add clone --shared feature

.gitignore

@@ -4,3 +4,4 @@ coverage.txt
 profile.out
 .tmp/
 .git-dist/
+.vscode

options.go

@@ -78,6 +78,15 @@ type CloneOptions struct {
 	CABundle []byte
 	// ProxyOptions provides info required for connecting to a proxy.
 	ProxyOptions transport.ProxyOptions
+	// When the repository to clone is on the local machine, instead of
+	// using hard links, automatically setup .git/objects/info/alternates
+	// to share the objects with the source repository.
+	// The resulting repository starts out without any object of its own.
+	// NOTE: this is a possibly dangerous operation; do not use it unless
+	// you understand what it does.
+	//
+	// [Reference]: https://git-scm.com/docs/git-clone#Documentation/git-clone.txt---shared
+	Shared bool
 }
 
 // Validate validates the fields and sets the default values.

plumbing/storer/object.go

@@ -42,6 +42,7 @@ type EncodedObjectStorer interface {
 	HasEncodedObject(plumbing.Hash) error
 	// EncodedObjectSize returns the plaintext size of the encoded object.
 	EncodedObjectSize(plumbing.Hash) (int64, error)
+	AddAlternate(remote string) error
 }
 
 // DeltaObjectStorer is an EncodedObjectStorer that can return delta

plumbing/storer/object_test.go

@@ -168,3 +168,7 @@ func (o *MockObjectStorage) IterEncodedObjects(t plumbing.ObjectType) (EncodedOb
 func (o *MockObjectStorage) Begin() Transaction {
 	return nil
 }
+
+func (o *MockObjectStorage) AddAlternate(remote string) error {
+	return nil
+}

repository.go

@@ -22,6 +22,7 @@ import (
 	"github.com/go-git/go-git/v5/config"
 	"github.com/go-git/go-git/v5/internal/path_util"
 	"github.com/go-git/go-git/v5/internal/revision"
+	"github.com/go-git/go-git/v5/internal/url"
 	"github.com/go-git/go-git/v5/plumbing"
 	"github.com/go-git/go-git/v5/plumbing/cache"
 	formatcfg "github.com/go-git/go-git/v5/plumbing/format/config"
@@ -62,6 +63,7 @@ var (
 	ErrUnableToResolveCommit     = errors.New("unable to resolve commit")
 	ErrPackedObjectsNotSupported = errors.New("packed objects not supported")
 	ErrSHA256NotSupported        = errors.New("go-git was not compiled with SHA256 support")
+	ErrAlternatePathNotSupported = errors.New("alternate path must use the file scheme")
 )
 
 // Repository represents a git repository
@@ -887,6 +889,30 @@ func (r *Repository) clone(ctx context.Context, o *CloneOptions) error {
 		return err
 	}
 
+	// When the repository to clone is on the local machine,
+	// instead of using hard links, automatically setup .git/objects/info/alternates
+	// to share the objects with the source repository
+	if o.Shared {
+		if !url.IsLocalEndpoint(o.URL) {
+			return ErrAlternatePathNotSupported
+		}
+		altpath := o.URL
+		remoteRepo, err := PlainOpen(o.URL)
+		if err != nil {
+			return fmt.Errorf("failed to open remote repository: %w", err)
+		}
+		conf, err := remoteRepo.Config()
+		if err != nil {
+			return fmt.Errorf("failed to read remote repository configuration: %w", err)
+		}
+		if !conf.Core.IsBare {
+			altpath = path.Join(altpath, GitDirName)
+		}
+		if err := r.Storer.AddAlternate(altpath); err != nil {
+			return fmt.Errorf("failed to add alternate file to git objects dir: %w", err)
+		}
+	}
+
 	ref, err := r.fetchAndUpdateReferences(ctx, &FetchOptions{
 		RefSpecs:        c.Fetch,
 		Depth:           o.Depth,

repository_test.go

@@ -9,6 +9,7 @@ import (
 	"os"
 	"os/exec"
 	"os/user"
+	"path"
 	"path/filepath"
 	"regexp"
 	"strings"
@@ -791,6 +792,101 @@ func (s *RepositorySuite) TestPlainClone(c *C) {
 	c.Assert(cfg.Branches["master"].Name, Equals, "master")
 }
 
+func (s *RepositorySuite) TestPlainCloneBareAndShared(c *C) {
+	dir, clean := s.TemporalDir()
+	defer clean()
+
+	remote := s.GetBasicLocalRepositoryURL()
+
+	r, err := PlainClone(dir, true, &CloneOptions{
+		URL:    remote,
+		Shared: true,
+	})
+	c.Assert(err, IsNil)
+
+	altpath := path.Join(dir, "objects", "info", "alternates")
+	_, err = os.Stat(altpath)
+	c.Assert(err, IsNil)
+
+	data, err := os.ReadFile(altpath)
+	c.Assert(err, IsNil)
+
+	line := path.Join(remote, GitDirName, "objects") + "\n"
+	c.Assert(string(data), Equals, line)
+
+	cfg, err := r.Config()
+	c.Assert(err, IsNil)
+	c.Assert(cfg.Branches, HasLen, 1)
+	c.Assert(cfg.Branches["master"].Name, Equals, "master")
+}
+
+func (s *RepositorySuite) TestPlainCloneShared(c *C) {
+	dir, clean := s.TemporalDir()
+	defer clean()
+
+	remote := s.GetBasicLocalRepositoryURL()
+
+	r, err := PlainClone(dir, false, &CloneOptions{
+		URL:    remote,
+		Shared: true,
+	})
+	c.Assert(err, IsNil)
+
+	altpath := path.Join(dir, GitDirName, "objects", "info", "alternates")
+	_, err = os.Stat(altpath)
+	c.Assert(err, IsNil)
+
+	data, err := os.ReadFile(altpath)
+	c.Assert(err, IsNil)
+
+	line := path.Join(remote, GitDirName, "objects") + "\n"
+	c.Assert(string(data), Equals, line)
+
+	cfg, err := r.Config()
+	c.Assert(err, IsNil)
+	c.Assert(cfg.Branches, HasLen, 1)
+	c.Assert(cfg.Branches["master"].Name, Equals, "master")
+}
+
+func (s *RepositorySuite) TestPlainCloneSharedHttpShouldReturnError(c *C) {
+	dir, clean := s.TemporalDir()
+	defer clean()
+
+	remote := "http://somerepo"
+
+	_, err := PlainClone(dir, false, &CloneOptions{
+		URL:    remote,
+		Shared: true,
+	})
+	c.Assert(err, Equals, ErrAlternatePathNotSupported)
+}
+
+func (s *RepositorySuite) TestPlainCloneSharedHttpsShouldReturnError(c *C) {
+	dir, clean := s.TemporalDir()
+	defer clean()
+
+	remote := "https://somerepo"
+
+	_, err := PlainClone(dir, false, &CloneOptions{
+		URL:    remote,
+		Shared: true,
+	})
+	c.Assert(err, Equals, ErrAlternatePathNotSupported)
+}
+
+func (s *RepositorySuite) TestPlainCloneSharedSSHShouldReturnError(c *C) {
+	dir, clean := s.TemporalDir()
+	defer clean()
+
+	remote := "ssh://somerepo"
+
+	_, err := PlainClone(dir, false, &CloneOptions{
+		URL:    remote,
+		Shared: true,
+	})
+	c.Assert(err, Equals, ErrAlternatePathNotSupported)
+}
+
 func (s *RepositorySuite) TestPlainCloneWithRemoteName(c *C) {
 	dir, clean := s.TemporalDir()
 	defer clean()

storage/filesystem/dotgit/dotgit.go

@@ -8,7 +8,9 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"path"
 	"path/filepath"
+	"runtime"
 	"sort"
 	"strings"
 	"time"
@@ -38,6 +40,7 @@ const (
 	remotesPath    = "remotes"
 	logsPath       = "logs"
 	worktreesPath  = "worktrees"
+	alternatesPath = "alternates"
 
 	tmpPackedRefsPrefix = "._packed-refs"
 
@@ -1105,10 +1108,38 @@ func (d *DotGit) Module(name string) (billy.Filesystem, error) {
 	return d.fs.Chroot(d.fs.Join(modulePath, name))
 }
 
+func (d *DotGit) AddAlternate(remote string) error {
+	altpath := d.fs.Join(objectsPath, infoPath, alternatesPath)
+
+	f, err := d.fs.OpenFile(altpath, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0640)
+	if err != nil {
+		return fmt.Errorf("cannot open file: %w", err)
+	}
+	defer f.Close()
+
+	// locking in windows throws an error, based on comments
+	// https://github.com/go-git/go-git/pull/860#issuecomment-1751823044
+	// do not lock on windows platform.
+	if runtime.GOOS != "windows" {
+		if err = f.Lock(); err != nil {
+			return fmt.Errorf("cannot lock file: %w", err)
+		}
+		defer f.Unlock()
+	}
+
+	line := path.Join(remote, objectsPath) + "\n"
+	_, err = io.WriteString(f, line)
+	if err != nil {
+		return fmt.Errorf("error writing 'alternates' file: %w", err)
+	}
+
+	return nil
+}
+
 // 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")
+	altpath := d.fs.Join(objectsPath, infoPath, alternatesPath)
 	f, err := d.fs.Open(altpath)
 	if err != nil {
 		return nil, err

storage/filesystem/storage.go

@@ -74,3 +74,7 @@ func (s *Storage) Filesystem() billy.Filesystem {
 func (s *Storage) Init() error {
 	return s.dir.Initialize()
 }
+
+func (s *Storage) AddAlternate(remote string) error {
+	return s.dir.AddAlternate(remote)
+}

storage/memory/storage.go

@@ -202,6 +202,10 @@ func (o *ObjectStorage) DeleteLooseObject(plumbing.Hash) error {
 	return errNotSupported
 }
 
+func (o *ObjectStorage) AddAlternate(remote string) error {
+	return errNotSupported
+}
+
 type TxObjectStorage struct {
 	Storage *ObjectStorage
 	Objects map[plumbing.Hash]plumbing.EncodedObject

storage/transactional/object.go

@@ -82,3 +82,7 @@ func (o *ObjectStorage) Commit() error {
 		return err
 	})
 }
+
+func (o *ObjectStorage) AddAlternate(remote string) error {
+	return o.temporal.AddAlternate(remote)
+}