9 files changed, 2112 insertions, 0 deletions
diff --git a/storage/filesystem/dotgit/dotgit.go b/storage/filesystem/dotgit/dotgit.go
new file mode 100644
index 0000000..52b621c
--- /dev/null
+++ b/storage/filesystem/dotgit/dotgit.go
@@ -0,0 +1,808 @@
+// https://github.com/git/git/blob/master/Documentation/gitrepository-layout.txt
+package dotgit
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"io"
+	stdioutil "io/ioutil"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+
+	"gopkg.in/src-d/go-billy.v4/osfs"
+	"gopkg.in/src-d/go-git.v4/plumbing"
+	"gopkg.in/src-d/go-git.v4/utils/ioutil"
+
+	"gopkg.in/src-d/go-billy.v4"
+)
+
+const (
+	suffix         = ".git"
+	packedRefsPath = "packed-refs"
+	configPath     = "config"
+	indexPath      = "index"
+	shallowPath    = "shallow"
+	modulePath     = "modules"
+	objectsPath    = "objects"
+	packPath       = "pack"
+	refsPath       = "refs"
+
+	tmpPackedRefsPrefix = "._packed-refs"
+
+	packExt = ".pack"
+	idxExt  = ".idx"
+)
+
+var (
+	// ErrNotFound is returned by New when the path is not found.
+	ErrNotFound = errors.New("path not found")
+	// ErrIdxNotFound is returned by Idxfile when the idx file is not found
+	ErrIdxNotFound = errors.New("idx file not found")
+	// ErrPackfileNotFound is returned by Packfile when the packfile is not found
+	ErrPackfileNotFound = errors.New("packfile not found")
+	// ErrConfigNotFound is returned by Config when the config is not found
+	ErrConfigNotFound = errors.New("config file not found")
+	// ErrPackedRefsDuplicatedRef is returned when a duplicated reference is
+	// found in the packed-ref file. This is usually the case for corrupted git
+	// repositories.
+	ErrPackedRefsDuplicatedRef = errors.New("duplicated ref found in packed-ref file")
+	// ErrPackedRefsBadFormat is returned when the packed-ref file corrupt.
+	ErrPackedRefsBadFormat = errors.New("malformed packed-ref")
+	// ErrSymRefTargetNotFound is returned when a symbolic reference is
+	// targeting a non-existing object. This usually means the repository
+	// is corrupt.
+	ErrSymRefTargetNotFound = errors.New("symbolic reference target not found")
+)
+
+// The DotGit type represents a local git repository on disk. This
+// type is not zero-value-safe, use the New function to initialize it.
+type DotGit struct {
+	fs billy.Filesystem
+}
+
+// New returns a DotGit value ready to be used. The path argument must
+// be the absolute path of a git repository directory (e.g.
+// "/foo/bar/.git").
+func New(fs billy.Filesystem) *DotGit {
+	return &DotGit{fs: fs}
+}
+
+// Initialize creates all the folder scaffolding.
+func (d *DotGit) Initialize() error {
+	mustExists := []string{
+		d.fs.Join("objects", "info"),
+		d.fs.Join("objects", "pack"),
+		d.fs.Join("refs", "heads"),
+		d.fs.Join("refs", "tags"),
+	}
+
+	for _, path := range mustExists {
+		_, err := d.fs.Stat(path)
+		if err == nil {
+			continue
+		}
+
+		if !os.IsNotExist(err) {
+			return err
+		}
+
+		if err := d.fs.MkdirAll(path, os.ModeDir|os.ModePerm); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// ConfigWriter returns a file pointer for write to the config file
+func (d *DotGit) ConfigWriter() (billy.File, error) {
+	return d.fs.Create(configPath)
+}
+
+// Config returns a file pointer for read to the config file
+func (d *DotGit) Config() (billy.File, error) {
+	return d.fs.Open(configPath)
+}
+
+// IndexWriter returns a file pointer for write to the index file
+func (d *DotGit) IndexWriter() (billy.File, error) {
+	return d.fs.Create(indexPath)
+}
+
+// Index returns a file pointer for read to the index file
+func (d *DotGit) Index() (billy.File, error) {
+	return d.fs.Open(indexPath)
+}
+
+// ShallowWriter returns a file pointer for write to the shallow file
+func (d *DotGit) ShallowWriter() (billy.File, error) {
+	return d.fs.Create(shallowPath)
+}
+
+// Shallow returns a file pointer for read to the shallow file
+func (d *DotGit) Shallow() (billy.File, error) {
+	f, err := d.fs.Open(shallowPath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil
+		}
+
+		return nil, err
+	}
+
+	return f, nil
+}
+
+// NewObjectPack return a writer for a new packfile, it saves the packfile to
+// disk and also generates and save the index for the given packfile.
+func (d *DotGit) NewObjectPack() (*PackWriter, error) {
+	return newPackWrite(d.fs)
+}
+
+// ObjectPacks returns the list of availables packfiles
+func (d *DotGit) ObjectPacks() ([]plumbing.Hash, error) {
+	packDir := d.fs.Join(objectsPath, packPath)
+	files, err := d.fs.ReadDir(packDir)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil
+		}
+
+		return nil, err
+	}
+
+	var packs []plumbing.Hash
+	for _, f := range files {
+		if !strings.HasSuffix(f.Name(), packExt) {
+			continue
+		}
+
+		n := f.Name()
+		h := plumbing.NewHash(n[5 : len(n)-5]) //pack-(hash).pack
+		if h.IsZero() {
+			// Ignore files with badly-formatted names.
+			continue
+		}
+		packs = append(packs, h)
+	}
+
+	return packs, nil
+}
+
+func (d *DotGit) objectPackPath(hash plumbing.Hash, extension string) string {
+	return d.fs.Join(objectsPath, packPath, fmt.Sprintf("pack-%s.%s", hash.String(), extension))
+}
+
+func (d *DotGit) objectPackOpen(hash plumbing.Hash, extension string) (billy.File, error) {
+	pack, err := d.fs.Open(d.objectPackPath(hash, extension))
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, ErrPackfileNotFound
+		}
+
+		return nil, err
+	}
+
+	return pack, nil
+}
+
+// ObjectPack returns a fs.File of the given packfile
+func (d *DotGit) ObjectPack(hash plumbing.Hash) (billy.File, error) {
+	return d.objectPackOpen(hash, `pack`)
+}
+
+// ObjectPackIdx returns a fs.File of the index file for a given packfile
+func (d *DotGit) ObjectPackIdx(hash plumbing.Hash) (billy.File, error) {
+	return d.objectPackOpen(hash, `idx`)
+}
+
+func (d *DotGit) DeleteOldObjectPackAndIndex(hash plumbing.Hash, t time.Time) error {
+	path := d.objectPackPath(hash, `pack`)
+	if !t.IsZero() {
+		fi, err := d.fs.Stat(path)
+		if err != nil {
+			return err
+		}
+		// too new, skip deletion.
+		if !fi.ModTime().Before(t) {
+			return nil
+		}
+	}
+	err := d.fs.Remove(path)
+	if err != nil {
+		return err
+	}
+	return d.fs.Remove(d.objectPackPath(hash, `idx`))
+}
+
+// NewObject return a writer for a new object file.
+func (d *DotGit) NewObject() (*ObjectWriter, error) {
+	return newObjectWriter(d.fs)
+}
+
+// Objects returns a slice with the hashes of objects found under the
+// .git/objects/ directory.
+func (d *DotGit) Objects() ([]plumbing.Hash, error) {
+	var objects []plumbing.Hash
+	err := d.ForEachObjectHash(func(hash plumbing.Hash) error {
+		objects = append(objects, hash)
+		return nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	return objects, nil
+}
+
+// Objects returns a slice with the hashes of objects found under the
+// .git/objects/ directory.
+func (d *DotGit) ForEachObjectHash(fun func(plumbing.Hash) error) error {
+	files, err := d.fs.ReadDir(objectsPath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil
+		}
+
+		return err
+	}
+
+	for _, f := range files {
+		if f.IsDir() && len(f.Name()) == 2 && isHex(f.Name()) {
+			base := f.Name()
+			d, err := d.fs.ReadDir(d.fs.Join(objectsPath, base))
+			if err != nil {
+				return err
+			}
+
+			for _, o := range d {
+				h := plumbing.NewHash(base + o.Name())
+				if h.IsZero() {
+					// Ignore files with badly-formatted names.
+					continue
+				}
+				err = fun(h)
+				if err != nil {
+					return err
+				}
+			}
+		}
+	}
+
+	return nil
+}
+
+func (d *DotGit) objectPath(h plumbing.Hash) string {
+	hash := h.String()
+	return d.fs.Join(objectsPath, hash[0:2], hash[2:40])
+}
+
+// Object returns a fs.File pointing the object file, if exists
+func (d *DotGit) Object(h plumbing.Hash) (billy.File, error) {
+	return d.fs.Open(d.objectPath(h))
+}
+
+// ObjectStat returns a os.FileInfo pointing the object file, if exists
+func (d *DotGit) ObjectStat(h plumbing.Hash) (os.FileInfo, error) {
+	return d.fs.Stat(d.objectPath(h))
+}
+
+// ObjectDelete removes the object file, if exists
+func (d *DotGit) ObjectDelete(h plumbing.Hash) error {
+	return d.fs.Remove(d.objectPath(h))
+}
+
+func (d *DotGit) readReferenceFrom(rd io.Reader, name string) (ref *plumbing.Reference, err error) {
+	b, err := stdioutil.ReadAll(rd)
+	if err != nil {
+		return nil, err
+	}
+
+	line := strings.TrimSpace(string(b))
+	return plumbing.NewReferenceFromStrings(name, line), nil
+}
+
+func (d *DotGit) checkReferenceAndTruncate(f billy.File, old *plumbing.Reference) error {
+	if old == nil {
+		return nil
+	}
+	ref, err := d.readReferenceFrom(f, old.Name().String())
+	if err != nil {
+		return err
+	}
+	if ref.Hash() != old.Hash() {
+		return fmt.Errorf("reference has changed concurrently")
+	}
+	_, err = f.Seek(0, io.SeekStart)
+	if err != nil {
+		return err
+	}
+	return f.Truncate(0)
+}
+
+func (d *DotGit) SetRef(r, old *plumbing.Reference) error {
+	var content string
+	switch r.Type() {
+	case plumbing.SymbolicReference:
+		content = fmt.Sprintf("ref: %s\n", r.Target())
+	case plumbing.HashReference:
+		content = fmt.Sprintln(r.Hash().String())
+	}
+
+	fileName := r.Name().String()
+
+	return d.setRef(fileName, content, old)
+}
+
+// Refs scans the git directory collecting references, which it returns.
+// Symbolic references are resolved and included in the output.
+func (d *DotGit) Refs() ([]*plumbing.Reference, error) {
+	var refs []*plumbing.Reference
+	var seen = make(map[plumbing.ReferenceName]bool)
+	if err := d.addRefsFromRefDir(&refs, seen); err != nil {
+		return nil, err
+	}
+
+	if err := d.addRefsFromPackedRefs(&refs, seen); err != nil {
+		return nil, err
+	}
+
+	if err := d.addRefFromHEAD(&refs); err != nil {
+		return nil, err
+	}
+
+	return refs, nil
+}
+
+// Ref returns the reference for a given reference name.
+func (d *DotGit) Ref(name plumbing.ReferenceName) (*plumbing.Reference, error) {
+	ref, err := d.readReferenceFile(".", name.String())
+	if err == nil {
+		return ref, nil
+	}
+
+	return d.packedRef(name)
+}
+
+func (d *DotGit) findPackedRefsInFile(f billy.File) ([]*plumbing.Reference, error) {
+	s := bufio.NewScanner(f)
+	var refs []*plumbing.Reference
+	for s.Scan() {
+		ref, err := d.processLine(s.Text())
+		if err != nil {
+			return nil, err
+		}
+
+		if ref != nil {
+			refs = append(refs, ref)
+		}
+	}
+
+	return refs, s.Err()
+}
+
+func (d *DotGit) findPackedRefs() (r []*plumbing.Reference, err error) {
+	f, err := d.fs.Open(packedRefsPath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil
+		}
+		return nil, err
+	}
+
+	defer ioutil.CheckClose(f, &err)
+	return d.findPackedRefsInFile(f)
+}
+
+func (d *DotGit) packedRef(name plumbing.ReferenceName) (*plumbing.Reference, error) {
+	refs, err := d.findPackedRefs()
+	if err != nil {
+		return nil, err
+	}
+
+	for _, ref := range refs {
+		if ref.Name() == name {
+			return ref, nil
+		}
+	}
+
+	return nil, plumbing.ErrReferenceNotFound
+}
+
+// RemoveRef removes a reference by name.
+func (d *DotGit) RemoveRef(name plumbing.ReferenceName) error {
+	path := d.fs.Join(".", name.String())
+	_, err := d.fs.Stat(path)
+	if err == nil {
+		err = d.fs.Remove(path)
+		// Drop down to remove it from the packed refs file, too.
+	}
+
+	if err != nil && !os.IsNotExist(err) {
+		return err
+	}
+
+	return d.rewritePackedRefsWithoutRef(name)
+}
+
+func (d *DotGit) addRefsFromPackedRefs(refs *[]*plumbing.Reference, seen map[plumbing.ReferenceName]bool) (err error) {
+	packedRefs, err := d.findPackedRefs()
+	if err != nil {
+		return err
+	}
+
+	for _, ref := range packedRefs {
+		if !seen[ref.Name()] {
+			*refs = append(*refs, ref)
+			seen[ref.Name()] = true
+		}
+	}
+	return nil
+}
+
+func (d *DotGit) addRefsFromPackedRefsFile(refs *[]*plumbing.Reference, f billy.File, seen map[plumbing.ReferenceName]bool) (err error) {
+	packedRefs, err := d.findPackedRefsInFile(f)
+	if err != nil {
+		return err
+	}
+
+	for _, ref := range packedRefs {
+		if !seen[ref.Name()] {
+			*refs = append(*refs, ref)
+			seen[ref.Name()] = true
+		}
+	}
+	return nil
+}
+
+func (d *DotGit) openAndLockPackedRefs(doCreate bool) (
+	pr billy.File, err error) {
+	var f billy.File
+	defer func() {
+		if err != nil && f != nil {
+			ioutil.CheckClose(f, &err)
+		}
+	}()
+
+	// File mode is retrieved from a constant defined in the target specific
+	// files (dotgit_rewrite_packed_refs_*). Some modes are not available
+	// in all filesystems.
+	openFlags := openAndLockPackedRefsMode
+	if doCreate {
+		openFlags |= os.O_CREATE
+	}
+
+	// Keep trying to open and lock the file until we're sure the file
+	// didn't change between the open and the lock.
+	for {
+		f, err = d.fs.OpenFile(packedRefsPath, openFlags, 0600)
+		if err != nil {
+			if os.IsNotExist(err) && !doCreate {
+				return nil, nil
+			}
+
+			return nil, err
+		}
+		fi, err := d.fs.Stat(packedRefsPath)
+		if err != nil {
+			return nil, err
+		}
+		mtime := fi.ModTime()
+
+		err = f.Lock()
+		if err != nil {
+			return nil, err
+		}
+
+		fi, err = d.fs.Stat(packedRefsPath)
+		if err != nil {
+			return nil, err
+		}
+		if mtime.Equal(fi.ModTime()) {
+			break
+		}
+		// The file has changed since we opened it.  Close and retry.
+		err = f.Close()
+		if err != nil {
+			return nil, err
+		}
+	}
+	return f, nil
+}
+
+func (d *DotGit) rewritePackedRefsWithoutRef(name plumbing.ReferenceName) (err error) {
+	pr, err := d.openAndLockPackedRefs(false)
+	if err != nil {
+		return err
+	}
+	if pr == nil {
+		return nil
+	}
+	defer ioutil.CheckClose(pr, &err)
+
+	// Creating the temp file in the same directory as the target file
+	// improves our chances for rename operation to be atomic.
+	tmp, err := d.fs.TempFile("", tmpPackedRefsPrefix)
+	if err != nil {
+		return err
+	}
+	tmpName := tmp.Name()
+	defer func() {
+		ioutil.CheckClose(tmp, &err)
+		_ = d.fs.Remove(tmpName) // don't check err, we might have renamed it
+	}()
+
+	s := bufio.NewScanner(pr)
+	found := false
+	for s.Scan() {
+		line := s.Text()
+		ref, err := d.processLine(line)
+		if err != nil {
+			return err
+		}
+
+		if ref != nil && ref.Name() == name {
+			found = true
+			continue
+		}
+
+		if _, err := fmt.Fprintln(tmp, line); err != nil {
+			return err
+		}
+	}
+
+	if err := s.Err(); err != nil {
+		return err
+	}
+
+	if !found {
+		return nil
+	}
+
+	return d.rewritePackedRefsWhileLocked(tmp, pr)
+}
+
+// process lines from a packed-refs file
+func (d *DotGit) processLine(line string) (*plumbing.Reference, error) {
+	if len(line) == 0 {
+		return nil, nil
+	}
+
+	switch line[0] {
+	case '#': // comment - ignore
+		return nil, nil
+	case '^': // annotated tag commit of the previous line - ignore
+		return nil, nil
+	default:
+		ws := strings.Split(line, " ") // hash then ref
+		if len(ws) != 2 {
+			return nil, ErrPackedRefsBadFormat
+		}
+
+		return plumbing.NewReferenceFromStrings(ws[1], ws[0]), nil
+	}
+}
+
+func (d *DotGit) addRefsFromRefDir(refs *[]*plumbing.Reference, seen map[plumbing.ReferenceName]bool) error {
+	return d.walkReferencesTree(refs, []string{refsPath}, seen)
+}
+
+func (d *DotGit) walkReferencesTree(refs *[]*plumbing.Reference, relPath []string, seen map[plumbing.ReferenceName]bool) error {
+	files, err := d.fs.ReadDir(d.fs.Join(relPath...))
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil
+		}
+
+		return err
+	}
+
+	for _, f := range files {
+		newRelPath := append(append([]string(nil), relPath...), f.Name())
+		if f.IsDir() {
+			if err = d.walkReferencesTree(refs, newRelPath, seen); err != nil {
+				return err
+			}
+
+			continue
+		}
+
+		ref, err := d.readReferenceFile(".", strings.Join(newRelPath, "/"))
+		if err != nil {
+			return err
+		}
+
+		if ref != nil && !seen[ref.Name()] {
+			*refs = append(*refs, ref)
+			seen[ref.Name()] = true
+		}
+	}
+
+	return nil
+}
+
+func (d *DotGit) addRefFromHEAD(refs *[]*plumbing.Reference) error {
+	ref, err := d.readReferenceFile(".", "HEAD")
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil
+		}
+
+		return err
+	}
+
+	*refs = append(*refs, ref)
+	return nil
+}
+
+func (d *DotGit) readReferenceFile(path, name string) (ref *plumbing.Reference, err error) {
+	path = d.fs.Join(path, d.fs.Join(strings.Split(name, "/")...))
+	f, err := d.fs.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer ioutil.CheckClose(f, &err)
+
+	return d.readReferenceFrom(f, name)
+}
+
+func (d *DotGit) CountLooseRefs() (int, error) {
+	var refs []*plumbing.Reference
+	var seen = make(map[plumbing.ReferenceName]bool)
+	if err := d.addRefsFromRefDir(&refs, seen); err != nil {
+		return 0, err
+	}
+
+	return len(refs), nil
+}
+
+// PackRefs packs all loose refs into the packed-refs file.
+//
+// This implementation only works under the assumption that the view
+// of the file system won't be updated during this operation.  This
+// strategy would not work on a general file system though, without
+// locking each loose reference and checking it again before deleting
+// the file, because otherwise an updated reference could sneak in and
+// then be deleted by the packed-refs process.  Alternatively, every
+// ref update could also lock packed-refs, so only one lock is
+// required during ref-packing.  But that would worsen performance in
+// the common case.
+//
+// TODO: add an "all" boolean like the `git pack-refs --all` flag.
+// When `all` is false, it would only pack refs that have already been
+// packed, plus all tags.
+func (d *DotGit) PackRefs() (err error) {
+	// Lock packed-refs, and create it if it doesn't exist yet.
+	f, err := d.openAndLockPackedRefs(true)
+	if err != nil {
+		return err
+	}
+	defer ioutil.CheckClose(f, &err)
+
+	// Gather all refs using addRefsFromRefDir and addRefsFromPackedRefs.
+	var refs []*plumbing.Reference
+	seen := make(map[plumbing.ReferenceName]bool)
+	if err = d.addRefsFromRefDir(&refs, seen); err != nil {
+		return err
+	}
+	if len(refs) == 0 {
+		// Nothing to do!
+		return nil
+	}
+	numLooseRefs := len(refs)
+	if err = d.addRefsFromPackedRefsFile(&refs, f, seen); err != nil {
+		return err
+	}
+
+	// Write them all to a new temp packed-refs file.
+	tmp, err := d.fs.TempFile("", tmpPackedRefsPrefix)
+	if err != nil {
+		return err
+	}
+	tmpName := tmp.Name()
+	defer func() {
+		ioutil.CheckClose(tmp, &err)
+		_ = d.fs.Remove(tmpName) // don't check err, we might have renamed it
+	}()
+
+	w := bufio.NewWriter(tmp)
+	for _, ref := range refs {
+		_, err = w.WriteString(ref.String() + "\n")
+		if err != nil {
+			return err
+		}
+	}
+	err = w.Flush()
+	if err != nil {
+		return err
+	}
+
+	// Rename the temp packed-refs file.
+	err = d.rewritePackedRefsWhileLocked(tmp, f)
+	if err != nil {
+		return err
+	}
+
+	// Delete all the loose refs, while still holding the packed-refs
+	// lock.
+	for _, ref := range refs[:numLooseRefs] {
+		path := d.fs.Join(".", ref.Name().String())
+		err = d.fs.Remove(path)
+		if err != nil && !os.IsNotExist(err) {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// Module return a billy.Filesystem pointing to the module folder
+func (d *DotGit) Module(name string) (billy.Filesystem, error) {
+	return d.fs.Chroot(d.fs.Join(modulePath, name))
+}
+
+// Alternates returns DotGit(s) based off paths in objects/info/alternates if
+// available. This can be used to checks if it's a shared repository.
+func (d *DotGit) Alternates() ([]*DotGit, error) {
+	altpath := d.fs.Join("objects", "info", "alternates")
+	f, err := d.fs.Open(altpath)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	var alternates []*DotGit
+
+	// Read alternate paths line-by-line and create DotGit objects.
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		path := scanner.Text()
+		if !filepath.IsAbs(path) {
+			// For relative paths, we can perform an internal conversion to
+			// slash so that they work cross-platform.
+			slashPath := filepath.ToSlash(path)
+			// If the path is not absolute, it must be relative to object
+			// database (.git/objects/info).
+			// https://www.kernel.org/pub/software/scm/git/docs/gitrepository-layout.html
+			// Hence, derive a path relative to DotGit's root.
+			// "../../../reponame/.git/" -> "../../reponame/.git"
+			// Remove the first ../
+			relpath := filepath.Join(strings.Split(slashPath, "/")[1:]...)
+			normalPath := filepath.FromSlash(relpath)
+			path = filepath.Join(d.fs.Root(), normalPath)
+		}
+		fs := osfs.New(filepath.Dir(path))
+		alternates = append(alternates, New(fs))
+	}
+
+	if err = scanner.Err(); err != nil {
+		return nil, err
+	}
+
+	return alternates, nil
+}
+
+func isHex(s string) bool {
+	for _, b := range []byte(s) {
+		if isNum(b) {
+			continue
+		}
+		if isHexAlpha(b) {
+			continue
+		}
+
+		return false
+	}
+
+	return true
+}
+
+func isNum(b byte) bool {
+	return b >= '0' && b <= '9'
+}
+
+func isHexAlpha(b byte) bool {
+	return b >= 'a' && b <= 'f' || b >= 'A' && b <= 'F'
+}
diff --git a/storage/filesystem/dotgit/dotgit_rewrite_packed_refs_nix.go b/storage/filesystem/dotgit/dotgit_rewrite_packed_refs_nix.go
new file mode 100644
index 0000000..c760793
--- /dev/null
+++ b/storage/filesystem/dotgit/dotgit_rewrite_packed_refs_nix.go
@@ -0,0 +1,17 @@
+// +build !windows,!norwfs
+
+package dotgit
+
+import (
+	"os"
+
+	"gopkg.in/src-d/go-billy.v4"
+)
+
+const openAndLockPackedRefsMode = os.O_RDWR
+
+func (d *DotGit) rewritePackedRefsWhileLocked(
+	tmp billy.File, pr billy.File) error {
+	// On non-Windows platforms, we can have atomic rename.
+	return d.fs.Rename(tmp.Name(), pr.Name())
+}
diff --git a/storage/filesystem/dotgit/dotgit_rewrite_packed_refs_norwfs.go b/storage/filesystem/dotgit/dotgit_rewrite_packed_refs_norwfs.go
new file mode 100644
index 0000000..6e43b42
--- /dev/null
+++ b/storage/filesystem/dotgit/dotgit_rewrite_packed_refs_norwfs.go
@@ -0,0 +1,34 @@
+// +build norwfs
+
+package dotgit
+
+import (
+	"io"
+	"os"
+
+	"gopkg.in/src-d/go-billy.v4"
+)
+
+const openAndLockPackedRefsMode = os.O_RDONLY
+
+// Instead of renaming that can not be supported in simpler filesystems
+// a full copy is done.
+func (d *DotGit) rewritePackedRefsWhileLocked(
+	tmp billy.File, pr billy.File) error {
+
+	prWrite, err := d.fs.Create(pr.Name())
+	if err != nil {
+		return err
+	}
+
+	defer prWrite.Close()
+
+	_, err = tmp.Seek(0, io.SeekStart)
+	if err != nil {
+		return err
+	}
+
+	_, err = io.Copy(prWrite, tmp)
+
+	return err
+}
diff --git a/storage/filesystem/dotgit/dotgit_rewrite_packed_refs_windows.go b/storage/filesystem/dotgit/dotgit_rewrite_packed_refs_windows.go
new file mode 100644
index 0000000..897d2c9
--- /dev/null
+++ b/storage/filesystem/dotgit/dotgit_rewrite_packed_refs_windows.go
@@ -0,0 +1,42 @@
+// +build windows,!norwfs
+
+package dotgit
+
+import (
+	"io"
+	"os"
+
+	"gopkg.in/src-d/go-billy.v4"
+)
+
+const openAndLockPackedRefsMode = os.O_RDWR
+
+func (d *DotGit) rewritePackedRefsWhileLocked(
+	tmp billy.File, pr billy.File) error {
+	// If we aren't using the bare Windows filesystem as the storage
+	// layer, we might be able to get away with a rename over a locked
+	// file.
+	err := d.fs.Rename(tmp.Name(), pr.Name())
+	if err == nil {
+		return nil
+	}
+
+	// Otherwise, Windows doesn't let us rename over a locked file, so
+	// we have to do a straight copy.  Unfortunately this could result
+	// in a partially-written file if the process fails before the
+	// copy completes.
+	_, err = pr.Seek(0, io.SeekStart)
+	if err != nil {
+		return err
+	}
+	err = pr.Truncate(0)
+	if err != nil {
+		return err
+	}
+	_, err = tmp.Seek(0, io.SeekStart)
+	if err != nil {
+		return err
+	}
+	_, err = io.Copy(pr, tmp)
+	return err
+}
diff --git a/storage/filesystem/dotgit/dotgit_setref.go b/storage/filesystem/dotgit/dotgit_setref.go
new file mode 100644
index 0000000..d27c1a3
--- /dev/null
+++ b/storage/filesystem/dotgit/dotgit_setref.go
@@ -0,0 +1,43 @@
+// +build !norwfs
+
+package dotgit
+
+import (
+	"os"
+
+	"gopkg.in/src-d/go-git.v4/plumbing"
+	"gopkg.in/src-d/go-git.v4/utils/ioutil"
+)
+
+func (d *DotGit) setRef(fileName, content string, old *plumbing.Reference) (err error) {
+	// If we are not checking an old ref, just truncate the file.
+	mode := os.O_RDWR | os.O_CREATE
+	if old == nil {
+		mode |= os.O_TRUNC
+	}
+
+	f, err := d.fs.OpenFile(fileName, mode, 0666)
+	if err != nil {
+		return err
+	}
+
+	defer ioutil.CheckClose(f, &err)
+
+	// Lock is unlocked by the deferred Close above. This is because Unlock
+	// does not imply a fsync and thus there would be a race between
+	// Unlock+Close and other concurrent writers. Adding Sync to go-billy
+	// could work, but this is better (and avoids superfluous syncs).
+	err = f.Lock()
+	if err != nil {
+		return err
+	}
+
+	// this is a no-op to call even when old is nil.
+	err = d.checkReferenceAndTruncate(f, old)
+	if err != nil {
+		return err
+	}
+
+	_, err = f.Write([]byte(content))
+	return err
+}
diff --git a/storage/filesystem/dotgit/dotgit_setref_norwfs.go b/storage/filesystem/dotgit/dotgit_setref_norwfs.go
new file mode 100644
index 0000000..5695bd3
--- /dev/null
+++ b/storage/filesystem/dotgit/dotgit_setref_norwfs.go
@@ -0,0 +1,47 @@
+// +build norwfs
+
+package dotgit
+
+import (
+	"fmt"
+
+	"gopkg.in/src-d/go-git.v4/plumbing"
+)
+
+// There are some filesystems that don't support opening files in RDWD mode.
+// In these filesystems the standard SetRef function can not be used as i
+// reads the reference file to check that it's not modified before updating it.
+//
+// This version of the function writes the reference without extra checks
+// making it compatible with these simple filesystems. This is usually not
+// a problem as they should be accessed by only one process at a time.
+func (d *DotGit) setRef(fileName, content string, old *plumbing.Reference) error {
+	_, err := d.fs.Stat(fileName)
+	if err == nil && old != nil {
+		fRead, err := d.fs.Open(fileName)
+		if err != nil {
+			return err
+		}
+
+		ref, err := d.readReferenceFrom(fRead, old.Name().String())
+		fRead.Close()
+
+		if err != nil {
+			return err
+		}
+
+		if ref.Hash() != old.Hash() {
+			return fmt.Errorf("reference has changed concurrently")
+		}
+	}
+
+	f, err := d.fs.Create(fileName)
+	if err != nil {
+		return err
+	}
+
+	defer f.Close()
+
+	_, err = f.Write([]byte(content))
+	return err
+}
diff --git a/storage/filesystem/dotgit/dotgit_test.go b/storage/filesystem/dotgit/dotgit_test.go
new file mode 100644
index 0000000..7733eef
--- /dev/null
+++ b/storage/filesystem/dotgit/dotgit_test.go
@@ -0,0 +1,683 @@
+package dotgit
+
+import (
+	"bufio"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+
+	"gopkg.in/src-d/go-git.v4/plumbing"
+
+	. "gopkg.in/check.v1"
+	"gopkg.in/src-d/go-billy.v4/osfs"
+	"gopkg.in/src-d/go-git-fixtures.v3"
+)
+
+func Test(t *testing.T) { TestingT(t) }
+
+type SuiteDotGit struct {
+	fixtures.Suite
+}
+
+var _ = Suite(&SuiteDotGit{})
+
+func (s *SuiteDotGit) TestInitialize(c *C) {
+	tmp, err := ioutil.TempDir("", "dot-git")
+	c.Assert(err, IsNil)
+	defer os.RemoveAll(tmp)
+
+	fs := osfs.New(tmp)
+	dir := New(fs)
+
+	err = dir.Initialize()
+	c.Assert(err, IsNil)
+
+	_, err = fs.Stat(fs.Join("objects", "info"))
+	c.Assert(err, IsNil)
+
+	_, err = fs.Stat(fs.Join("objects", "pack"))
+	c.Assert(err, IsNil)
+
+	_, err = fs.Stat(fs.Join("refs", "heads"))
+	c.Assert(err, IsNil)
+
+	_, err = fs.Stat(fs.Join("refs", "tags"))
+	c.Assert(err, IsNil)
+}
+
+func (s *SuiteDotGit) TestSetRefs(c *C) {
+	tmp, err := ioutil.TempDir("", "dot-git")
+	c.Assert(err, IsNil)
+	defer os.RemoveAll(tmp)
+
+	fs := osfs.New(tmp)
+	dir := New(fs)
+
+	firstFoo := plumbing.NewReferenceFromStrings(
+		"refs/heads/foo",
+		"e8d3ffab552895c19b9fcf7aa264d277cde33881",
+	)
+	err = dir.SetRef(firstFoo, nil)
+
+	c.Assert(err, IsNil)
+
+	err = dir.SetRef(plumbing.NewReferenceFromStrings(
+		"refs/heads/symbolic",
+		"ref: refs/heads/foo",
+	), nil)
+
+	c.Assert(err, IsNil)
+
+	err = dir.SetRef(plumbing.NewReferenceFromStrings(
+		"bar",
+		"e8d3ffab552895c19b9fcf7aa264d277cde33881",
+	), nil)
+	c.Assert(err, IsNil)
+
+	refs, err := dir.Refs()
+	c.Assert(err, IsNil)
+	c.Assert(refs, HasLen, 2)
+
+	ref := findReference(refs, "refs/heads/foo")
+	c.Assert(ref, NotNil)
+	c.Assert(ref.Hash().String(), Equals, "e8d3ffab552895c19b9fcf7aa264d277cde33881")
+
+	ref = findReference(refs, "refs/heads/symbolic")
+	c.Assert(ref, NotNil)
+	c.Assert(ref.Target().String(), Equals, "refs/heads/foo")
+
+	ref = findReference(refs, "bar")
+	c.Assert(ref, IsNil)
+
+	ref, err = dir.Ref("refs/heads/foo")
+	c.Assert(err, IsNil)
+	c.Assert(ref, NotNil)
+	c.Assert(ref.Hash().String(), Equals, "e8d3ffab552895c19b9fcf7aa264d277cde33881")
+
+	ref, err = dir.Ref("refs/heads/symbolic")
+	c.Assert(err, IsNil)
+	c.Assert(ref, NotNil)
+	c.Assert(ref.Target().String(), Equals, "refs/heads/foo")
+
+	ref, err = dir.Ref("bar")
+	c.Assert(err, IsNil)
+	c.Assert(ref, NotNil)
+	c.Assert(ref.Hash().String(), Equals, "e8d3ffab552895c19b9fcf7aa264d277cde33881")
+
+	// Check that SetRef with a non-nil `old` works.
+	err = dir.SetRef(plumbing.NewReferenceFromStrings(
+		"refs/heads/foo",
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5",
+	), firstFoo)
+	c.Assert(err, IsNil)
+
+	// `firstFoo` is no longer the right `old` reference, so this
+	// should fail.
+	err = dir.SetRef(plumbing.NewReferenceFromStrings(
+		"refs/heads/foo",
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5",
+	), firstFoo)
+	c.Assert(err, NotNil)
+}
+
+func (s *SuiteDotGit) TestRefsFromPackedRefs(c *C) {
+	fs := fixtures.Basic().ByTag(".git").One().DotGit()
+	dir := New(fs)
+
+	refs, err := dir.Refs()
+	c.Assert(err, IsNil)
+
+	ref := findReference(refs, "refs/remotes/origin/branch")
+	c.Assert(ref, NotNil)
+	c.Assert(ref.Hash().String(), Equals, "e8d3ffab552895c19b9fcf7aa264d277cde33881")
+
+}
+
+func (s *SuiteDotGit) TestRefsFromReferenceFile(c *C) {
+	fs := fixtures.Basic().ByTag(".git").One().DotGit()
+	dir := New(fs)
+
+	refs, err := dir.Refs()
+	c.Assert(err, IsNil)
+
+	ref := findReference(refs, "refs/remotes/origin/HEAD")
+	c.Assert(ref, NotNil)
+	c.Assert(ref.Type(), Equals, plumbing.SymbolicReference)
+	c.Assert(string(ref.Target()), Equals, "refs/remotes/origin/master")
+
+}
+
+func BenchmarkRefMultipleTimes(b *testing.B) {
+	fixtures.Init()
+	fs := fixtures.Basic().ByTag(".git").One().DotGit()
+	refname := plumbing.ReferenceName("refs/remotes/origin/branch")
+
+	dir := New(fs)
+	_, err := dir.Ref(refname)
+	if err != nil {
+		b.Fatalf("unexpected error: %s", err)
+	}
+
+	for i := 0; i < b.N; i++ {
+		_, err := dir.Ref(refname)
+		if err != nil {
+			b.Fatalf("unexpected error: %s", err)
+		}
+	}
+}
+
+func (s *SuiteDotGit) TestRemoveRefFromReferenceFile(c *C) {
+	fs := fixtures.Basic().ByTag(".git").One().DotGit()
+	dir := New(fs)
+
+	name := plumbing.ReferenceName("refs/remotes/origin/HEAD")
+	err := dir.RemoveRef(name)
+	c.Assert(err, IsNil)
+
+	refs, err := dir.Refs()
+	c.Assert(err, IsNil)
+
+	ref := findReference(refs, string(name))
+	c.Assert(ref, IsNil)
+}
+
+func (s *SuiteDotGit) TestRemoveRefFromPackedRefs(c *C) {
+	fs := fixtures.Basic().ByTag(".git").One().DotGit()
+	dir := New(fs)
+
+	name := plumbing.ReferenceName("refs/remotes/origin/master")
+	err := dir.RemoveRef(name)
+	c.Assert(err, IsNil)
+
+	b, err := ioutil.ReadFile(filepath.Join(fs.Root(), packedRefsPath))
+	c.Assert(err, IsNil)
+
+	c.Assert(string(b), Equals, ""+
+		"# pack-refs with: peeled fully-peeled \n"+
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 refs/heads/master\n"+
+		"e8d3ffab552895c19b9fcf7aa264d277cde33881 refs/remotes/origin/branch\n")
+}
+
+func (s *SuiteDotGit) TestRemoveRefFromReferenceFileAndPackedRefs(c *C) {
+	fs := fixtures.Basic().ByTag(".git").One().DotGit()
+	dir := New(fs)
+
+	// Make a ref file for a ref that's already in `packed-refs`.
+	err := dir.SetRef(plumbing.NewReferenceFromStrings(
+		"refs/remotes/origin/branch",
+		"e8d3ffab552895c19b9fcf7aa264d277cde33881",
+	), nil)
+
+	// Make sure it only appears once in the refs list.
+	refs, err := dir.Refs()
+	c.Assert(err, IsNil)
+	found := false
+	for _, ref := range refs {
+		if ref.Name() == "refs/remotes/origin/branch" {
+			c.Assert(found, Equals, false)
+			found = true
+		}
+	}
+
+	name := plumbing.ReferenceName("refs/remotes/origin/branch")
+	err = dir.RemoveRef(name)
+	c.Assert(err, IsNil)
+
+	b, err := ioutil.ReadFile(filepath.Join(fs.Root(), packedRefsPath))
+	c.Assert(err, IsNil)
+
+	c.Assert(string(b), Equals, ""+
+		"# pack-refs with: peeled fully-peeled \n"+
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 refs/heads/master\n"+
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5 refs/remotes/origin/master\n")
+
+	refs, err = dir.Refs()
+	c.Assert(err, IsNil)
+
+	ref := findReference(refs, string(name))
+	c.Assert(ref, IsNil)
+}
+
+func (s *SuiteDotGit) TestRemoveRefNonExistent(c *C) {
+	fs := fixtures.Basic().ByTag(".git").One().DotGit()
+	dir := New(fs)
+
+	packedRefs := filepath.Join(fs.Root(), packedRefsPath)
+	before, err := ioutil.ReadFile(packedRefs)
+	c.Assert(err, IsNil)
+
+	name := plumbing.ReferenceName("refs/heads/nonexistent")
+	err = dir.RemoveRef(name)
+	c.Assert(err, IsNil)
+
+	after, err := ioutil.ReadFile(packedRefs)
+	c.Assert(err, IsNil)
+
+	c.Assert(string(before), Equals, string(after))
+}
+
+func (s *SuiteDotGit) TestRemoveRefInvalidPackedRefs(c *C) {
+	fs := fixtures.Basic().ByTag(".git").One().DotGit()
+	dir := New(fs)
+
+	packedRefs := filepath.Join(fs.Root(), packedRefsPath)
+	brokenContent := "BROKEN STUFF REALLY BROKEN"
+
+	err := ioutil.WriteFile(packedRefs, []byte(brokenContent), os.FileMode(0755))
+	c.Assert(err, IsNil)
+
+	name := plumbing.ReferenceName("refs/heads/nonexistent")
+	err = dir.RemoveRef(name)
+	c.Assert(err, NotNil)
+
+	after, err := ioutil.ReadFile(filepath.Join(fs.Root(), packedRefsPath))
+	c.Assert(err, IsNil)
+
+	c.Assert(brokenContent, Equals, string(after))
+}
+
+func (s *SuiteDotGit) TestRemoveRefInvalidPackedRefs2(c *C) {
+	fs := fixtures.Basic().ByTag(".git").One().DotGit()
+	dir := New(fs)
+
+	packedRefs := filepath.Join(fs.Root(), packedRefsPath)
+	brokenContent := strings.Repeat("a", bufio.MaxScanTokenSize*2)
+
+	err := ioutil.WriteFile(packedRefs, []byte(brokenContent), os.FileMode(0755))
+	c.Assert(err, IsNil)
+
+	name := plumbing.ReferenceName("refs/heads/nonexistent")
+	err = dir.RemoveRef(name)
+	c.Assert(err, NotNil)
+
+	after, err := ioutil.ReadFile(filepath.Join(fs.Root(), packedRefsPath))
+	c.Assert(err, IsNil)
+
+	c.Assert(brokenContent, Equals, string(after))
+}
+
+func (s *SuiteDotGit) TestRefsFromHEADFile(c *C) {
+	fs := fixtures.Basic().ByTag(".git").One().DotGit()
+	dir := New(fs)
+
+	refs, err := dir.Refs()
+	c.Assert(err, IsNil)
+
+	ref := findReference(refs, "HEAD")
+	c.Assert(ref, NotNil)
+	c.Assert(ref.Type(), Equals, plumbing.SymbolicReference)
+	c.Assert(string(ref.Target()), Equals, "refs/heads/master")
+}
+
+func (s *SuiteDotGit) TestConfig(c *C) {
+	fs := fixtures.Basic().ByTag(".git").One().DotGit()
+	dir := New(fs)
+
+	file, err := dir.Config()
+	c.Assert(err, IsNil)
+	c.Assert(filepath.Base(file.Name()), Equals, "config")
+}
+
+func (s *SuiteDotGit) TestConfigWriteAndConfig(c *C) {
+	tmp, err := ioutil.TempDir("", "dot-git")
+	c.Assert(err, IsNil)
+	defer os.RemoveAll(tmp)
+
+	fs := osfs.New(tmp)
+	dir := New(fs)
+
+	f, err := dir.ConfigWriter()
+	c.Assert(err, IsNil)
+
+	_, err = f.Write([]byte("foo"))
+	c.Assert(err, IsNil)
+
+	f, err = dir.Config()
+	c.Assert(err, IsNil)
+
+	cnt, err := ioutil.ReadAll(f)
+	c.Assert(err, IsNil)
+
+	c.Assert(string(cnt), Equals, "foo")
+}
+
+func (s *SuiteDotGit) TestIndex(c *C) {
+	fs := fixtures.Basic().ByTag(".git").One().DotGit()
+	dir := New(fs)
+
+	idx, err := dir.Index()
+	c.Assert(err, IsNil)
+	c.Assert(idx, NotNil)
+}
+
+func (s *SuiteDotGit) TestIndexWriteAndIndex(c *C) {
+	tmp, err := ioutil.TempDir("", "dot-git")
+	c.Assert(err, IsNil)
+	defer os.RemoveAll(tmp)
+
+	fs := osfs.New(tmp)
+	dir := New(fs)
+
+	f, err := dir.IndexWriter()
+	c.Assert(err, IsNil)
+
+	_, err = f.Write([]byte("foo"))
+	c.Assert(err, IsNil)
+
+	f, err = dir.Index()
+	c.Assert(err, IsNil)
+
+	cnt, err := ioutil.ReadAll(f)
+	c.Assert(err, IsNil)
+
+	c.Assert(string(cnt), Equals, "foo")
+}
+
+func (s *SuiteDotGit) TestShallow(c *C) {
+	fs := fixtures.Basic().ByTag(".git").One().DotGit()
+	dir := New(fs)
+
+	file, err := dir.Shallow()
+	c.Assert(err, IsNil)
+	c.Assert(file, IsNil)
+}
+
+func (s *SuiteDotGit) TestShallowWriteAndShallow(c *C) {
+	tmp, err := ioutil.TempDir("", "dot-git")
+	c.Assert(err, IsNil)
+	defer os.RemoveAll(tmp)
+
+	fs := osfs.New(tmp)
+	dir := New(fs)
+
+	f, err := dir.ShallowWriter()
+	c.Assert(err, IsNil)
+
+	_, err = f.Write([]byte("foo"))
+	c.Assert(err, IsNil)
+
+	f, err = dir.Shallow()
+	c.Assert(err, IsNil)
+
+	cnt, err := ioutil.ReadAll(f)
+	c.Assert(err, IsNil)
+
+	c.Assert(string(cnt), Equals, "foo")
+}
+
+func findReference(refs []*plumbing.Reference, name string) *plumbing.Reference {
+	n := plumbing.ReferenceName(name)
+	for _, ref := range refs {
+		if ref.Name() == n {
+			return ref
+		}
+	}
+
+	return nil
+}
+
+func (s *SuiteDotGit) TestObjectPacks(c *C) {
+	f := fixtures.Basic().ByTag(".git").One()
+	fs := f.DotGit()
+	dir := New(fs)
+
+	hashes, err := dir.ObjectPacks()
+	c.Assert(err, IsNil)
+	c.Assert(hashes, HasLen, 1)
+	c.Assert(hashes[0], Equals, f.PackfileHash)
+
+	// Make sure that a random file in the pack directory doesn't
+	// break everything.
+	badFile, err := fs.Create("objects/pack/OOPS_THIS_IS_NOT_RIGHT.pack")
+	c.Assert(err, IsNil)
+	err = badFile.Close()
+	c.Assert(err, IsNil)
+
+	hashes2, err := dir.ObjectPacks()
+	c.Assert(err, IsNil)
+	c.Assert(hashes2, HasLen, 1)
+	c.Assert(hashes[0], Equals, hashes2[0])
+}
+
+func (s *SuiteDotGit) TestObjectPack(c *C) {
+	f := fixtures.Basic().ByTag(".git").One()
+	fs := f.DotGit()
+	dir := New(fs)
+
+	pack, err := dir.ObjectPack(f.PackfileHash)
+	c.Assert(err, IsNil)
+	c.Assert(filepath.Ext(pack.Name()), Equals, ".pack")
+}
+
+func (s *SuiteDotGit) TestObjectPackIdx(c *C) {
+	f := fixtures.Basic().ByTag(".git").One()
+	fs := f.DotGit()
+	dir := New(fs)
+
+	idx, err := dir.ObjectPackIdx(f.PackfileHash)
+	c.Assert(err, IsNil)
+	c.Assert(filepath.Ext(idx.Name()), Equals, ".idx")
+	c.Assert(idx.Close(), IsNil)
+}
+
+func (s *SuiteDotGit) TestObjectPackNotFound(c *C) {
+	fs := fixtures.Basic().ByTag(".git").One().DotGit()
+	dir := New(fs)
+
+	pack, err := dir.ObjectPack(plumbing.ZeroHash)
+	c.Assert(err, Equals, ErrPackfileNotFound)
+	c.Assert(pack, IsNil)
+
+	idx, err := dir.ObjectPackIdx(plumbing.ZeroHash)
+	c.Assert(err, Equals, ErrPackfileNotFound)
+	c.Assert(idx, IsNil)
+}
+
+func (s *SuiteDotGit) TestNewObject(c *C) {
+	tmp, err := ioutil.TempDir("", "dot-git")
+	c.Assert(err, IsNil)
+	defer os.RemoveAll(tmp)
+
+	fs := osfs.New(tmp)
+	dir := New(fs)
+	w, err := dir.NewObject()
+	c.Assert(err, IsNil)
+
+	err = w.WriteHeader(plumbing.BlobObject, 14)
+	c.Assert(err, IsNil)
+	n, err := w.Write([]byte("this is a test"))
+	c.Assert(err, IsNil)
+	c.Assert(n, Equals, 14)
+
+	c.Assert(w.Hash().String(), Equals, "a8a940627d132695a9769df883f85992f0ff4a43")
+
+	err = w.Close()
+	c.Assert(err, IsNil)
+
+	i, err := fs.Stat("objects/a8/a940627d132695a9769df883f85992f0ff4a43")
+	c.Assert(err, IsNil)
+	c.Assert(i.Size(), Equals, int64(34))
+}
+
+func (s *SuiteDotGit) TestObjects(c *C) {
+	fs := fixtures.ByTag(".git").ByTag("unpacked").One().DotGit()
+	dir := New(fs)
+
+	hashes, err := dir.Objects()
+	c.Assert(err, IsNil)
+	c.Assert(hashes, HasLen, 187)
+	c.Assert(hashes[0].String(), Equals, "0097821d427a3c3385898eb13b50dcbc8702b8a3")
+	c.Assert(hashes[1].String(), Equals, "01d5fa556c33743006de7e76e67a2dfcd994ca04")
+	c.Assert(hashes[2].String(), Equals, "03db8e1fbe133a480f2867aac478fd866686d69e")
+}
+
+func (s *SuiteDotGit) TestObjectsNoFolder(c *C) {
+	tmp, err := ioutil.TempDir("", "dot-git")
+	c.Assert(err, IsNil)
+	defer os.RemoveAll(tmp)
+
+	fs := osfs.New(tmp)
+	dir := New(fs)
+	hash, err := dir.Objects()
+	c.Assert(err, IsNil)
+	c.Assert(hash, HasLen, 0)
+}
+
+func (s *SuiteDotGit) TestObject(c *C) {
+	fs := fixtures.ByTag(".git").ByTag("unpacked").One().DotGit()
+	dir := New(fs)
+
+	hash := plumbing.NewHash("03db8e1fbe133a480f2867aac478fd866686d69e")
+	file, err := dir.Object(hash)
+	c.Assert(err, IsNil)
+	c.Assert(strings.HasSuffix(
+		file.Name(), fs.Join("objects", "03", "db8e1fbe133a480f2867aac478fd866686d69e")),
+		Equals, true,
+	)
+}
+
+func (s *SuiteDotGit) TestObjectNotFound(c *C) {
+	fs := fixtures.ByTag(".git").ByTag("unpacked").One().DotGit()
+	dir := New(fs)
+
+	hash := plumbing.NewHash("not-found-object")
+	file, err := dir.Object(hash)
+	c.Assert(err, NotNil)
+	c.Assert(file, IsNil)
+}
+
+func (s *SuiteDotGit) TestSubmodules(c *C) {
+	fs := fixtures.ByTag("submodule").One().DotGit()
+	dir := New(fs)
+
+	m, err := dir.Module("basic")
+	c.Assert(err, IsNil)
+	c.Assert(strings.HasSuffix(m.Root(), m.Join(".git", "modules", "basic")), Equals, true)
+}
+
+func (s *SuiteDotGit) TestPackRefs(c *C) {
+	tmp, err := ioutil.TempDir("", "dot-git")
+	c.Assert(err, IsNil)
+	defer os.RemoveAll(tmp)
+
+	fs := osfs.New(tmp)
+	dir := New(fs)
+
+	err = dir.SetRef(plumbing.NewReferenceFromStrings(
+		"refs/heads/foo",
+		"e8d3ffab552895c19b9fcf7aa264d277cde33881",
+	), nil)
+	c.Assert(err, IsNil)
+	err = dir.SetRef(plumbing.NewReferenceFromStrings(
+		"refs/heads/bar",
+		"a8d3ffab552895c19b9fcf7aa264d277cde33881",
+	), nil)
+	c.Assert(err, IsNil)
+
+	refs, err := dir.Refs()
+	c.Assert(err, IsNil)
+	c.Assert(refs, HasLen, 2)
+	looseCount, err := dir.CountLooseRefs()
+	c.Assert(err, IsNil)
+	c.Assert(looseCount, Equals, 2)
+
+	err = dir.PackRefs()
+	c.Assert(err, IsNil)
+
+	// Make sure the refs are still there, but no longer loose.
+	refs, err = dir.Refs()
+	c.Assert(err, IsNil)
+	c.Assert(refs, HasLen, 2)
+	looseCount, err = dir.CountLooseRefs()
+	c.Assert(err, IsNil)
+	c.Assert(looseCount, Equals, 0)
+
+	ref, err := dir.Ref("refs/heads/foo")
+	c.Assert(err, IsNil)
+	c.Assert(ref, NotNil)
+	c.Assert(ref.Hash().String(), Equals, "e8d3ffab552895c19b9fcf7aa264d277cde33881")
+	ref, err = dir.Ref("refs/heads/bar")
+	c.Assert(err, IsNil)
+	c.Assert(ref, NotNil)
+	c.Assert(ref.Hash().String(), Equals, "a8d3ffab552895c19b9fcf7aa264d277cde33881")
+
+	// Now update one of them, re-pack, and check again.
+	err = dir.SetRef(plumbing.NewReferenceFromStrings(
+		"refs/heads/foo",
+		"b8d3ffab552895c19b9fcf7aa264d277cde33881",
+	), nil)
+	c.Assert(err, IsNil)
+	looseCount, err = dir.CountLooseRefs()
+	c.Assert(err, IsNil)
+	c.Assert(looseCount, Equals, 1)
+	err = dir.PackRefs()
+	c.Assert(err, IsNil)
+
+	// Make sure the refs are still there, but no longer loose.
+	refs, err = dir.Refs()
+	c.Assert(err, IsNil)
+	c.Assert(refs, HasLen, 2)
+	looseCount, err = dir.CountLooseRefs()
+	c.Assert(err, IsNil)
+	c.Assert(looseCount, Equals, 0)
+
+	ref, err = dir.Ref("refs/heads/foo")
+	c.Assert(err, IsNil)
+	c.Assert(ref, NotNil)
+	c.Assert(ref.Hash().String(), Equals, "b8d3ffab552895c19b9fcf7aa264d277cde33881")
+}
+
+func (s *SuiteDotGit) TestAlternates(c *C) {
+	tmp, err := ioutil.TempDir("", "dot-git")
+	c.Assert(err, IsNil)
+	defer os.RemoveAll(tmp)
+
+	// Create a new billy fs.
+	fs := osfs.New(tmp)
+
+	// Create a new dotgit object and initialize.
+	dir := New(fs)
+	err = dir.Initialize()
+	c.Assert(err, IsNil)
+
+	// Create alternates file.
+	altpath := filepath.Join("objects", "info", "alternates")
+	f, err := fs.Create(altpath)
+	c.Assert(err, IsNil)
+
+	// Multiple alternates.
+	var strContent string
+	if runtime.GOOS == "windows" {
+		strContent = "C:\\Users\\username\\repo1\\.git\\objects\r\n..\\..\\..\\rep2\\.git\\objects"
+	} else {
+		strContent = "/Users/username/rep1//.git/objects\n../../../rep2//.git/objects"
+	}
+	content := []byte(strContent)
+	f.Write(content)
+	f.Close()
+
+	dotgits, err := dir.Alternates()
+	c.Assert(err, IsNil)
+	if runtime.GOOS == "windows" {
+		c.Assert(dotgits[0].fs.Root(), Equals, "C:\\Users\\username\\repo1\\.git")
+	} else {
+		c.Assert(dotgits[0].fs.Root(), Equals, "/Users/username/rep1/.git")
+	}
+
+	// For relative path:
+	// /some/absolute/path/to/dot-git -> /some/absolute/path
+	pathx := strings.Split(tmp, string(filepath.Separator))
+	pathx = pathx[:len(pathx)-2]
+	// Use string.Join() to avoid malformed absolutepath on windows
+	// C:Users\\User\\... instead of C:\\Users\\appveyor\\... .
+	resolvedPath := strings.Join(pathx, string(filepath.Separator))
+	// Append the alternate path to the resolvedPath
+	expectedPath := filepath.Join(string(filepath.Separator), resolvedPath, "rep2", ".git")
+	if runtime.GOOS == "windows" {
+		expectedPath = filepath.Join(resolvedPath, "rep2", ".git")
+	}
+	c.Assert(dotgits[1].fs.Root(), Equals, expectedPath)
+}
diff --git a/storage/filesystem/dotgit/writers.go b/storage/filesystem/dotgit/writers.go
new file mode 100644
index 0000000..c2b420f
--- /dev/null
+++ b/storage/filesystem/dotgit/writers.go
@@ -0,0 +1,282 @@
+package dotgit
+
+import (
+	"fmt"
+	"io"
+	"sync/atomic"
+
+	"gopkg.in/src-d/go-git.v4/plumbing"
+	"gopkg.in/src-d/go-git.v4/plumbing/format/idxfile"
+	"gopkg.in/src-d/go-git.v4/plumbing/format/objfile"
+	"gopkg.in/src-d/go-git.v4/plumbing/format/packfile"
+
+	"gopkg.in/src-d/go-billy.v4"
+)
+
+// PackWriter is a io.Writer that generates the packfile index simultaneously,
+// a packfile.Decoder is used with a file reader to read the file being written
+// this operation is synchronized with the write operations.
+// The packfile is written in a temp file, when Close is called this file
+// is renamed/moved (depends on the Filesystem implementation) to the final
+// location, if the PackWriter is not used, nothing is written
+type PackWriter struct {
+	Notify func(plumbing.Hash, *packfile.Index)
+
+	fs       billy.Filesystem
+	fr, fw   billy.File
+	synced   *syncedReader
+	checksum plumbing.Hash
+	index    *packfile.Index
+	result   chan error
+}
+
+func newPackWrite(fs billy.Filesystem) (*PackWriter, error) {
+	fw, err := fs.TempFile(fs.Join(objectsPath, packPath), "tmp_pack_")
+	if err != nil {
+		return nil, err
+	}
+
+	fr, err := fs.Open(fw.Name())
+	if err != nil {
+		return nil, err
+	}
+
+	writer := &PackWriter{
+		fs:     fs,
+		fw:     fw,
+		fr:     fr,
+		synced: newSyncedReader(fw, fr),
+		result: make(chan error),
+	}
+
+	go writer.buildIndex()
+	return writer, nil
+}
+
+func (w *PackWriter) buildIndex() {
+	s := packfile.NewScanner(w.synced)
+	d, err := packfile.NewDecoder(s, nil)
+	if err != nil {
+		w.result <- err
+		return
+	}
+
+	checksum, err := d.Decode()
+	if err != nil {
+		w.result <- err
+		return
+	}
+
+	w.checksum = checksum
+	w.index = d.Index()
+	w.result <- err
+}
+
+// waitBuildIndex waits until buildIndex function finishes, this can terminate
+// with a packfile.ErrEmptyPackfile, this means that nothing was written so we
+// ignore the error
+func (w *PackWriter) waitBuildIndex() error {
+	err := <-w.result
+	if err == packfile.ErrEmptyPackfile {
+		return nil
+	}
+
+	return err
+}
+
+func (w *PackWriter) Write(p []byte) (int, error) {
+	return w.synced.Write(p)
+}
+
+// Close closes all the file descriptors and save the final packfile, if nothing
+// was written, the tempfiles are deleted without writing a packfile.
+func (w *PackWriter) Close() error {
+	defer func() {
+		if w.Notify != nil && w.index != nil && w.index.Size() > 0 {
+			w.Notify(w.checksum, w.index)
+		}
+
+		close(w.result)
+	}()
+
+	if err := w.synced.Close(); err != nil {
+		return err
+	}
+
+	if err := w.waitBuildIndex(); err != nil {
+		return err
+	}
+
+	if err := w.fr.Close(); err != nil {
+		return err
+	}
+
+	if err := w.fw.Close(); err != nil {
+		return err
+	}
+
+	if w.index == nil || w.index.Size() == 0 {
+		return w.clean()
+	}
+
+	return w.save()
+}
+
+func (w *PackWriter) clean() error {
+	return w.fs.Remove(w.fw.Name())
+}
+
+func (w *PackWriter) save() error {
+	base := w.fs.Join(objectsPath, packPath, fmt.Sprintf("pack-%s", w.checksum))
+	idx, err := w.fs.Create(fmt.Sprintf("%s.idx", base))
+	if err != nil {
+		return err
+	}
+
+	if err := w.encodeIdx(idx); err != nil {
+		return err
+	}
+
+	if err := idx.Close(); err != nil {
+		return err
+	}
+
+	return w.fs.Rename(w.fw.Name(), fmt.Sprintf("%s.pack", base))
+}
+
+func (w *PackWriter) encodeIdx(writer io.Writer) error {
+	idx := w.index.ToIdxFile()
+	idx.PackfileChecksum = w.checksum
+	idx.Version = idxfile.VersionSupported
+	e := idxfile.NewEncoder(writer)
+	_, err := e.Encode(idx)
+	return err
+}
+
+type syncedReader struct {
+	w io.Writer
+	r io.ReadSeeker
+
+	blocked, done uint32
+	written, read uint64
+	news          chan bool
+}
+
+func newSyncedReader(w io.Writer, r io.ReadSeeker) *syncedReader {
+	return &syncedReader{
+		w:    w,
+		r:    r,
+		news: make(chan bool),
+	}
+}
+
+func (s *syncedReader) Write(p []byte) (n int, err error) {
+	defer func() {
+		written := atomic.AddUint64(&s.written, uint64(n))
+		read := atomic.LoadUint64(&s.read)
+		if written > read {
+			s.wake()
+		}
+	}()
+
+	n, err = s.w.Write(p)
+	return
+}
+
+func (s *syncedReader) Read(p []byte) (n int, err error) {
+	defer func() { atomic.AddUint64(&s.read, uint64(n)) }()
+
+	for {
+		s.sleep()
+		n, err = s.r.Read(p)
+		if err == io.EOF && !s.isDone() && n == 0 {
+			continue
+		}
+
+		break
+	}
+
+	return
+}
+
+func (s *syncedReader) isDone() bool {
+	return atomic.LoadUint32(&s.done) == 1
+}
+
+func (s *syncedReader) isBlocked() bool {
+	return atomic.LoadUint32(&s.blocked) == 1
+}
+
+func (s *syncedReader) wake() {
+	if s.isBlocked() {
+		//	fmt.Println("wake")
+		atomic.StoreUint32(&s.blocked, 0)
+		s.news <- true
+	}
+}
+
+func (s *syncedReader) sleep() {
+	read := atomic.LoadUint64(&s.read)
+	written := atomic.LoadUint64(&s.written)
+	if read >= written {
+		atomic.StoreUint32(&s.blocked, 1)
+		//	fmt.Println("sleep", read, written)
+		<-s.news
+	}
+
+}
+
+func (s *syncedReader) Seek(offset int64, whence int) (int64, error) {
+	if whence == io.SeekCurrent {
+		return s.r.Seek(offset, whence)
+	}
+
+	p, err := s.r.Seek(offset, whence)
+	atomic.StoreUint64(&s.read, uint64(p))
+
+	return p, err
+}
+
+func (s *syncedReader) Close() error {
+	atomic.StoreUint32(&s.done, 1)
+	close(s.news)
+	return nil
+}
+
+type ObjectWriter struct {
+	objfile.Writer
+	fs billy.Filesystem
+	f  billy.File
+}
+
+func newObjectWriter(fs billy.Filesystem) (*ObjectWriter, error) {
+	f, err := fs.TempFile(fs.Join(objectsPath, packPath), "tmp_obj_")
+	if err != nil {
+		return nil, err
+	}
+
+	return &ObjectWriter{
+		Writer: (*objfile.NewWriter(f)),
+		fs:     fs,
+		f:      f,
+	}, nil
+}
+
+func (w *ObjectWriter) Close() error {
+	if err := w.Writer.Close(); err != nil {
+		return err
+	}
+
+	if err := w.f.Close(); err != nil {
+		return err
+	}
+
+	return w.save()
+}
+
+func (w *ObjectWriter) save() error {
+	hash := w.Hash().String()
+	file := w.fs.Join(objectsPath, hash[0:2], hash[2:40])
+
+	return w.fs.Rename(w.f.Name(), file)
+}
diff --git a/storage/filesystem/dotgit/writers_test.go b/storage/filesystem/dotgit/writers_test.go
new file mode 100644
index 0000000..bf00762
--- /dev/null
+++ b/storage/filesystem/dotgit/writers_test.go
@@ -0,0 +1,156 @@
+package dotgit
+
+import (
+	"fmt"
+	"io"
+	"io/ioutil"
+	"log"
+	"os"
+	"strconv"
+
+	"gopkg.in/src-d/go-git.v4/plumbing"
+	"gopkg.in/src-d/go-git.v4/plumbing/format/packfile"
+
+	. "gopkg.in/check.v1"
+	"gopkg.in/src-d/go-billy.v4/osfs"
+	"gopkg.in/src-d/go-git-fixtures.v3"
+)
+
+func (s *SuiteDotGit) TestNewObjectPack(c *C) {
+	f := fixtures.Basic().One()
+
+	dir, err := ioutil.TempDir("", "example")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	defer os.RemoveAll(dir)
+
+	fs := osfs.New(dir)
+	dot := New(fs)
+
+	w, err := dot.NewObjectPack()
+	c.Assert(err, IsNil)
+
+	_, err = io.Copy(w, f.Packfile())
+	c.Assert(err, IsNil)
+
+	c.Assert(w.Close(), IsNil)
+
+	pfPath := fmt.Sprintf("objects/pack/pack-%s.pack", f.PackfileHash)
+	idxPath := fmt.Sprintf("objects/pack/pack-%s.idx", f.PackfileHash)
+
+	stat, err := fs.Stat(pfPath)
+	c.Assert(err, IsNil)
+	c.Assert(stat.Size(), Equals, int64(84794))
+
+	stat, err = fs.Stat(idxPath)
+	c.Assert(err, IsNil)
+	c.Assert(stat.Size(), Equals, int64(1940))
+
+	pf, err := fs.Open(pfPath)
+	c.Assert(err, IsNil)
+	pfs := packfile.NewScanner(pf)
+	_, objects, err := pfs.Header()
+	c.Assert(err, IsNil)
+	for i := uint32(0); i < objects; i++ {
+		_, err := pfs.NextObjectHeader()
+		if err != nil {
+			c.Assert(err, IsNil)
+			break
+		}
+	}
+	c.Assert(pfs.Close(), IsNil)
+}
+
+func (s *SuiteDotGit) TestNewObjectPackUnused(c *C) {
+	dir, err := ioutil.TempDir("", "example")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	defer os.RemoveAll(dir)
+
+	fs := osfs.New(dir)
+	dot := New(fs)
+
+	w, err := dot.NewObjectPack()
+	c.Assert(err, IsNil)
+
+	c.Assert(w.Close(), IsNil)
+
+	info, err := fs.ReadDir("objects/pack")
+	c.Assert(err, IsNil)
+	c.Assert(info, HasLen, 0)
+
+	// check clean up of temporary files
+	info, err = fs.ReadDir("")
+	c.Assert(err, IsNil)
+	for _, fi := range info {
+		c.Assert(fi.IsDir(), Equals, true)
+	}
+}
+
+func (s *SuiteDotGit) TestSyncedReader(c *C) {
+	tmpw, err := ioutil.TempFile("", "example")
+	c.Assert(err, IsNil)
+
+	tmpr, err := os.Open(tmpw.Name())
+	c.Assert(err, IsNil)
+
+	defer func() {
+		tmpw.Close()
+		tmpr.Close()
+		os.Remove(tmpw.Name())
+	}()
+
+	synced := newSyncedReader(tmpw, tmpr)
+
+	go func() {
+		for i := 0; i < 281; i++ {
+			_, err := synced.Write([]byte(strconv.Itoa(i) + "\n"))
+			c.Assert(err, IsNil)
+		}
+
+		synced.Close()
+	}()
+
+	o, err := synced.Seek(1002, io.SeekStart)
+	c.Assert(err, IsNil)
+	c.Assert(o, Equals, int64(1002))
+
+	head := make([]byte, 3)
+	n, err := io.ReadFull(synced, head)
+	c.Assert(err, IsNil)
+	c.Assert(n, Equals, 3)
+	c.Assert(string(head), Equals, "278")
+
+	o, err = synced.Seek(1010, io.SeekStart)
+	c.Assert(err, IsNil)
+	c.Assert(o, Equals, int64(1010))
+
+	n, err = io.ReadFull(synced, head)
+	c.Assert(err, IsNil)
+	c.Assert(n, Equals, 3)
+	c.Assert(string(head), Equals, "280")
+}
+
+func (s *SuiteDotGit) TestPackWriterUnusedNotify(c *C) {
+	dir, err := ioutil.TempDir("", "example")
+	if err != nil {
+		c.Assert(err, IsNil)
+	}
+
+	defer os.RemoveAll(dir)
+
+	fs := osfs.New(dir)
+
+	w, err := newPackWrite(fs)
+	c.Assert(err, IsNil)
+
+	w.Notify = func(h plumbing.Hash, idx *packfile.Index) {
+		c.Fatal("unexpected call to PackWriter.Notify")
+	}
+
+	c.Assert(w.Close(), IsNil)
+}