Path: backend, Env: env, Stderr: os.Stdout}).ServeHTTP(w, r)

	if write {
		// A push that failed leaves nothing to collect.
		gcAuto(filepath.Join(f.store.reposDir, dir))
	}
}

// gitViewer reads Basic auth carrying a scoped push token, never a password:
// git cannot be asked for a second factor, and a token is separately scoped
// and separately revoked. No credential is the anonymous viewer.
func (f *Forge) gitViewer(w http.ResponseWriter, r *http.Request, path string) (Viewer, bool) {
	given, secret, sent := r.BasicAuth()
	if !sent {
		return Viewer{}, true
	}
	name, u, ok := f.checkToken(given, secret, path)
	if !ok {
		f.gitChallenge(w, "bad account name or token")
		return Viewer{}, false
	}
	// adminInEffect, not the raw flag: the web and git have to agree.
	return Viewer{Name: name, Admin: u.adminInEffect()}, true
}

func (f *Forge) gitChallenge(w http.ResponseWriter, msg string) {
	w.Header().Set("WWW-Authenticate", `Basic realm="`+f.siteName()+`", charset="UTF-8"`)
	http.Error(w, msg+"\n", http.StatusUnauthorized)
}

// isPush reports whether this request is the start or the body of a push.
func isPush(r *http.Request, rest string) bool {
	return rest == "git-receive-pack" || r.URL.Query().Get("service") == "git-receive-pack"
}

func gitSuffix(kind string) string {
	if kind == "" {
		return ".git"
	}
	return "." + kind + ".git"
}