env: refactor IsolatedEnv

The `IsolatedEnv` is made responsible for env creation to allow pip
to implement custom env creation logic.

`IsolatedEnvBuilder` takes an `IsolatedEnv` class as an argument
so that bringing your own `IsolatedEnv` won't require re-implementing
the builder.

src/build/env.py

@@ -38,26 +38,34 @@
 class IsolatedEnv(metaclass=abc.ABCMeta):
     """Abstract base of isolated build environments, as required by the build project."""
 
+    @abc.abstractmethod
+    def __init__(self, path: str, logging_fn: Callable[[str], None]) -> None:
+        """
+        :param path: The path where the environment will be created.
+        :param logging_fn: Logging function.
+        """
+
     @property
     @abc.abstractmethod
-    def executable(self) -> str:
-        """The executable of the isolated build environment."""
-        raise NotImplementedError
+    def python_executable(self) -> str:
+        """The Python executable of the isolated environment."""
 
     @property
     @abc.abstractmethod
     def scripts_dir(self) -> str:
-        """The scripts directory of the isolated build environment."""
-        raise NotImplementedError
+        """The scripts directory of the isolated environment."""
 
     @abc.abstractmethod
-    def install(self, requirements: Iterable[str]) -> None:
+    def create(self) -> None:
+        """Create the isolated environment."""
+
+    @abc.abstractmethod
+    def install_packages(self, requirements: Iterable[str]) -> None:
         """
-        Install packages from PEP 508 requirements in the isolated build environment.
+        Install packages from PEP 508 requirements in the isolated environment.
 
         :param requirements: PEP 508 requirements
         """
-        raise NotImplementedError
 
 
 @functools.lru_cache(maxsize=None)
@@ -82,10 +90,13 @@ def _subprocess(cmd: List[str]) -> None:
 
 
 class IsolatedEnvBuilder:
-    """Builder object for isolated environments."""
+    """Create and dispose of isolated build environments."""
 
-    def __init__(self) -> None:
-        self._path: Optional[str] = None
+    def __init__(self, isolated_env_class: Optional[Type[IsolatedEnv]] = None) -> None:
+        if isolated_env_class is not None:
+            self._isolated_env_class = isolated_env_class
+        else:
+            self._isolated_env_class = _DefaultIsolatedEnv
 
     def __enter__(self) -> IsolatedEnv:
         """
@@ -95,19 +106,9 @@ def __enter__(self) -> IsolatedEnv:
         """
         self._path = tempfile.mkdtemp(prefix='build-env-')
         try:
-            # use virtualenv when available (as it's faster than venv)
-            if _should_use_virtualenv():
-                self.log('Creating virtualenv isolated environment...')
-                executable, scripts_dir = _create_isolated_env_virtualenv(self._path)
-            else:
-                self.log('Creating venv isolated environment...')
-                executable, scripts_dir = _create_isolated_env_venv(self._path)
-            return _IsolatedEnvVenvPip(
-                path=self._path,
-                python_executable=executable,
-                scripts_dir=scripts_dir,
-                log=self.log,
-            )
+            isolated_env = self._isolated_env_class(self._path, self.log)
+            isolated_env.create()
+            return isolated_env
         except Exception:  # cleanup folder if creation fails
             self.__exit__(*sys.exc_info())
             raise
@@ -122,72 +123,57 @@ def __exit__(
         :param exc_val: The value of exception raised (if any)
         :param exc_tb: The traceback of exception raised (if any)
         """
-        if self._path is not None and os.path.exists(self._path):  # in case the user already deleted skip remove
+        if os.path.exists(self._path):  # in case the user already deleted skip remove
             shutil.rmtree(self._path)
 
     @staticmethod
     def log(message: str) -> None:
         """
-        Prints message
+        Log a message.
 
         The default implementation uses the logging module but this function can be
         overwritten by users to have a different implementation.
 
-        :param msg: Message to output
+        :param msg: Message to log
         """
         if sys.version_info >= (3, 8):
             _logger.log(logging.INFO, message, stacklevel=2)
         else:
             _logger.log(logging.INFO, message)
 
 
-class _IsolatedEnvVenvPip(IsolatedEnv):
+class _DefaultIsolatedEnv(IsolatedEnv):
     """
-    Isolated build environment context manager
+    An isolated environment which combines virtualenv and venv with pip.
 
     Non-standard paths injected directly to sys.path will still be passed to the environment.
     """
 
-    def __init__(
-        self,
-        path: str,
-        python_executable: str,
-        scripts_dir: str,
-        log: Callable[[str], None],
-    ) -> None:
-        """
-        :param path: The path where the environment exists
-        :param python_executable: The python executable within the environment
-        :param log: Log function
-        """
+    def __init__(self, path: str, logging_fn: Callable[[str], None]) -> None:
         self._path = path
-        self._python_executable = python_executable
-        self._scripts_dir = scripts_dir
-        self._log = log
-
-    @property
-    def path(self) -> str:
-        """The location of the isolated build environment."""
-        return self._path
+        self._log = logging_fn
 
     @property
-    def executable(self) -> str:
-        """The python executable of the isolated build environment."""
+    def python_executable(self) -> str:
         return self._python_executable
 
     @property
     def scripts_dir(self) -> str:
         return self._scripts_dir
 
-    def install(self, requirements: Iterable[str]) -> None:
-        """
-        Install packages from PEP 508 requirements in the isolated build environment.
+    def create(self) -> None:
+        # use virtualenv when available (as it's faster than venv)
+        if _should_use_virtualenv():
+            self._log('Creating virtualenv isolated environment...')
+            self._python_executable, self._scripts_dir = _create_isolated_env_virtualenv(self._path)
+        else:
+            self._log('Creating venv isolated environment...')
+            self._python_executable, self._scripts_dir = _create_isolated_env_venv(self._path)
 
-        :param requirements: PEP 508 requirement specification to install
+    def install_packages(self, requirements: Iterable[str]) -> None:
+        # Passing non-PEP 508 strings will result in undefined behavior, you *should not* rely on it. It is
+        # merely an implementation detail, it may change any time without warning.
 
-        :note: Passing non-PEP 508 strings will result in undefined behavior, you *should not* rely on it. It is
-               merely an implementation detail, it may change any time without warning.
-        """
         if not requirements:
             return
 
@@ -199,7 +185,7 @@ def install(self, requirements: Iterable[str]) -> None:
             req_file.write(os.linesep.join(requirements))
         try:
             cmd = [
-                self.executable,
+                self._python_executable,
                 '-Im',
                 'pip',
                 'install',
@@ -255,8 +241,12 @@ def _create_isolated_env_venv(path: str) -> Tuple[str, str]:
     import venv
 
     venv.EnvBuilder(with_pip=True, symlinks=_fs_supports_symlink()).create(path)
-    executable, script_dir, purelib = _find_executable_and_scripts(path)
+    executable, scripts_dir, purelib = _find_executable_and_scripts(path)
+    _post_create_isolated_env_venv(executable, purelib)
+    return executable, scripts_dir
 
+
+def _post_create_isolated_env_venv(executable: str, purelib: str) -> None:
     # Get the version of pip in the environment
     pip_distribution = next(iter(metadata.distributions(name='pip', path=[purelib])))  # type: ignore[no-untyped-call]
     current_pip_version = packaging.version.Version(pip_distribution.version)
@@ -275,15 +265,14 @@ def _create_isolated_env_venv(path: str) -> Tuple[str, str]:
 
     # Avoid the setuptools from ensurepip to break the isolation
     _subprocess([executable, '-m', 'pip', 'uninstall', 'setuptools', '-y'])
-    return executable, script_dir
 
 
 def _find_executable_and_scripts(path: str) -> Tuple[str, str, str]:
     """
-    Detect the Python executable and script folder of a virtual environment.
+    Detect the Python executable and sysconfig paths of a venv.
 
-    :param path: The location of the virtual environment
-    :return: The Python executable, script folder, and purelib folder
+    :param path: venv path
+    :return: The Python executable, scripts directory, and purelib directory
     """
     config_vars = sysconfig.get_config_vars().copy()  # globally cached, copy before altering it
     config_vars['base'] = path