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.

Author: obilaniu

Lib/shutil.py

@@ -359,6 +359,9 @@ def lookup(name):
     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 lookup(name):
                     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.

Lib/test/test_shutil.py

@@ -531,12 +531,20 @@ def _raise_on_src(fname, *, follow_symlinks=True):
 
         # 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

Misc/ACKS

@@ -149,6 +149,7 @@ Stephen Bevan
 Ron Bickers
 Natalia B. Bidart
 Adrian von Bidder
+Olexa Bilaniuk
 David Binger
 Dominic Binks
 Philippe Biondi

Misc/NEWS.d/next/Library/2019-05-09-08-35-18.bpo-24538.WK8Y-k.rst

@@ -0,0 +1,3 @@
+In `shutil.copystat()`, first copy extended file attributes and then file
+permissions, since extended attributes can only be set on the destination
+while it is still writeable.