Test metadata roundtrip when using pypa/wheel parsing techniques

Author: abravalheri

setuptools/tests/test_core_metadata.py

@@ -4,7 +4,10 @@
 import importlib
 import io
 from email import message_from_string
+from email.generator import Generator
 from email.message import Message
+from email.parser import Parser
+from email.policy import EmailPolicy
 from pathlib import Path
 from unittest.mock import Mock
 
@@ -403,6 +406,31 @@ def test_equivalent_output(self, tmp_path, dist):
         # Compare setuptools PKG-INFO x pypa/wheel METADATA
         assert metadata_str == pkg_info_str
 
+        # Make sure it parses/serializes well in pypa/wheel
+        _assert_roundtrip_message(pkg_info)
+
+
+def _assert_roundtrip_message(metadata: str) -> None:
+    """Emulate the way wheel.bdist_wheel parses and regenerates the message,
+    then ensures the metadata generated by setuptools is compatible.
+    """
+    with io.StringIO(metadata) as buffer:
+        msg = Parser().parse(buffer)
+
+    serialization_policy = EmailPolicy(
+        utf8=True,
+        mangle_from_=False,
+        max_line_length=0,
+    )
+    with io.BytesIO() as buffer:
+        out = io.TextIOWrapper(buffer, encoding="utf-8")
+        Generator(out, policy=serialization_policy).flatten(msg)
+        out.flush()
+        regenerated = buffer.getvalue()
+
+    raw_metadata = bytes(metadata, "utf-8")
+    assert regenerated == raw_metadata
+
 
 def _normalize_metadata(msg: Message) -> str:
     """Allow equivalent metadata to be compared directly"""