Bug 1632870 - [mozbuild] Allow making copies of 'mozbuild.util.ReadOnlyDict', r=firefox-build-system-reviewers,mhentges

The copy and deepcopy operations currently use 'setitem' which obviously fails
for 'ReadOnlyDict'. But copying the dict yields a new instance so there should
be no reason this is prevented.

Specifically, I'd like to make certain subsets of task configuration read-only.
But copying is needed when we split the task into multiple tasks (e.g for
chunking).

Differential Revision: https://phabricator.services.mozilla.com/D131284

python/mozbuild/mozbuild/test/test_util.py

@@ -5,6 +5,7 @@
 
 from __future__ import absolute_import, print_function, unicode_literals
 
+import copy
 import hashlib
 import io
 import itertools
@@ -15,6 +16,7 @@ import string
 import sys
 import textwrap
 
+import pytest
 from mozfile.mozfile import NamedTemporaryFile
 from mozunit import main
 
@@ -32,6 +34,7 @@ from mozbuild.util import (
     HierarchicalStringList,
     EnumString,
     EnumStringComparisonError,
+    ReadOnlyDict,
     StrictOrderingOnAppendList,
     StrictOrderingOnAppendListWithAction,
     StrictOrderingOnAppendListWithFlagsFactory,
@@ -927,5 +930,32 @@ class TestHexDump(unittest.TestCase):
         )
 
 
+def test_read_only_dict():
+    d = ReadOnlyDict(foo="bar")
+    with pytest.raises(Exception):
+        d["foo"] = "baz"
+
+    with pytest.raises(Exception):
+        d.update({"foo": "baz"})
+
+    with pytest.raises(Exception):
+        del d["foo"]
+
+    # ensure copy still works
+    d_copy = d.copy()
+    assert d == d_copy
+    # TODO Returning a dict here feels like a bug, but there are places in-tree
+    # relying on this behaviour.
+    assert isinstance(d_copy, dict)
+
+    d_copy = copy.copy(d)
+    assert d == d_copy
+    assert isinstance(d_copy, ReadOnlyDict)
+
+    d_copy = copy.deepcopy(d)
+    assert d == d_copy
+    assert isinstance(d_copy, ReadOnlyDict)
+
+
 if __name__ == "__main__":
     main()

python/mozbuild/mozbuild/util.py

@@ -10,6 +10,7 @@ from __future__ import absolute_import, print_function, unicode_literals
 import argparse
 import collections
 import collections.abc
+import copy
 import ctypes
 import difflib
 import errno
@@ -133,6 +134,16 @@ class ReadOnlyDict(dict):
     def update(self, *args, **kwargs):
         raise Exception("Object does not support update.")
 
+    def __copy__(self, *args, **kwargs):
+        return ReadOnlyDict(**dict.copy(self, *args, **kwargs))
+
+    def __deepcopy__(self, memo):
+        result = {}
+        for k, v in self.items():
+            result[k] = copy.deepcopy(v, memo)
+
+        return ReadOnlyDict(**result)
+
 
 class undefined_default(object):
     """Represents an undefined argument value that isn't None."""