3 files changed, 72 insertions, 9 deletions

diff --git a/plumbing/difftree/difftree.go b/plumbing/difftree/difftree.go
index 76c5f27..ff1ceaf 100644
--- a/plumbing/difftree/difftree.go
+++ b/plumbing/difftree/difftree.go
@@ -2,20 +2,19 @@ package difftree
 
 import (
 	"bytes"
+	"os"
 
 	"srcd.works/go-git.v4/plumbing/object"
 	"srcd.works/go-git.v4/utils/merkletrie"
 	"srcd.works/go-git.v4/utils/merkletrie/noder"
 )
 
+// DiffTree compares the content and mode of the blobs found via two
+// tree objects.
 func DiffTree(a, b *object.Tree) ([]*Change, error) {
 	from := newTreeNoder(a)
 	to := newTreeNoder(b)
 
-	hashEqual := func(a, b noder.Hasher) bool {
-		return bytes.Equal(a.Hash(), b.Hash())
-	}
-
 	merkletrieChanges, err := merkletrie.DiffTree(from, to, hashEqual)
 	if err != nil {
 		return nil, err
@@ -23,3 +22,42 @@ func DiffTree(a, b *object.Tree) ([]*Change, error) {
 
 	return newChanges(merkletrieChanges)
 }
+
+// check if the hash of the contents is different, if not, check if
+// the permissions are different (but taking into account deprecated
+// file modes).  On a treenoder, the hash of the contents is codified
+// in the first 20 bytes of the data returned by Hash() and the last
+// 4 bytes is the mode.
+func hashEqual(a, b noder.Hasher) bool {
+	hashA, hashB := a.Hash(), b.Hash()
+	contentsA, contentsB := hashA[:20], hashB[:20]
+
+	sameContents := bytes.Equal(contentsA, contentsB)
+	if !sameContents {
+		return false
+	}
+
+	modeA, modeB := hashA[20:], hashB[20:]
+
+	return equivalentMode(modeA, modeB)
+}
+
+func equivalentMode(a, b []byte) bool {
+	if isFilish(a) && isFilish(b) {
+		return true
+	}
+	return bytes.Equal(a, b)
+}
+
+var (
+	file           = modeToBytes(object.FileMode)
+	fileDeprecated = modeToBytes(object.FileModeDeprecated)
+	// remove this by fixing plumbing.Object mode ASAP
+	fileGoGit = modeToBytes(os.FileMode(0644))
+)
+
+func isFilish(b []byte) bool {
+	return bytes.Equal(b, file) ||
+		bytes.Equal(b, fileDeprecated) ||
+		bytes.Equal(b, fileGoGit)
+}
diff --git a/plumbing/difftree/difftree_test.go b/plumbing/difftree/difftree_test.go
index e2519b3..24817f1 100644
--- a/plumbing/difftree/difftree_test.go
+++ b/plumbing/difftree/difftree_test.go
@@ -1,6 +1,7 @@
 package difftree
 
 import (
+	"os"
 	"sort"
 	"testing"
 
@@ -353,3 +354,26 @@ func (s *DiffTreeSuite) TestDiffTree(c *C) {
 		assertChanges(obtained, c)
 	}
 }
+
+func (s *DiffTreeSuite) TestIssue279(c *C) {
+	// HashEqual should ignore files if the only change is from a 100664
+	// mode to a 100644 or vice versa.
+	from := &treeNoder{
+		hash: plumbing.NewHash("d08e895238bac36d8220586fdc28c27e1a7a76d3"),
+		mode: os.FileMode(0100664),
+	}
+	to := &treeNoder{
+		hash: plumbing.NewHash("d08e895238bac36d8220586fdc28c27e1a7a76d3"),
+		mode: os.FileMode(0100644),
+	}
+	c.Assert(hashEqual(from, to), Equals, true)
+	c.Assert(hashEqual(to, from), Equals, true)
+
+	// but should detect if the contents of the file also changed.
+	to = &treeNoder{
+		hash: plumbing.NewHash("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"),
+		mode: os.FileMode(0100644),
+	}
+	c.Assert(hashEqual(from, to), Equals, false)
+	c.Assert(hashEqual(to, from), Equals, false)
+}
diff --git a/plumbing/object/tree.go b/plumbing/object/tree.go
index 27d8578..436ac32 100644
--- a/plumbing/object/tree.go
+++ b/plumbing/object/tree.go
@@ -19,11 +19,12 @@ const (
 	maxTreeDepth      = 1024
 	startingStackSize = 8
 
-	FileMode       os.FileMode = 0100644
-	ExecutableMode os.FileMode = 0100755
-	SubmoduleMode  os.FileMode = 0160000
-	SymlinkMode    os.FileMode = 0120000
-	TreeMode       os.FileMode = 0040000
+	FileMode           os.FileMode = 0100644
+	FileModeDeprecated os.FileMode = 0100664
+	ExecutableMode     os.FileMode = 0100755
+	SubmoduleMode      os.FileMode = 0160000
+	SymlinkMode        os.FileMode = 0120000
+	TreeMode           os.FileMode = 0040000
 )
 
 // New errors defined by this package.