bpo-24538: Fix bug in shutil involving the copying of xattrs to read-only files. (PR-13212)

Extended attributes can only be set on user-writeable files, but shutil previously
first chmod()ed the destination file to the source's permissions and then tried to
copy xattrs. This will cause failures if attempting to copy read-only files with
xattrs, as occurs with Git clones on Lustre FS.

2 files changed, 11 insertions, 1 deletions

diff --git a/Lib/shutil.py b/Lib/shutil.py
index 6cfe373..b2e8f5f 100644
--- a/Lib/shutil.py
+++ b/Lib/shutil.py
@@ -359,6 +359,9 @@ def copystat(src, dst, *, follow_symlinks=True):
     mode = stat.S_IMODE(st.st_mode)
     lookup("utime")(dst, ns=(st.st_atime_ns, st.st_mtime_ns),
         follow_symlinks=follow)
+    # We must copy extended attributes before the file is (potentially)
+    # chmod()'ed read-only, otherwise setxattr() will error with -EACCES.
+    _copyxattr(src, dst, follow_symlinks=follow)
     try:
         lookup("chmod")(dst, mode, follow_symlinks=follow)
     except NotImplementedError:
@@ -382,7 +385,6 @@ def copystat(src, dst, *, follow_symlinks=True):
                     break
             else:
                 raise
-    _copyxattr(src, dst, follow_symlinks=follow)
 
 def copy(src, dst, *, follow_symlinks=True):
     """Copy data and mode bits ("cp src dst"). Return the file's destination.
diff --git a/Lib/test/test_shutil.py b/Lib/test/test_shutil.py
index e709a56..eeebb97 100644
--- a/Lib/test/test_shutil.py
+++ b/Lib/test/test_shutil.py
@@ -531,12 +531,20 @@ class TestShutil(unittest.TestCase):
 
         # test that shutil.copystat copies xattrs
         src = os.path.join(tmp_dir, 'the_original')
+        srcro = os.path.join(tmp_dir, 'the_original_ro')
         write_file(src, src)
+        write_file(srcro, srcro)
         os.setxattr(src, 'user.the_value', b'fiddly')
+        os.setxattr(srcro, 'user.the_value', b'fiddly')
+        os.chmod(srcro, 0o444)
         dst = os.path.join(tmp_dir, 'the_copy')
+        dstro = os.path.join(tmp_dir, 'the_copy_ro')
         write_file(dst, dst)
+        write_file(dstro, dstro)
         shutil.copystat(src, dst)
+        shutil.copystat(srcro, dstro)
         self.assertEqual(os.getxattr(dst, 'user.the_value'), b'fiddly')
+        self.assertEqual(os.getxattr(dstro, 'user.the_value'), b'fiddly')
 
     @support.skip_unless_symlink
     @support.skip_unless_xattr