Added FollowLink() method to TreeEntry

error.go

@@ -40,6 +40,16 @@ func (err ErrNotExist) Error() string {
 	return fmt.Sprintf("object does not exist [id: %s, rel_path: %s]", err.ID, err.RelPath)
 }
 
+// ErrBadLink entry.FollowLink error
+type ErrBadLink struct {
+	Name    string
+	Message string
+}
+
+func (err ErrBadLink) Error() string {
+	return fmt.Sprintf("%s: %s", err.Name, err.Message)
+}
+
 // ErrUnsupportedVersion error when required git version not matched
 type ErrUnsupportedVersion struct {
 	Required string

tests/repos/repo1_bare/objects/6f/bd69e9823458e6c4a2fc5c0f6bc022b2f2acd1

@@ -0,0 +1 @@
+x
��1n�0E3���-�&����H&F+�ա�o�#d����/��X:��zS�i��Z�b1�ؔSa��g�@�F�35];̈�'Ɇ%s�D��5������R,��w��_m�೶�k�C�����v�"?��}m�#�8"�#|xDt����f��ET �z���z��/��N�

tests/repos/repo1_bare/refs/heads/master

@@ -1 +1 @@
-8d92fc957a4d7cfd98bc375f0b7bb189a0d6c9f2
+6fbd69e9823458e6c4a2fc5c0f6bc022b2f2acd1

tree_entry.go

@@ -5,6 +5,7 @@
 package git
 
 import (
+	"io"
 	"sort"
 	"strconv"
 	"strings"
@@ -90,6 +91,45 @@ func (te *TreeEntry) Blob() *Blob {
 	}
 }
 
+// FollowLink returns the entry pointed to by a symlink
+func (te *TreeEntry) FollowLink() (*TreeEntry, error) {
+	if !te.IsLink() {
+		return nil, ErrBadLink{te.Name(), "not a symlink"}
+	}
+
+	// read the link
+	r, err := te.Blob().Data()
+	if err != nil {
+		return nil, err
+	}
+	buf := make([]byte, te.Size())
+	_, err = io.ReadFull(r, buf)
+	if err != nil {
+		return nil, err
+	}
+
+	lnk := string(buf)
+	t := te.ptree
+
+	// traverse up directories
+	for ; t != nil && lnk[:3] == "../"; lnk = lnk[3:] {
+		t = t.ptree
+	}
+
+	if t == nil {
+		return nil, ErrBadLink{te.Name(), "points outside of repo"}
+	}
+
+	target, err := t.GetTreeEntryByPath(lnk)
+	if err != nil {
+		if IsErrNotExist(err) {
+			return nil, ErrBadLink{te.Name(), "broken link"}
+		}
+		return nil, err
+	}
+	return target, nil
+}
+
 // GetSubJumpablePathName return the full path of subdirectory jumpable ( contains only one directory )
 func (te *TreeEntry) GetSubJumpablePathName() string {
 	if te.IsSubModule() || !te.IsDir() {

tree_entry_test.go

@@ -50,3 +50,43 @@ func TestEntriesCustomSort(t *testing.T) {
 	assert.Equal(t, "bcd", entries[6].Name())
 	assert.Equal(t, "abc", entries[7].Name())
 }
+
+func TestFollowLink(t *testing.T) {
+	r, err := OpenRepository("tests/repos/repo1_bare")
+	assert.NoError(t, err)
+
+	commit, err := r.GetCommit("6fbd69e9823458e6c4a2fc5c0f6bc022b2f2acd1")
+	assert.NoError(t, err)
+
+	// get the symlink
+	lnk, err := commit.Tree.GetTreeEntryByPath("foo/bar/link_to_hello")
+	assert.NoError(t, err)
+	assert.True(t, lnk.IsLink())
+
+	// should be able to dereference to target
+	target, err := lnk.FollowLink()
+	assert.NoError(t, err)
+	assert.Equal(t, target.Name(), "hello")
+	assert.False(t, target.IsLink())
+	assert.Equal(t, target.ID.String(), "b14df6442ea5a1b382985a6549b85d435376c351")
+
+	// should error when called on normal file
+	target, err = commit.Tree.GetTreeEntryByPath("file1.txt")
+	assert.NoError(t, err)
+	_, err = target.FollowLink()
+	assert.Equal(t, err.Error(), "file1.txt: not a symlink")
+
+	// should error for broken links
+	target, err = commit.Tree.GetTreeEntryByPath("foo/broken_link")
+	assert.NoError(t, err)
+	assert.True(t, target.IsLink())
+	_, err = target.FollowLink()
+	assert.Equal(t, err.Error(), "broken_link: broken link")
+
+	// should error for external links
+	target, err = commit.Tree.GetTreeEntryByPath("foo/outside_repo")
+	assert.NoError(t, err)
+	assert.True(t, target.IsLink())
+	_, err = target.FollowLink()
+	assert.Equal(t, err.Error(), "outside_repo: points outside of repo")
+}