diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 4275bbde7a..092c0dccf8 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -67,7 +67,13 @@ jobs:
             ${{ matrix.python }}
 
   test_cygwin:
-    runs-on: windows-latest
+    strategy:
+      matrix:
+        python:
+        - 39
+        platform:
+        - windows-latest
+    runs-on: ${{ matrix.platform }}
     timeout-minutes: 75
     steps:
       - uses: actions/checkout@v2
@@ -76,19 +82,14 @@ jobs:
         with:
           platform: x86_64
           packages: >-
-            git,
+            python${{ matrix.python }},
+            python${{ matrix.python }}-devel,
+            python${{ matrix.python }}-tox,
             gcc-core,
-            python38,
-            python38-devel,
-            python38-pip
-      - name: Install tox
-        shell: C:\cygwin\bin\env.exe CYGWIN_NOWINPATH=1 CHERE_INVOKING=1 C:\cygwin\bin\bash.exe -leo pipefail -o igncr {0}
-        run: |
-          python3.8 -m pip install tox
+            git,
       - name: Run tests
         shell: C:\cygwin\bin\env.exe CYGWIN_NOWINPATH=1 CHERE_INVOKING=1 C:\cygwin\bin\bash.exe -leo pipefail -o igncr {0}
-        run: |
-          tox -- --cov-report xml
+        run: tox
 
   integration-test:
     needs: test
diff --git a/changelog.d/3299.change.rst b/changelog.d/3299.change.rst
index c84d7f0fd8..a5b6a8e7be 100644
--- a/changelog.d/3299.change.rst
+++ b/changelog.d/3299.change.rst
@@ -1 +1 @@
-Optional metadata fields are now truly optional.
+Optional metadata fields are now truly optional. Includes merge with pypa/distutils@a7cfb56 per pypa/distutils#138.
diff --git a/docs/deprecated/distutils/examples.rst b/docs/deprecated/distutils/examples.rst
index d0984655df..00eef73fa9 100644
--- a/docs/deprecated/distutils/examples.rst
+++ b/docs/deprecated/distutils/examples.rst
@@ -253,9 +253,7 @@ Running the ``check`` command will display some warnings:
 
     $ python setup.py check
     running check
-    warning: check: missing required meta-data: version, url
-    warning: check: missing meta-data: either (author and author_email) or
-             (maintainer and maintainer_email) should be supplied
+    warning: check: missing required meta-data: version
 
 
 If you use the reStructuredText syntax in the ``long_description`` field and
diff --git a/docs/deprecated/distutils/setupscript.rst b/docs/deprecated/distutils/setupscript.rst
index f49c4f893f..ec9cf34ed7 100644
--- a/docs/deprecated/distutils/setupscript.rst
+++ b/docs/deprecated/distutils/setupscript.rst
@@ -582,7 +582,7 @@ This information includes:
 | ``maintainer_email`` | email address of the      | email address   | \(3)   |
 |                      | package maintainer        |                 |        |
 +----------------------+---------------------------+-----------------+--------+
-| ``url``              | home page for the package | URL             | \(1)   |
+| ``url``              | home page for the package | URL             |        |
 +----------------------+---------------------------+-----------------+--------+
 | ``description``      | short, summary            | short string    |        |
 |                      | description of the        |                 |        |
@@ -612,8 +612,8 @@ Notes:
     It is recommended that versions take the form *major.minor[.patch[.sub]]*.
 
 (3)
-    Either the author or the maintainer must be identified. If maintainer is
-    provided, distutils lists it as the author in :file:`PKG-INFO`.
+    If maintainer is provided and author is not, distutils lists maintainer as
+    the author in :file:`PKG-INFO`.
 
 (4)
     The ``long_description`` field is used by PyPI when you publish a package,
diff --git a/setuptools/_distutils/_functools.py b/setuptools/_distutils/_functools.py
new file mode 100644
index 0000000000..e7053bac12
--- /dev/null
+++ b/setuptools/_distutils/_functools.py
@@ -0,0 +1,20 @@
+import functools
+
+
+# from jaraco.functools 3.5
+def pass_none(func):
+    """
+    Wrap func so it's not called if its first param is None
+
+    >>> print_text = pass_none(print)
+    >>> print_text('text')
+    text
+    >>> print_text(None)
+    """
+
+    @functools.wraps(func)
+    def wrapper(param, *args, **kwargs):
+        if param is not None:
+            return func(param, *args, **kwargs)
+
+    return wrapper
diff --git a/setuptools/_distutils/command/bdist_msi.py b/setuptools/_distutils/command/bdist_msi.py
index 1525953241..56c4b9883a 100644
--- a/setuptools/_distutils/command/bdist_msi.py
+++ b/setuptools/_distutils/command/bdist_msi.py
@@ -231,11 +231,7 @@ def run(self):
         if os.path.exists(installer_name): os.unlink(installer_name)
 
         metadata = self.distribution.metadata
-        author = metadata.author
-        if not author:
-            author = metadata.maintainer
-        if not author:
-            author = "UNKNOWN"
+        author = metadata.author or metadata.maintainer
         version = metadata.get_version()
         # ProductVersion must be strictly numeric
         # XXX need to deal with prerelease versions
diff --git a/setuptools/_distutils/command/bdist_rpm.py b/setuptools/_distutils/command/bdist_rpm.py
index 550cbfa1e2..a2a9e8e588 100644
--- a/setuptools/_distutils/command/bdist_rpm.py
+++ b/setuptools/_distutils/command/bdist_rpm.py
@@ -399,7 +399,7 @@ def _make_spec_file(self):
             '%define unmangled_version ' + self.distribution.get_version(),
             '%define release ' + self.release.replace('-','_'),
             '',
-            'Summary: ' + self.distribution.get_description(),
+            'Summary: ' + (self.distribution.get_description() or "UNKNOWN"),
             ]
 
         # Workaround for #14443 which affects some RPM based systems such as
@@ -438,7 +438,7 @@ def _make_spec_file(self):
             spec_file.append('Source0: %{name}-%{unmangled_version}.tar.gz')
 
         spec_file.extend([
-            'License: ' + self.distribution.get_license(),
+            'License: ' + (self.distribution.get_license() or "UNKNOWN"),
             'Group: ' + self.group,
             'BuildRoot: %{_tmppath}/%{name}-%{version}-%{release}-buildroot',
             'Prefix: %{_prefix}', ])
@@ -464,7 +464,7 @@ def _make_spec_file(self):
                 spec_file.append('%s: %s' % (field, val))
 
 
-        if self.distribution.get_url() != 'UNKNOWN':
+        if self.distribution.get_url():
             spec_file.append('Url: ' + self.distribution.get_url())
 
         if self.distribution_name:
@@ -483,7 +483,7 @@ def _make_spec_file(self):
         spec_file.extend([
             '',
             '%description',
-            self.distribution.get_long_description()
+            self.distribution.get_long_description() or "",
             ])
 
         # put locale descriptions into spec file
diff --git a/setuptools/_distutils/command/check.py b/setuptools/_distutils/command/check.py
index af311ca90e..8a02dbca7d 100644
--- a/setuptools/_distutils/command/check.py
+++ b/setuptools/_distutils/command/check.py
@@ -82,54 +82,19 @@ def check_metadata(self):
         """Ensures that all required elements of meta-data are supplied.
 
         Required fields:
-            name, version, URL
-
-        Recommended fields:
-            (author and author_email) or (maintainer and maintainer_email))
+            name, version
 
         Warns if any are missing.
         """
         metadata = self.distribution.metadata
 
         missing = []
-        for attr in ('name', 'version', 'url'):
-            if not (hasattr(metadata, attr) and getattr(metadata, attr)):
+        for attr in 'name', 'version':
+            if not getattr(metadata, attr, None):
                 missing.append(attr)
 
         if missing:
-            self.warn("missing required meta-data: %s"  % ', '.join(missing))
-        if not (
-            self._check_contact("author", metadata) or
-            self._check_contact("maintainer", metadata)
-        ):
-            self.warn("missing meta-data: either (author and author_email) " +
-                      "or (maintainer and maintainer_email) " +
-                      "should be supplied")
-
-    def _check_contact(self, kind, metadata):
-        """
-        Returns True if the contact's name is specified and False otherwise.
-        This function will warn if the contact's email is not specified.
-        """
-        name = getattr(metadata, kind) or ''
-        email = getattr(metadata, kind + '_email') or ''
-
-        msg = ("missing meta-data: if '{}' supplied, " +
-               "'{}' should be supplied too")
-
-        if name and email:
-            return True
-
-        if name:
-            self.warn(msg.format(kind, kind + '_email'))
-            return True
-
-        addresses = [(alias, addr) for alias, addr in getaddresses([email])]
-        if any(alias and addr for alias, addr in addresses):
-            # The contact's name can be encoded in the email: `Name <email>`
-            return True
-
-        return False
+            self.warn("missing required meta-data: %s" % ', '.join(missing))
 
     def check_restructuredtext(self):
         """Checks if the long string fields are reST-compliant."""
diff --git a/setuptools/_distutils/dist.py b/setuptools/_distutils/dist.py
index 37db4d6cd7..45024975b9 100644
--- a/setuptools/_distutils/dist.py
+++ b/setuptools/_distutils/dist.py
@@ -1064,9 +1064,8 @@ def read_pkg_file(self, file):
 
         def _read_field(name):
             value = msg[name]
-            if value == 'UNKNOWN':
-                return None
-            return value
+            if value and value != "UNKNOWN":
+                return value
 
         def _read_list(name):
             values = msg.get_all(name, None)
@@ -1125,23 +1124,24 @@ def write_pkg_file(self, file):
                 self.classifiers or self.download_url):
             version = '1.1'
 
+        # required fields
         file.write('Metadata-Version: %s\n' % version)
         file.write('Name: %s\n' % self.get_name())
         file.write('Version: %s\n' % self.get_version())
-        file.write('Summary: %s\n' % self.get_description())
-        file.write('Home-page: %s\n' % self.get_url())
-        file.write('Author: %s\n' % self.get_contact())
-        file.write('Author-email: %s\n' % self.get_contact_email())
-        file.write('License: %s\n' % self.get_license())
-        if self.download_url:
-            file.write('Download-URL: %s\n' % self.download_url)
 
-        long_desc = rfc822_escape(self.get_long_description())
-        file.write('Description: %s\n' % long_desc)
+        def maybe_write(header, val):
+            if val:
+                file.write("{}: {}\n".format(header, val))
 
-        keywords = ','.join(self.get_keywords())
-        if keywords:
-            file.write('Keywords: %s\n' % keywords)
+        # optional fields
+        maybe_write("Summary", self.get_description())
+        maybe_write("Home-page", self.get_url())
+        maybe_write("Author", self.get_contact())
+        maybe_write("Author-email", self.get_contact_email())
+        maybe_write("License", self.get_license())
+        maybe_write("Download-URL", self.download_url)
+        maybe_write("Description", rfc822_escape(self.get_long_description() or ""))
+        maybe_write("Keywords", ",".join(self.get_keywords()))
 
         self._write_list(file, 'Platform', self.get_platforms())
         self._write_list(file, 'Classifier', self.get_classifiers())
@@ -1152,6 +1152,7 @@ def write_pkg_file(self, file):
         self._write_list(file, 'Obsoletes', self.get_obsoletes())
 
     def _write_list(self, file, name, values):
+        values = values or []
         for value in values:
             file.write('%s: %s\n' % (name, value))
 
@@ -1167,35 +1168,35 @@ def get_fullname(self):
         return "%s-%s" % (self.get_name(), self.get_version())
 
     def get_author(self):
-        return self.author or "UNKNOWN"
+        return self.author
 
     def get_author_email(self):
-        return self.author_email or "UNKNOWN"
+        return self.author_email
 
     def get_maintainer(self):
-        return self.maintainer or "UNKNOWN"
+        return self.maintainer
 
     def get_maintainer_email(self):
-        return self.maintainer_email or "UNKNOWN"
+        return self.maintainer_email
 
     def get_contact(self):
-        return self.maintainer or self.author or "UNKNOWN"
+        return self.maintainer or self.author
 
     def get_contact_email(self):
-        return self.maintainer_email or self.author_email or "UNKNOWN"
+        return self.maintainer_email or self.author_email
 
     def get_url(self):
-        return self.url or "UNKNOWN"
+        return self.url
 
     def get_license(self):
-        return self.license or "UNKNOWN"
+        return self.license
     get_licence = get_license
 
     def get_description(self):
-        return self.description or "UNKNOWN"
+        return self.description
 
     def get_long_description(self):
-        return self.long_description or "UNKNOWN"
+        return self.long_description
 
     def get_keywords(self):
         return self.keywords or []
@@ -1204,7 +1205,7 @@ def set_keywords(self, value):
         self.keywords = _ensure_list(value, 'keywords')
 
     def get_platforms(self):
-        return self.platforms or ["UNKNOWN"]
+        return self.platforms
 
     def set_platforms(self, value):
         self.platforms = _ensure_list(value, 'platforms')
@@ -1216,7 +1217,7 @@ def set_classifiers(self, value):
         self.classifiers = _ensure_list(value, 'classifiers')
 
     def get_download_url(self):
-        return self.download_url or "UNKNOWN"
+        return self.download_url
 
     # PEP 314
     def get_requires(self):
diff --git a/setuptools/_distutils/sysconfig.py b/setuptools/_distutils/sysconfig.py
index 55a42e169d..7543f794cb 100644
--- a/setuptools/_distutils/sysconfig.py
+++ b/setuptools/_distutils/sysconfig.py
@@ -16,6 +16,7 @@
 
 from .errors import DistutilsPlatformError
 from . import py39compat
+from ._functools import pass_none
 
 IS_PYPY = '__pypy__' in sys.builtin_module_names
 
@@ -51,12 +52,25 @@ def _is_python_source_dir(d):
 
 _sys_home = getattr(sys, '_home', None)
 
+
+def _is_parent(dir_a, dir_b):
+    """
+    Return True if a is a parent of b.
+    """
+    return os.path.normcase(dir_a).startswith(os.path.normcase(dir_b))
+
+
 if os.name == 'nt':
+    @pass_none
     def _fix_pcbuild(d):
-        if d and os.path.normcase(d).startswith(
-                os.path.normcase(os.path.join(PREFIX, "PCbuild"))):
-            return PREFIX
-        return d
+        # In a venv, sys._home will be inside BASE_PREFIX rather than PREFIX.
+        prefixes = PREFIX, BASE_PREFIX
+        matched = (
+            prefix
+            for prefix in prefixes
+            if _is_parent(d, os.path.join(prefix, "PCbuild"))
+        )
+        return next(matched, d)
     project_base = _fix_pcbuild(project_base)
     _sys_home = _fix_pcbuild(_sys_home)
 
diff --git a/setuptools/_distutils/tests/test_check.py b/setuptools/_distutils/tests/test_check.py
index b41dba3d0a..2414d6eb5e 100644
--- a/setuptools/_distutils/tests/test_check.py
+++ b/setuptools/_distutils/tests/test_check.py
@@ -43,7 +43,7 @@ def test_check_metadata(self):
         # by default, check is checking the metadata
         # should have some warnings
         cmd = self._run()
-        self.assertEqual(cmd._warnings, 2)
+        self.assertEqual(cmd._warnings, 1)
 
         # now let's add the required fields
         # and run it again, to make sure we don't get
@@ -81,17 +81,16 @@ def test_check_author_maintainer(self):
             cmd = self._run(metadata)
             self.assertEqual(cmd._warnings, 0)
 
-            # the check should warn if only email is given and it does not
-            # contain the name
+            # the check should not warn if only email is given
             metadata[kind + '_email'] = 'name@email.com'
             cmd = self._run(metadata)
-            self.assertEqual(cmd._warnings, 1)
+            self.assertEqual(cmd._warnings, 0)
 
-            # the check should warn if only the name is given
+            # the check should not warn if only the name is given
             metadata[kind] = "Name"
             del metadata[kind + '_email']
             cmd = self._run(metadata)
-            self.assertEqual(cmd._warnings, 1)
+            self.assertEqual(cmd._warnings, 0)
 
     @unittest.skipUnless(HAS_DOCUTILS, "won't test without docutils")
     def test_check_document(self):
diff --git a/setuptools/_distutils/tests/test_dist.py b/setuptools/_distutils/tests/test_dist.py
index 36155be152..9132bc040b 100644
--- a/setuptools/_distutils/tests/test_dist.py
+++ b/setuptools/_distutils/tests/test_dist.py
@@ -519,7 +519,7 @@ def test_read_metadata(self):
         self.assertEqual(metadata.description, "xxx")
         self.assertEqual(metadata.download_url, 'http://example.com')
         self.assertEqual(metadata.keywords, ['one', 'two'])
-        self.assertEqual(metadata.platforms, ['UNKNOWN'])
+        self.assertEqual(metadata.platforms, None)
         self.assertEqual(metadata.obsoletes, None)
         self.assertEqual(metadata.requires, ['foo'])
 
diff --git a/setuptools/_distutils/tests/test_register.py b/setuptools/_distutils/tests/test_register.py
index 5770ed58ae..4556768645 100644
--- a/setuptools/_distutils/tests/test_register.py
+++ b/setuptools/_distutils/tests/test_register.py
@@ -154,8 +154,8 @@ def _no_way(prompt=''):
         req1 = dict(self.conn.reqs[0].headers)
         req2 = dict(self.conn.reqs[1].headers)
 
-        self.assertEqual(req1['Content-length'], '1374')
-        self.assertEqual(req2['Content-length'], '1374')
+        self.assertEqual(req1['Content-length'], '1359')
+        self.assertEqual(req2['Content-length'], '1359')
         self.assertIn(b'xxx', self.conn.reqs[1].data)
 
     def test_password_not_in_file(self):
diff --git a/setuptools/_distutils/tests/test_sdist.py b/setuptools/_distutils/tests/test_sdist.py
index 4c51717ce6..aa04dd0546 100644
--- a/setuptools/_distutils/tests/test_sdist.py
+++ b/setuptools/_distutils/tests/test_sdist.py
@@ -251,7 +251,7 @@ def test_metadata_check_option(self):
         cmd.run()
         warnings = [msg for msg in self.get_logs(WARN) if
                     msg.startswith('warning: check:')]
-        self.assertEqual(len(warnings), 2)
+        self.assertEqual(len(warnings), 1)
 
         # trying with a complete set of metadata
         self.clear_logs()
diff --git a/setuptools/_distutils/tests/test_sysconfig.py b/setuptools/_distutils/tests/test_sysconfig.py
index e671f9e09b..1c88cc85f7 100644
--- a/setuptools/_distutils/tests/test_sysconfig.py
+++ b/setuptools/_distutils/tests/test_sysconfig.py
@@ -7,6 +7,9 @@
 import textwrap
 import unittest
 
+import jaraco.envs
+
+import distutils
 from distutils import sysconfig
 from distutils.ccompiler import get_default_compiler
 from distutils.unixccompiler import UnixCCompiler
@@ -309,6 +312,31 @@ def test_win_ext_suffix(self):
         self.assertTrue(sysconfig.get_config_var("EXT_SUFFIX").endswith(".pyd"))
         self.assertNotEqual(sysconfig.get_config_var("EXT_SUFFIX"), ".pyd")
 
+    @unittest.skipUnless(
+        sys.platform == 'win32',
+        'Testing Windows build layout')
+    @unittest.skipUnless(
+        sys.implementation.name == 'cpython',
+        'Need cpython for this test')
+    @unittest.skipUnless(
+        '\\PCbuild\\'.casefold() in sys.executable.casefold(),
+        'Need sys.executable to be in a source tree')
+    def test_win_build_venv_from_source_tree(self):
+        """Ensure distutils.sysconfig detects venvs from source tree builds."""
+        env = jaraco.envs.VEnv()
+        env.create_opts = env.clean_opts
+        env.root = TESTFN
+        env.ensure_env()
+        cmd = [
+            env.exe(),
+            "-c",
+            "import distutils.sysconfig; print(distutils.sysconfig.python_build)"
+        ]
+        distutils_path = os.path.dirname(os.path.dirname(distutils.__file__))
+        out = subprocess.check_output(cmd, env={**os.environ, "PYTHONPATH": distutils_path})
+        assert out == "True"
+
+
 def test_suite():
     suite = unittest.TestSuite()
     suite.addTest(unittest.TestLoader().loadTestsFromTestCase(SysconfigTestCase))
diff --git a/tox.ini b/tox.ini
index 973f3763a6..bb2e7cb17d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,6 +1,6 @@
 [tox]
 envlist = python
-minversion = 3.2
+minversion = 3.25
 # https://github.com/jaraco/skeleton/issues/6
 tox_pip_extensions_ext_venv_update = true
 toxworkdir={env:TOX_WORK_DIR:.tox}
@@ -20,10 +20,6 @@ passenv =
 	windir  # required for test_pkg_resources
 	# honor git config in pytest-perf
 	HOME
-	# workaround for tox-dev/tox#2382
-	PROGRAMDATA
-	PROGRAMFILES
-	PROGRAMFILES(x86)
 
 [testenv:integration]
 deps = {[testenv]deps}
@@ -31,10 +27,6 @@ extras = testing-integration
 passenv =
 	{[testenv]passenv}
 	DOWNLOAD_PATH
-	# workaround for tox-dev/tox#2382
-	PROGRAMDATA
-	PROGRAMFILES
-	PROGRAMFILES(x86)
 setenv =
     PROJECT_ROOT = {toxinidir}
 commands =