Improve atomicity when writing PKG-INFO

For the time being, when `setuptools.build_meta` is called,
`egg_info.egg_base` is accidentally set to the project root between the
several calls to the different build hooks.

This means that if the hooks are called, they will try to overwrite
`setuptools.egg-info/PKG-INFO`, and for a very short interval of time it
will be an empty file.

Another process may then try to simultaneously use `importlib.metadata`
to list entry-points. However to sort entry-points, `importlib.metadata`
will try to read the `Name` field in the `PKG-INFO/METADATA` files and
will raise an error/warning if that file is empty.

This commit tries to use `os.replace` to avoid having an empty PKG-INFO
while `importlib.metadata` is used.

setuptools/_core_metadata.py

@@ -3,9 +3,12 @@
 
 See: https://packaging.python.org/en/latest/specifications/core-metadata/
 """
+import os
+import stat
 import textwrap
 from email import message_from_file
 from email.message import Message
+from tempfile import NamedTemporaryFile
 from typing import Optional, List
 
 from distutils.util import rfc822_escape
@@ -119,6 +122,24 @@ def single_line(val):
     return val
 
 
+def write_pkg_info(self, base_dir):
+    """Write the PKG-INFO file into the release tree."""
+    temp = ""
+    final = os.path.join(base_dir, 'PKG-INFO')
+    try:
+        # Use a temporary file while writing to avoid race conditions
+        # (e.g. `importlib.metadata` reading `.egg-info/PKG-INFO`):
+        with NamedTemporaryFile("w", encoding="utf-8", dir=base_dir, delete=False) as f:
+            temp = f.name
+            self.write_pkg_file(f)
+        permissions = stat.S_IMODE(os.lstat(temp).st_mode)
+        os.chmod(temp, permissions | stat.S_IRGRP | stat.S_IROTH)
+        os.replace(temp, final)  # atomic operation.
+    finally:
+        if temp and os.path.exists(temp):
+            os.remove(temp)
+
+
 # Based on Python 3.5 version
 def write_pkg_file(self, file):  # noqa: C901  # is too complex (14)  # FIXME
     """Write the PKG-INFO format data to a file object."""

setuptools/monkey.py

@@ -102,7 +102,12 @@ def _patch_distribution_metadata():
     from . import _core_metadata
 
     """Patch write_pkg_file and read_pkg_file for higher metadata standards"""
-    for attr in ('write_pkg_file', 'read_pkg_file', 'get_metadata_version'):
+    for attr in (
+        'write_pkg_info',
+        'write_pkg_file',
+        'read_pkg_file',
+        'get_metadata_version',
+    ):
         new_val = getattr(_core_metadata, attr)
         setattr(distutils.dist.DistributionMetadata, attr, new_val)