storage: filesystem, add support for git alternates (#663)

This change adds a new method Alternates() in DotGit to check and
query alternate source.

3 files changed, 118 insertions, 1 deletions

diff --git a/storage/filesystem/internal/dotgit/dotgit.go b/storage/filesystem/internal/dotgit/dotgit.go
index 11b2512..ebc6bad 100644
--- a/storage/filesystem/internal/dotgit/dotgit.go
+++ b/storage/filesystem/internal/dotgit/dotgit.go
@@ -8,9 +8,11 @@ import (
 	"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"
 
@@ -756,11 +758,52 @@ func (d *DotGit) PackRefs() (err error) {
 	return nil
 }
 
-// Module return a billy.Filesystem poiting to the module folder
+// 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) {
diff --git a/storage/filesystem/internal/dotgit/dotgit_test.go b/storage/filesystem/internal/dotgit/dotgit_test.go
index 0ed9ec5..2c43295 100644
--- a/storage/filesystem/internal/dotgit/dotgit_test.go
+++ b/storage/filesystem/internal/dotgit/dotgit_test.go
@@ -5,6 +5,7 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"testing"
 
@@ -615,3 +616,55 @@ func (s *SuiteDotGit) TestPackRefs(c *C) {
 	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/object.go b/storage/filesystem/object.go
index 6ca67cc..fd52ed5 100644
--- a/storage/filesystem/object.go
+++ b/storage/filesystem/object.go
@@ -160,6 +160,27 @@ func (s *ObjectStorage) EncodedObject(t plumbing.ObjectType, h plumbing.Hash) (p
 		obj, err = s.getFromPackfile(h, false)
 	}
 
+	// If the error is still object not found, check if it's a shared object
+	// repository.
+	if err == plumbing.ErrObjectNotFound {
+		dotgits, e := s.dir.Alternates()
+		if e == nil {
+			// Create a new object storage with the DotGit(s) and check for the
+			// required hash object. Skip when not found.
+			for _, dg := range dotgits {
+				o, oe := newObjectStorage(dg)
+				if oe != nil {
+					continue
+				}
+				enobj, enerr := o.EncodedObject(t, h)
+				if enerr != nil {
+					continue
+				}
+				return enobj, nil
+			}
+		}
+	}
+
 	if err != nil {
 		return nil, err
 	}