correctly handle uname-cmd that doesn't point to an executable file

.gitignore

@@ -10,6 +10,8 @@ __pycache__/
 # Transient editor files
 *.swp
 *~
+\#*#
+.#*#
 
 # Editor configuration
 nbproject

AUTHORS

@@ -55,5 +55,6 @@ Contributors are:
 -Eliah Kagan <eliah.kagan _at_ gmail.com>
 -Ethan Lin <et.repositories _at_ gmail.com>
 -Jonas Scharpf <jonas.scharpf _at_ checkmk.com>
+-Gordon Marx
 
 Portions derived from other open source works and are clearly marked.

git/util.py

@@ -464,6 +464,12 @@ def _is_cygwin_git(git_executable: str) -> bool:
 
             # Just a name given, not a real path.
             uname_cmd = osp.join(git_dir, "uname")
+
+            if not (pathlib.Path(uname_cmd).is_file() and os.access(uname_cmd, os.X_OK)):
+                _logger.debug(f"Failed checking if running in CYGWIN: {uname_cmd} is not an executable")
+                _is_cygwin_cache[git_executable] = is_cygwin
+                return is_cygwin
+
             process = subprocess.Popen([uname_cmd], stdout=subprocess.PIPE, universal_newlines=True)
             uname_out, _ = process.communicate()
             # retcode = process.poll()
@@ -484,7 +490,9 @@ def is_cygwin_git(git_executable: PathLike) -> bool: ...
 
 
 def is_cygwin_git(git_executable: Union[None, PathLike]) -> bool:
-    if sys.platform == "win32":  # TODO: See if we can use `sys.platform != "cygwin"`.
+    # TODO: when py3.7 support is dropped, use the new interpolation f"{variable=}"
+    _logger.debug(f"sys.platform={sys.platform!r}, git_executable={git_executable!r}")
+    if sys.platform != "cygwin":
         return False
     elif git_executable is None:
         return False

test/test_util.py

@@ -34,6 +34,7 @@
     LockFile,
     cygpath,
     decygpath,
+    is_cygwin_git,
     get_user_id,
     remove_password_if_present,
     rmtree,
@@ -349,6 +350,24 @@ def test_decygpath(self, wpath, cpath):
         assert wcpath == wpath.replace("/", "\\"), cpath
 
 
+class TestIsCygwinGit:
+    """Tests for :func:`is_cygwin_git`"""
+
+    def test_on_path_executable(self):
+        # Currently we assume tests run on Cygwin use Cygwin git. See #533 and #1455 for background.
+        if sys.platform == "cygwin":
+            assert is_cygwin_git("git")
+        else:
+            assert not is_cygwin_git("git")
+
+    def test_none_executable(self):
+        assert not is_cygwin_git(None)
+
+    def test_with_missing_uname(self):
+        """Test for handling when `uname` isn't in the same directory as `git`"""
+        assert not is_cygwin_git("/bogus_path/git")
+
+
 class _Member:
     """A member of an IterableList."""