4 files changed, 44 insertions, 1 deletions
diff --git a/storage/filesystem/dotgit/dotgit.go b/storage/filesystem/dotgit/dotgit.go
index e02e6dd..3080e4a 100644
--- a/storage/filesystem/dotgit/dotgit.go
+++ b/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
diff --git a/storage/filesystem/storage.go b/storage/filesystem/storage.go
index 7e7a2c5..2069d3a 100644
--- a/storage/filesystem/storage.go
+++ b/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)
+}
diff --git a/storage/memory/storage.go b/storage/memory/storage.go
index ef6a445..79211c7 100644
--- a/storage/memory/storage.go
+++ b/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
diff --git a/storage/transactional/object.go b/storage/transactional/object.go
index 5d102b0..b43c96d 100644
--- a/storage/transactional/object.go
+++ b/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)
+}