Don't trust modes if they can't be modified after a file has been created. Fixes: #719

NEWS

@@ -18,6 +18,9 @@
  * Implement ``RefsContainer.__iter__``
    (Jelmer Vernooij, #717)
 
+ * Don't trust modes if they can't be modified after a file has been created.
+   (Jelmer Vernooij, #719)
+
 0.19.11	2019-02-07
 
  IMPROVEMENTS

dulwich/repo.py

@@ -994,7 +994,10 @@ class Repo(BaseRepo):
             f.write('')
 
         st1 = os.lstat(fname)
-        os.chmod(fname, st1.st_mode ^ stat.S_IXUSR)
+        try:
+            os.chmod(fname, st1.st_mode ^ stat.S_IXUSR)
+        except PermissionError:
+            return False
         st2 = os.lstat(fname)
 
         os.unlink(fname)