gbvendor/repo.go - third_party/FiloSottile/gvt - Git at Google

 package vendor

 import (
 	"bytes"
 	"fmt"
 	"io"
 	"io/ioutil"
 	"log"
 	"net/url"
 	"os"
 	"os/exec"
 	"path/filepath"
 	"regexp"
 	"strings"

 	"github.com/constabulary/gb/fileutils"
 )

 // RemoteRepo describes a remote dvcs repository.
 type RemoteRepo interface {

 	// Checkout checks out a specific branch, tag, or revision.
 	// The interpretation of these three values is impementation
 	// specific.
 	Checkout(branch, tag, revision string) (WorkingCopy, error)

 	// URL returns the URL the clone was taken from. It should
 	// only be called after Clone.
 	URL() string
 }

 // WorkingCopy represents a local copy of a remote dvcs repository.
 type WorkingCopy interface {

 	// Dir is the root of this working copy.
 	Dir() string

 	// Revision returns the revision of this working copy.
 	Revision() (string, error)

 	// Branch returns the branch to which this working copy belongs.
 	Branch() (string, error)

 	// Destroy removes the working copy and cleans path to the working copy.
 	Destroy() error
 }

 var (
 	ghregex   = regexp.MustCompile(`^(?P<root>github\.com/([A-Za-z0-9_.\-]+/[A-Za-z0-9_.\-]+))(/[A-Za-z0-9_.\-]+)*$`)
 	bbregex   = regexp.MustCompile(`^(?P<root>bitbucket\.org/(?P<bitname>[A-Za-z0-9_.\-]+/[A-Za-z0-9_.\-]+))(/[A-Za-z0-9_.\-]+)*$`)
 	lpregex   = regexp.MustCompile(`^launchpad.net/([A-Za-z0-9-._]+)(/[A-Za-z0-9-._]+)?(/.+)?`)
 	gcregex   = regexp.MustCompile(`^(?P<root>code\.google\.com/[pr]/(?P<project>[a-z0-9\-]+)(\.(?P<subrepo>[a-z0-9\-]+))?)(/[A-Za-z0-9_.\-]+)*$`)
 	genericre = regexp.MustCompile(`^(?P<root>(?P<repo>([a-z0-9.\-]+\.)+[a-z0-9.\-]+(:[0-9]+)?/[A-Za-z0-9_.\-/]*?)\.(?P<vcs>bzr|git|hg|svn))([/A-Za-z0-9_.\-]+)*$`)
 )

 // DeduceRemoteRepo takes a potential import path and returns a RemoteRepo
 // representing the remote location of the source of an import path.
 // Remote repositories can be bare import paths, or urls including a checkout scheme.
 // If deduction would cause traversal of an insecure host, a message will be
 // printed and the travelsal path will be ignored.
 func DeduceRemoteRepo(path string, insecure bool) (RemoteRepo, string, error) {
 	u, err := url.Parse(path)
 	if err != nil {
 		return nil, "", fmt.Errorf("%q is not a valid import path", path)
 	}

 	var schemes []string
 	if u.Scheme != "" {
 		schemes = append(schemes, u.Scheme)
 	}

 	path = u.Host + u.Path
 	if !regexp.MustCompile(`^([A-Za-z0-9-]+)(.[A-Za-z0-9-]+)+(/[A-Za-z0-9-_.]+)+$`).MatchString(path) {
 		return nil, "", fmt.Errorf("%q is not a valid import path", path)
 	}

 	switch {
 	case ghregex.MatchString(path):
 		v := ghregex.FindStringSubmatch(path)
 		url := &url.URL{
 			Host: "github.com",
 			Path: v[2],
 		}
 		repo, err := Gitrepo(url, insecure, schemes...)
 		return repo, v[0][len(v[1]):], err
 	case bbregex.MatchString(path):
 		v := bbregex.FindStringSubmatch(path)
 		url := &url.URL{
 			Host: "bitbucket.org",
 			Path: v[2],
 		}
 		repo, err := Gitrepo(url, insecure, schemes...)
 		if err == nil {
 			return repo, v[0][len(v[1]):], nil
 		}
 		repo, err = Hgrepo(url, insecure)
 		if err == nil {
 			return repo, v[0][len(v[1]):], nil
 		}
 		return nil, "", fmt.Errorf("unknown repository type")
 	case gcregex.MatchString(path):
 		v := gcregex.FindStringSubmatch(path)
 		url := &url.URL{
 			Host: "code.google.com",
 			Path: "p/" + v[2],
 		}
 		repo, err := Hgrepo(url, insecure, schemes...)
 		if err == nil {
 			return repo, v[0][len(v[1]):], nil
 		}
 		repo, err = Gitrepo(url, insecure, schemes...)
 		if err == nil {
 			return repo, v[0][len(v[1]):], nil
 		}
 		return nil, "", fmt.Errorf("unknown repository type")
 	case lpregex.MatchString(path):
 		v := lpregex.FindStringSubmatch(path)
 		v = append(v, "", "")
 		if v[2] == "" {
 			// launchpad.net/project"
 			repo, err := Bzrrepo(fmt.Sprintf("https://launchpad.net/%v", v[1]))
 			return repo, "", err
 		}
 		// launchpad.net/project/series"
 		repo, err := Bzrrepo(fmt.Sprintf("https://launchpad.net/%s/%s", v[1], v[2]))
 		return repo, v[3], err
 	}

 	// try the general syntax
 	if genericre.MatchString(path) {
 		v := genericre.FindStringSubmatch(path)
 		switch v[5] {
 		case "git":
 			x := strings.SplitN(v[1], "/", 2)
 			url := &url.URL{
 				Host: x[0],
 				Path: x[1],
 			}
 			repo, err := Gitrepo(url, insecure, schemes...)
 			return repo, v[6], err
 		case "hg":
 			x := strings.SplitN(v[1], "/", 2)
 			url := &url.URL{
 				Host: x[0],
 				Path: x[1],
 			}
 			repo, err := Hgrepo(url, insecure, schemes...)
 			return repo, v[6], err
 		case "bzr":
 			repo, err := Bzrrepo("https://" + v[1])
 			return repo, v[6], err
 		default:
 			return nil, "", fmt.Errorf("unknown repository type: %q", v[5])

 		}
 	}

 	// no idea, try to resolve as a vanity import
 	importpath, vcs, reporoot, err := ParseMetadata(path, insecure)
 	if err != nil {
 		return nil, "", err
 	}
 	u, err = url.Parse(reporoot)
 	if err != nil {
 		return nil, "", err
 	}
 	extra := path[len(importpath):]
 	switch vcs {
 	case "git":
 		u.Path = u.Path[1:]
 		repo, err := Gitrepo(u, insecure, u.Scheme)
 		return repo, extra, err
 	case "hg":
 		u.Path = u.Path[1:]
 		repo, err := Hgrepo(u, insecure, u.Scheme)
 		return repo, extra, err
 	case "bzr":
 		repo, err := Bzrrepo(reporoot)
 		return repo, extra, err
 	default:
 		return nil, "", fmt.Errorf("unknown repository type: %q", vcs)
 	}
 }

 // Gitrepo returns a RemoteRepo representing a remote git repository.
 func Gitrepo(url *url.URL, insecure bool, schemes ...string) (RemoteRepo, error) {
 	if len(schemes) == 0 {
 		schemes = []string{"https", "git", "ssh", "http"}
 	}
 	u, err := probeGitUrl(url, insecure, schemes)
 	if err != nil {
 		return nil, err
 	}
 	return &gitrepo{
 		url: u,
 	}, nil
 }

 func probeGitUrl(u *url.URL, insecure bool, schemes []string) (string, error) {
 	git := func(url *url.URL) error {
 		out, err := run("git", "ls-remote", url.String(), "HEAD")
 		if err != nil {
 			return err
 		}

 		if !bytes.Contains(out, []byte("HEAD")) {
 			return fmt.Errorf("not a git repo")
 		}
 		return nil
 	}
 	return probe(git, u, insecure, schemes...)
 }

 func probeHgUrl(u *url.URL, insecure bool, schemes []string) (string, error) {
 	hg := func(url *url.URL) error {
 		_, err := run("hg", "identify", url.String())
 		return err
 	}
 	return probe(hg, u, insecure, schemes...)
 }

 func probeBzrUrl(u string) error {
 	bzr := func(url *url.URL) error {
 		_, err := run("bzr", "info", url.String())
 		return err
 	}
 	url, err := url.Parse(u)
 	if err != nil {
 		return err
 	}
 	_, err = probe(bzr, url, false, "https")
 	return err
 }

 // probe calls the supplied vcs function to probe a variety of url constructions.
 // If vcs returns non nil, it is assumed that the url is not a valid repo.
 func probe(vcs func(*url.URL) error, url *url.URL, insecure bool, schemes ...string) (string, error) {
 	var unsuccessful []string
 	for _, scheme := range schemes {

 		// make copy of url and apply scheme
 		url := *url
 		url.Scheme = scheme

 		switch url.Scheme {
 		case "https", "ssh":
 			if err := vcs(&url); err == nil {
 				return url.String(), nil
 			}
 		case "http", "git":
 			if !insecure {
 				log.Printf("skipping insecure protocol: %s", url.String())
 				continue
 			}
 			if err := vcs(&url); err == nil {
 				return url.String(), nil
 			}
 		default:
 			return "", fmt.Errorf("unsupported scheme: %v", url.Scheme)
 		}
 		unsuccessful = append(unsuccessful, url.String())
 	}
 	return "", fmt.Errorf("vcs probe failed, tried: %s", strings.Join(unsuccessful, ","))
 }

 // gitrepo is a git RemoteRepo.
 type gitrepo struct {

 	// remote repository url, see man 1 git-clone
 	url string
 }

 func (g *gitrepo) URL() string {
 	return g.url
 }

 // Checkout fetchs the remote branch, tag, or revision. If more than one is
 // supplied, an error is returned. If the branch is blank,
 // then the default remote branch will be used. If the branch is "HEAD", an
 // error will be returned.
 func (g *gitrepo) Checkout(branch, tag, revision string) (WorkingCopy, error) {
 	if branch == "HEAD" {
 		return nil, fmt.Errorf("cannot update %q as it has been previously fetched with -tag or -revision. Please use gvt delete then fetch again.", g.url)
 	}
 	if !atMostOne(branch, tag, revision) {
 		return nil, fmt.Errorf("only one of branch, tag or revision may be supplied")
 	}
 	dir, err := mktmp()
 	if err != nil {
 		return nil, err
 	}
 	wc := workingcopy{
 		path: dir,
 	}

 	args := []string{
 		"clone",
 		"-q", // silence progress report to stderr
 		g.url,
 		dir,
 	}
 	if branch != "" {
 		args = append(args, "--branch", branch)
 	}

 	if _, err := run("git", args...); err != nil {
 		wc.Destroy()
 		return nil, err
 	}

 	if revision != "" || tag != "" {
 		if err := runOutPath(os.Stderr, dir, "git", "checkout", "-q", oneOf(revision, tag)); err != nil {
 			wc.Destroy()
 			return nil, err
 		}
 	}

 	return &GitClone{wc}, nil
 }

 type workingcopy struct {
 	path string
 }

 func (w workingcopy) Dir() string { return w.path }

 func (w workingcopy) Destroy() error {
 	if err := fileutils.RemoveAll(w.path); err != nil {
 		return err
 	}
 	parent := filepath.Dir(w.path)
 	return cleanPath(parent)
 }

 // GitClone is a git WorkingCopy.
 type GitClone struct {
 	workingcopy
 }

 func (g *GitClone) Revision() (string, error) {
 	rev, err := runPath(g.path, "git", "rev-parse", "HEAD")
 	return strings.TrimSpace(string(rev)), err
 }

 func (g *GitClone) Branch() (string, error) {
 	rev, err := runPath(g.path, "git", "rev-parse", "--abbrev-ref", "HEAD")
 	return strings.TrimSpace(string(rev)), err
 }

 // Hgrepo returns a RemoteRepo representing a remote git repository.
 func Hgrepo(u *url.URL, insecure bool, schemes ...string) (RemoteRepo, error) {
 	if len(schemes) == 0 {
 		schemes = []string{"https", "http"}
 	}
 	url, err := probeHgUrl(u, insecure, schemes)
 	if err != nil {
 		return nil, err
 	}
 	return &hgrepo{
 		url: url,
 	}, nil
 }

 // hgrepo is a Mercurial repo.
 type hgrepo struct {

 	// remote repository url, see man 1 hg
 	url string
 }

 func (h *hgrepo) URL() string { return h.url }

 func (h *hgrepo) Checkout(branch, tag, revision string) (WorkingCopy, error) {
 	if !atMostOne(tag, revision) {
 		return nil, fmt.Errorf("only one of tag or revision may be supplied")
 	}
 	dir, err := mktmp()
 	if err != nil {
 		return nil, err
 	}
 	args := []string{
 		"clone",
 		h.url,
 		dir,
 		"--noninteractive",
 	}

 	if branch != "" {
 		args = append(args, "--branch", branch)
 	}
 	if err := runOut(os.Stderr, "hg", args...); err != nil {
 		fileutils.RemoveAll(dir)
 		return nil, err
 	}
 	if revision != "" {
 		if err := runOut(os.Stderr, "hg", "--cwd", dir, "update", "-r", revision); err != nil {
 			fileutils.RemoveAll(dir)
 			return nil, err
 		}
 	}

 	return &HgClone{
 		workingcopy{
 			path: dir,
 		},
 	}, nil
 }

 // HgClone is a mercurial WorkingCopy.
 type HgClone struct {
 	workingcopy
 }

 func (h *HgClone) Revision() (string, error) {
 	rev, err := run("hg", "--cwd", h.path, "id", "-i")
 	return strings.TrimSpace(string(rev)), err
 }

 func (h *HgClone) Branch() (string, error) {
 	rev, err := run("hg", "--cwd", h.path, "branch")
 	return strings.TrimSpace(string(rev)), err
 }

 // Bzrrepo returns a RemoteRepo representing a remote bzr repository.
 func Bzrrepo(url string) (RemoteRepo, error) {
 	if err := probeBzrUrl(url); err != nil {
 		return nil, err
 	}
 	return &bzrrepo{
 		url: url,
 	}, nil
 }

 // bzrrepo is a bzr RemoteRepo.
 type bzrrepo struct {

 	// remote repository url
 	url string
 }

 func (b *bzrrepo) URL() string {
 	return b.url
 }

 func (b *bzrrepo) Checkout(branch, tag, revision string) (WorkingCopy, error) {
 	if !atMostOne(tag, revision) {
 		return nil, fmt.Errorf("only one of tag or revision may be supplied")
 	}
 	dir, err := mktmp()
 	if err != nil {
 		return nil, err
 	}
 	wc := filepath.Join(dir, "wc")
 	if err := runOut(os.Stderr, "bzr", "branch", b.url, wc); err != nil {
 		fileutils.RemoveAll(dir)
 		return nil, err
 	}

 	return &BzrClone{
 		workingcopy{
 			path: wc,
 		},
 	}, nil
 }

 // BzrClone is a bazaar WorkingCopy.
 type BzrClone struct {
 	workingcopy
 }

 func (b *BzrClone) Revision() (string, error) {
 	return "1", nil
 }

 func (b *BzrClone) Branch() (string, error) {
 	return "master", nil
 }

 func cleanPath(path string) error {
 	if files, _ := ioutil.ReadDir(path); len(files) > 0 || filepath.Base(path) == "src" {
 		return nil
 	}
 	parent := filepath.Dir(path)
 	if err := fileutils.RemoveAll(path); err != nil {
 		return err
 	}
 	return cleanPath(parent)
 }

 func mkdir(path string) error {
 	return os.MkdirAll(path, 0755)
 }

 func mktmp() (string, error) {
 	return ioutil.TempDir("", "gvt-")
 }

 func run(c string, args ...string) ([]byte, error) {
 	var buf bytes.Buffer
 	err := runOut(&buf, c, args...)
 	return buf.Bytes(), err
 }

 func runOut(w io.Writer, c string, args ...string) error {
 	cmd := exec.Command(c, args...)
 	cmd.Stdin = nil
 	cmd.Stdout = w
 	cmd.Stderr = os.Stderr
 	return cmd.Run()
 }

 func runPath(path string, c string, args ...string) ([]byte, error) {
 	var buf bytes.Buffer
 	err := runOutPath(&buf, path, c, args...)
 	return buf.Bytes(), err
 }

 func runOutPath(w io.Writer, path string, c string, args ...string) error {
 	cmd := exec.Command(c, args...)
 	cmd.Dir = path
 	cmd.Stdin = nil
 	cmd.Stdout = w
 	cmd.Stderr = os.Stderr
 	return cmd.Run()
 }

 // atMostOne returns true if no more than one string supplied is not empty.
 func atMostOne(args ...string) bool {
 	var c int
 	for _, arg := range args {
 		if arg != "" {
 			c++
 		}
 	}
 	return c < 2
 }

 // oneof returns the first non empty string
 func oneOf(args ...string) string {
 	for _, arg := range args {
 		if arg != "" {
 			return arg
 		}
 	}
 	return ""
 }
	package vendor

	import (
	"bytes"
	"fmt"
	"io"
	"io/ioutil"
	"log"
	"net/url"
	"os"
	"os/exec"
	"path/filepath"
	"regexp"
	"strings"

	"github.com/constabulary/gb/fileutils"
	)

	// RemoteRepo describes a remote dvcs repository.
	type RemoteRepo interface {

	// Checkout checks out a specific branch, tag, or revision.
	// The interpretation of these three values is impementation
	// specific.
	Checkout(branch, tag, revision string) (WorkingCopy, error)

	// URL returns the URL the clone was taken from. It should
	// only be called after Clone.
	URL() string
	}

	// WorkingCopy represents a local copy of a remote dvcs repository.
	type WorkingCopy interface {

	// Dir is the root of this working copy.
	Dir() string

	// Revision returns the revision of this working copy.
	Revision() (string, error)

	// Branch returns the branch to which this working copy belongs.
	Branch() (string, error)

	// Destroy removes the working copy and cleans path to the working copy.
	Destroy() error
	}

	var (
	ghregex = regexp.MustCompile(`^(?P<root>github\.com/([A-Za-z0-9_.\-]+/[A-Za-z0-9_.\-]+))(/[A-Za-z0-9_.\-]+)*$`)
	bbregex = regexp.MustCompile(`^(?P<root>bitbucket\.org/(?P<bitname>[A-Za-z0-9_.\-]+/[A-Za-z0-9_.\-]+))(/[A-Za-z0-9_.\-]+)*$`)
	lpregex = regexp.MustCompile(`^launchpad.net/([A-Za-z0-9-._]+)(/[A-Za-z0-9-._]+)?(/.+)?`)
	gcregex = regexp.MustCompile(`^(?P<root>code\.google\.com/[pr]/(?P<project>[a-z0-9\-]+)(\.(?P<subrepo>[a-z0-9\-]+))?)(/[A-Za-z0-9_.\-]+)*$`)
	genericre = regexp.MustCompile(`^(?P<root>(?P<repo>([a-z0-9.\-]+\.)+[a-z0-9.\-]+(:[0-9]+)?/[A-Za-z0-9_.\-/]?)\.(?P<vcs>bzr\|git\|hg\|svn))([/A-Za-z0-9_.\-]+)$`)
	)

	// DeduceRemoteRepo takes a potential import path and returns a RemoteRepo
	// representing the remote location of the source of an import path.
	// Remote repositories can be bare import paths, or urls including a checkout scheme.
	// If deduction would cause traversal of an insecure host, a message will be
	// printed and the travelsal path will be ignored.
	func DeduceRemoteRepo(path string, insecure bool) (RemoteRepo, string, error) {
	u, err := url.Parse(path)
	if err != nil {
	return nil, "", fmt.Errorf("%q is not a valid import path", path)
	}

	var schemes []string
	if u.Scheme != "" {
	schemes = append(schemes, u.Scheme)
	}

	path = u.Host + u.Path
	if !regexp.MustCompile(`^([A-Za-z0-9-]+)(.[A-Za-z0-9-]+)+(/[A-Za-z0-9-_.]+)+$`).MatchString(path) {
	return nil, "", fmt.Errorf("%q is not a valid import path", path)
	}

	switch {
	case ghregex.MatchString(path):
	v := ghregex.FindStringSubmatch(path)
	url := &url.URL{
	Host: "github.com",
	Path: v[2],
	}
	repo, err := Gitrepo(url, insecure, schemes...)
	return repo, v[0][len(v[1]):], err
	case bbregex.MatchString(path):
	v := bbregex.FindStringSubmatch(path)
	url := &url.URL{
	Host: "bitbucket.org",
	Path: v[2],
	}
	repo, err := Gitrepo(url, insecure, schemes...)
	if err == nil {
	return repo, v[0][len(v[1]):], nil
	}
	repo, err = Hgrepo(url, insecure)
	if err == nil {
	return repo, v[0][len(v[1]):], nil
	}
	return nil, "", fmt.Errorf("unknown repository type")
	case gcregex.MatchString(path):
	v := gcregex.FindStringSubmatch(path)
	url := &url.URL{
	Host: "code.google.com",
	Path: "p/" + v[2],
	}
	repo, err := Hgrepo(url, insecure, schemes...)
	if err == nil {
	return repo, v[0][len(v[1]):], nil
	}
	repo, err = Gitrepo(url, insecure, schemes...)
	if err == nil {
	return repo, v[0][len(v[1]):], nil
	}
	return nil, "", fmt.Errorf("unknown repository type")
	case lpregex.MatchString(path):
	v := lpregex.FindStringSubmatch(path)
	v = append(v, "", "")
	if v[2] == "" {
	// launchpad.net/project"
	repo, err := Bzrrepo(fmt.Sprintf("https://launchpad.net/%v", v[1]))
	return repo, "", err
	}
	// launchpad.net/project/series"
	repo, err := Bzrrepo(fmt.Sprintf("https://launchpad.net/%s/%s", v[1], v[2]))
	return repo, v[3], err
	}

	// try the general syntax
	if genericre.MatchString(path) {
	v := genericre.FindStringSubmatch(path)
	switch v[5] {
	case "git":
	x := strings.SplitN(v[1], "/", 2)
	url := &url.URL{
	Host: x[0],
	Path: x[1],
	}
	repo, err := Gitrepo(url, insecure, schemes...)
	return repo, v[6], err
	case "hg":
	x := strings.SplitN(v[1], "/", 2)
	url := &url.URL{
	Host: x[0],
	Path: x[1],
	}
	repo, err := Hgrepo(url, insecure, schemes...)
	return repo, v[6], err
	case "bzr":
	repo, err := Bzrrepo("https://" + v[1])
	return repo, v[6], err
	default:
	return nil, "", fmt.Errorf("unknown repository type: %q", v[5])

	}
	}

	// no idea, try to resolve as a vanity import
	importpath, vcs, reporoot, err := ParseMetadata(path, insecure)
	if err != nil {
	return nil, "", err
	}
	u, err = url.Parse(reporoot)
	if err != nil {
	return nil, "", err
	}
	extra := path[len(importpath):]
	switch vcs {
	case "git":
	u.Path = u.Path[1:]
	repo, err := Gitrepo(u, insecure, u.Scheme)
	return repo, extra, err
	case "hg":
	u.Path = u.Path[1:]
	repo, err := Hgrepo(u, insecure, u.Scheme)
	return repo, extra, err
	case "bzr":
	repo, err := Bzrrepo(reporoot)
	return repo, extra, err
	default:
	return nil, "", fmt.Errorf("unknown repository type: %q", vcs)
	}
	}

	// Gitrepo returns a RemoteRepo representing a remote git repository.
	func Gitrepo(url *url.URL, insecure bool, schemes ...string) (RemoteRepo, error) {
	if len(schemes) == 0 {
	schemes = []string{"https", "git", "ssh", "http"}
	}
	u, err := probeGitUrl(url, insecure, schemes)
	if err != nil {
	return nil, err
	}
	return &gitrepo{
	url: u,
	}, nil
	}

	func probeGitUrl(u *url.URL, insecure bool, schemes []string) (string, error) {
	git := func(url *url.URL) error {
	out, err := run("git", "ls-remote", url.String(), "HEAD")
	if err != nil {
	return err
	}

	if !bytes.Contains(out, []byte("HEAD")) {
	return fmt.Errorf("not a git repo")
	}
	return nil
	}
	return probe(git, u, insecure, schemes...)
	}

	func probeHgUrl(u *url.URL, insecure bool, schemes []string) (string, error) {
	hg := func(url *url.URL) error {
	_, err := run("hg", "identify", url.String())
	return err
	}
	return probe(hg, u, insecure, schemes...)
	}

	func probeBzrUrl(u string) error {
	bzr := func(url *url.URL) error {
	_, err := run("bzr", "info", url.String())
	return err
	}
	url, err := url.Parse(u)
	if err != nil {
	return err
	}
	_, err = probe(bzr, url, false, "https")
	return err
	}

	// probe calls the supplied vcs function to probe a variety of url constructions.
	// If vcs returns non nil, it is assumed that the url is not a valid repo.
	func probe(vcs func(url.URL) error, url url.URL, insecure bool, schemes ...string) (string, error) {
	var unsuccessful []string
	for _, scheme := range schemes {

	// make copy of url and apply scheme
	url := *url
	url.Scheme = scheme

	switch url.Scheme {
	case "https", "ssh":
	if err := vcs(&url); err == nil {
	return url.String(), nil
	}
	case "http", "git":
	if !insecure {
	log.Printf("skipping insecure protocol: %s", url.String())
	continue
	}
	if err := vcs(&url); err == nil {
	return url.String(), nil
	}
	default:
	return "", fmt.Errorf("unsupported scheme: %v", url.Scheme)
	}
	unsuccessful = append(unsuccessful, url.String())
	}
	return "", fmt.Errorf("vcs probe failed, tried: %s", strings.Join(unsuccessful, ","))
	}

	// gitrepo is a git RemoteRepo.
	type gitrepo struct {

	// remote repository url, see man 1 git-clone
	url string
	}

	func (g *gitrepo) URL() string {
	return g.url
	}

	// Checkout fetchs the remote branch, tag, or revision. If more than one is
	// supplied, an error is returned. If the branch is blank,
	// then the default remote branch will be used. If the branch is "HEAD", an
	// error will be returned.
	func (g *gitrepo) Checkout(branch, tag, revision string) (WorkingCopy, error) {
	if branch == "HEAD" {
	return nil, fmt.Errorf("cannot update %q as it has been previously fetched with -tag or -revision. Please use gvt delete then fetch again.", g.url)
	}
	if !atMostOne(branch, tag, revision) {
	return nil, fmt.Errorf("only one of branch, tag or revision may be supplied")
	}
	dir, err := mktmp()
	if err != nil {
	return nil, err
	}
	wc := workingcopy{
	path: dir,
	}

	args := []string{
	"clone",
	"-q", // silence progress report to stderr
	g.url,
	dir,
	}
	if branch != "" {
	args = append(args, "--branch", branch)
	}

	if _, err := run("git", args...); err != nil {
	wc.Destroy()
	return nil, err
	}

	if revision != "" \|\| tag != "" {
	if err := runOutPath(os.Stderr, dir, "git", "checkout", "-q", oneOf(revision, tag)); err != nil {
	wc.Destroy()
	return nil, err
	}
	}

	return &GitClone{wc}, nil
	}

	type workingcopy struct {
	path string
	}

	func (w workingcopy) Dir() string { return w.path }

	func (w workingcopy) Destroy() error {
	if err := fileutils.RemoveAll(w.path); err != nil {
	return err
	}
	parent := filepath.Dir(w.path)
	return cleanPath(parent)
	}

	// GitClone is a git WorkingCopy.
	type GitClone struct {
	workingcopy
	}

	func (g *GitClone) Revision() (string, error) {
	rev, err := runPath(g.path, "git", "rev-parse", "HEAD")
	return strings.TrimSpace(string(rev)), err
	}

	func (g *GitClone) Branch() (string, error) {
	rev, err := runPath(g.path, "git", "rev-parse", "--abbrev-ref", "HEAD")
	return strings.TrimSpace(string(rev)), err
	}

	// Hgrepo returns a RemoteRepo representing a remote git repository.
	func Hgrepo(u *url.URL, insecure bool, schemes ...string) (RemoteRepo, error) {
	if len(schemes) == 0 {
	schemes = []string{"https", "http"}
	}
	url, err := probeHgUrl(u, insecure, schemes)
	if err != nil {
	return nil, err
	}
	return &hgrepo{
	url: url,
	}, nil
	}

	// hgrepo is a Mercurial repo.
	type hgrepo struct {

	// remote repository url, see man 1 hg
	url string
	}

	func (h *hgrepo) URL() string { return h.url }

	func (h *hgrepo) Checkout(branch, tag, revision string) (WorkingCopy, error) {
	if !atMostOne(tag, revision) {
	return nil, fmt.Errorf("only one of tag or revision may be supplied")
	}
	dir, err := mktmp()
	if err != nil {
	return nil, err
	}
	args := []string{
	"clone",
	h.url,
	dir,
	"--noninteractive",
	}

	if branch != "" {
	args = append(args, "--branch", branch)
	}
	if err := runOut(os.Stderr, "hg", args...); err != nil {
	fileutils.RemoveAll(dir)
	return nil, err
	}
	if revision != "" {
	if err := runOut(os.Stderr, "hg", "--cwd", dir, "update", "-r", revision); err != nil {
	fileutils.RemoveAll(dir)
	return nil, err
	}
	}

	return &HgClone{
	workingcopy{
	path: dir,
	},
	}, nil
	}

	// HgClone is a mercurial WorkingCopy.
	type HgClone struct {
	workingcopy
	}

	func (h *HgClone) Revision() (string, error) {
	rev, err := run("hg", "--cwd", h.path, "id", "-i")
	return strings.TrimSpace(string(rev)), err
	}

	func (h *HgClone) Branch() (string, error) {
	rev, err := run("hg", "--cwd", h.path, "branch")
	return strings.TrimSpace(string(rev)), err
	}

	// Bzrrepo returns a RemoteRepo representing a remote bzr repository.
	func Bzrrepo(url string) (RemoteRepo, error) {
	if err := probeBzrUrl(url); err != nil {
	return nil, err
	}
	return &bzrrepo{
	url: url,
	}, nil
	}

	// bzrrepo is a bzr RemoteRepo.
	type bzrrepo struct {

	// remote repository url
	url string
	}

	func (b *bzrrepo) URL() string {
	return b.url
	}

	func (b *bzrrepo) Checkout(branch, tag, revision string) (WorkingCopy, error) {
	if !atMostOne(tag, revision) {
	return nil, fmt.Errorf("only one of tag or revision may be supplied")
	}
	dir, err := mktmp()
	if err != nil {
	return nil, err
	}
	wc := filepath.Join(dir, "wc")
	if err := runOut(os.Stderr, "bzr", "branch", b.url, wc); err != nil {
	fileutils.RemoveAll(dir)
	return nil, err
	}

	return &BzrClone{
	workingcopy{
	path: wc,
	},
	}, nil
	}

	// BzrClone is a bazaar WorkingCopy.
	type BzrClone struct {
	workingcopy
	}

	func (b *BzrClone) Revision() (string, error) {
	return "1", nil
	}

	func (b *BzrClone) Branch() (string, error) {
	return "master", nil
	}

	func cleanPath(path string) error {
	if files, _ := ioutil.ReadDir(path); len(files) > 0 \|\| filepath.Base(path) == "src" {
	return nil
	}
	parent := filepath.Dir(path)
	if err := fileutils.RemoveAll(path); err != nil {
	return err
	}
	return cleanPath(parent)
	}

	func mkdir(path string) error {
	return os.MkdirAll(path, 0755)
	}

	func mktmp() (string, error) {
	return ioutil.TempDir("", "gvt-")
	}

	func run(c string, args ...string) ([]byte, error) {
	var buf bytes.Buffer
	err := runOut(&buf, c, args...)
	return buf.Bytes(), err
	}

	func runOut(w io.Writer, c string, args ...string) error {
	cmd := exec.Command(c, args...)
	cmd.Stdin = nil
	cmd.Stdout = w
	cmd.Stderr = os.Stderr
	return cmd.Run()
	}

	func runPath(path string, c string, args ...string) ([]byte, error) {
	var buf bytes.Buffer
	err := runOutPath(&buf, path, c, args...)
	return buf.Bytes(), err
	}

	func runOutPath(w io.Writer, path string, c string, args ...string) error {
	cmd := exec.Command(c, args...)
	cmd.Dir = path
	cmd.Stdin = nil
	cmd.Stdout = w
	cmd.Stderr = os.Stderr
	return cmd.Run()
	}

	// atMostOne returns true if no more than one string supplied is not empty.
	func atMostOne(args ...string) bool {
	var c int
	for _, arg := range args {
	if arg != "" {
	c++
	}
	}
	return c < 2
	}

	// oneof returns the first non empty string
	func oneOf(args ...string) string {
	for _, arg := range args {
	if arg != "" {
	return arg
	}
	}
	return ""
	}