env._find_executable_and_scripts(): Prefer the venv installation sche…

…me if it exists

Python distributors with custom default installation scheme can set a
scheme that can't be used to expand the paths in a venv.
This can happen if build itself is not installed in a venv.
The distributors are encouraged to set a "venv" scheme to be used for this.
See https://bugs.python.org/issue45413
and pypa/virtualenv#2208

Since Python that ships with the macOS developer tools does not have the "venv"
scheme yet, we keep the special case below. Once it gains the "venv" scheme,
it will be preferred.

Fixes #433

src/build/env.py

@@ -298,14 +298,21 @@ def _find_executable_and_scripts(path: str) -> Tuple[str, str, str]:
     """
     config_vars = sysconfig.get_config_vars().copy()  # globally cached, copy before altering it
     config_vars['base'] = path
+    # Python distributors with custom default installation scheme can set a
+    # scheme that can't be used to expand the paths in a venv.
+    # This can happen if build itself is not installed in a venv.
+    # The distributors are encouraged to set a "venv" scheme to be used for this.
+    # See https://bugs.python.org/issue45413
+    # and https://github.com/pypa/virtualenv/issues/2208
+    #
     # The Python that ships with the macOS developer tools varies the
     # default scheme depending on whether the ``sys.prefix`` is part of a framework.
-    # The framework "osx_framework_library" scheme
-    # can't be used to expand the paths in a venv, which
-    # can happen if build itself is not installed in a venv.
-    # If the Apple-custom "osx_framework_library" scheme is available
-    # we enforce "posix_prefix", the venv scheme, for isolated envs.
-    if 'osx_framework_library' in sysconfig.get_scheme_names():
+    # But it does not (yet) set the "venv" scheme.
+    # If the Apple-custom "osx_framework_library" scheme is available but "venv"
+    # is not, we use "posix_prefix" instead which is venv-compatible there.
+    if 'venv' in sysconfig.get_scheme_names():
+        paths = sysconfig.get_paths(scheme='venv', vars=config_vars)
+    elif 'osx_framework_library' in sysconfig.get_scheme_names():
         paths = sysconfig.get_paths(scheme='posix_prefix', vars=config_vars)
     else:
         paths = sysconfig.get_paths(vars=config_vars)