improve index mode for files with executable bit

The fix for #430 in bebc4f5 (Use correct mode for executable files,
2016-05-19) is incomplete.  It fails (in most cases) when files have
modes which are not exactly 0644 or 0755.

Git only cares whether the executable bit is set (or not).  Ensure the
mode we set for the index is either 100644 or 100755 based on whether
the executable bit is set for the file owner.  Do this similarly to how
upstream git does it in cache.h¹.

Add a test covering various file modes to help catch regressions.

Fixes #1253

¹ https://github.com/git/git/blob/v2.31.1/cache.h#L247

Author: tmzullinger

git/index/fun.py

@@ -11,6 +11,7 @@
     S_ISDIR,
     S_IFMT,
     S_IFREG,
+    S_IXUSR,
 )
 import subprocess
 
@@ -115,7 +116,7 @@ def stat_mode_to_index_mode(mode):
         return S_IFLNK
     if S_ISDIR(mode) or S_IFMT(mode) == S_IFGITLINK:    # submodules
         return S_IFGITLINK
-    return S_IFREG | 0o644 | (mode & 0o111)       # blobs with or without executable bit
+    return S_IFREG | (mode & S_IXUSR and 0o755 or 0o644)  # blobs with or without executable bit
 
 
 def write_cache(entries: Sequence[Union[BaseIndexEntry, 'IndexEntry']], stream: IO[bytes],

test/test_fun.py

@@ -1,13 +1,14 @@
 from io import BytesIO
-from stat import S_IFDIR, S_IFREG, S_IFLNK
+from stat import S_IFDIR, S_IFREG, S_IFLNK, S_IXUSR
 from os import stat
 import os.path as osp
 from unittest import SkipTest
 
 from git import Git
 from git.index import IndexFile
 from git.index.fun import (
-    aggressive_tree_merge
+    aggressive_tree_merge,
+    stat_mode_to_index_mode,
 )
 from git.objects.fun import (
     traverse_tree_recursive,
@@ -206,6 +207,16 @@ def assert_entries(entries, num_entries, has_conflict=False):
             assert_entries(aggressive_tree_merge(odb, trees), 2, True)
         # END handle ours, theirs
 
+    def test_stat_mode_to_index_mode(self):
+        modes = (
+            0o600, 0o611, 0o640, 0o641, 0o644, 0o650, 0o651,
+            0o700, 0o711, 0o740, 0o744, 0o750, 0o751, 0o755,
+        )
+        for mode in modes:
+            expected_mode = S_IFREG | (mode & S_IXUSR and 0o755 or 0o644)
+            assert stat_mode_to_index_mode(mode) == expected_mode
+        # END for each mode
+
     def _assert_tree_entries(self, entries, num_trees):
         for entry in entries:
             assert len(entry) == num_trees