multi: refactor isolated env

* `IsolatedEnv` reworked to subsume env creation logic.

* `IsolatedEnvBuilder` renamed to `IsolatedEnvManager`.

* `ProjectBuilder.from_isolated_env` added to auto-fill init args from
  an isolated env.

* Made the `IsolatedEnv` responsible for customising the environ as a prelude
  to fixing pypa#377.  Removed `ProjectBuilder.scripts_dir`.

* `ProjectBuilder`'s @properties made read-only. Mutating the builder is not
  necessary with the addition of `ProjectBuilder.from_isolated_env`.

src/build/__init__.py

@@ -3,6 +3,7 @@
 """
 build - A simple, correct PEP 517 build frontend
 """
+
 __version__ = '0.7.0'
 
 import contextlib
@@ -16,8 +17,8 @@
 import warnings
 import zipfile
 
-from collections import OrderedDict
 from typing import (
+    TYPE_CHECKING,
     AbstractSet,
     Any,
     Callable,
@@ -36,6 +37,11 @@
 import pep517.wrappers
 
 
+if TYPE_CHECKING:
+    # Avoid import cycle.
+    from . import env
+
+
 TOMLDecodeError: Type[Exception]
 toml_loads: Callable[[str], MutableMapping[str, Any]]
 
@@ -231,12 +237,10 @@ def __init__(
         self,
         srcdir: PathType,
         python_executable: str = sys.executable,
-        scripts_dir: Optional[str] = None,
         runner: RunnerType = pep517.wrappers.default_subprocess_runner,
     ) -> None:
         """
         :param srcdir: The source directory
-        :param scripts_dir: The location of the scripts dir (defaults to the folder where the python executable lives)
         :param python_executable: The python executable where the backend lives
         :param runner: An alternative runner for backend subprocesses
 
@@ -268,53 +272,39 @@ def __init__(
             raise BuildException(f'Failed to parse {spec_file}: {e} ')
 
         self._build_system = _parse_build_system_table(spec)
+        self._python_executable = python_executable
+        self._requires = set(self._build_system['requires'])
         self._backend = self._build_system['build-backend']
-        self._scripts_dir = scripts_dir
-        self._hook_runner = runner
         self._hook = pep517.wrappers.Pep517HookCaller(
             self.srcdir,
             self._backend,
             backend_path=self._build_system.get('backend-path'),
-            python_executable=python_executable,
-            runner=self._runner,
+            runner=runner,
+            python_executable=self._python_executable,
         )
 
-    def _runner(
-        self, cmd: Sequence[str], cwd: Optional[str] = None, extra_environ: Optional[Mapping[str, str]] = None
-    ) -> None:
-        # if script dir is specified must be inserted at the start of PATH (avoid duplicate path while doing so)
-        if self.scripts_dir is not None:
-            paths: Dict[str, None] = OrderedDict()
-            paths[str(self.scripts_dir)] = None
-            if 'PATH' in os.environ:
-                paths.update((i, None) for i in os.environ['PATH'].split(os.pathsep))
-            extra_environ = {} if extra_environ is None else dict(extra_environ)
-            extra_environ['PATH'] = os.pathsep.join(paths)
-        self._hook_runner(cmd, cwd, extra_environ)
-
-    @property
-    def python_executable(self) -> str:
+    @classmethod
+    def from_isolated_env(cls, isolated_env: 'env.IsolatedEnv', srcdir: PathType) -> 'ProjectBuilder':
         """
-        The Python executable used to invoke the backend.
+        Instantiate the builder from an isolated environment.
+
+        :param isolated_env: The isolated environment instance
+        :param srcdir: The project source directory
         """
-        # make mypy happy
-        exe: str = self._hook.python_executable
-        return exe
 
-    @python_executable.setter
-    def python_executable(self, value: str) -> None:
-        self._hook.python_executable = value
+        def runner(cmd: Sequence[str], cwd: Optional[str] = None, extra_environ: Optional[Mapping[str, str]] = None) -> None:
+            env = dict(isolated_env.prepare_environ())
+            if extra_environ:
+                env.update(extra_environ)
 
-    @property
-    def scripts_dir(self) -> Optional[str]:
-        """
-        The folder where the scripts are stored for the python executable.
-        """
-        return self._scripts_dir
+            subprocess.check_call(cmd, cwd=cwd, env=env)
 
-    @scripts_dir.setter
-    def scripts_dir(self, value: Optional[str]) -> None:
-        self._scripts_dir = value
+        return cls(srcdir, python_executable=isolated_env.python_executable, runner=runner)
+
+    @property
+    def python_executable(self) -> str:
+        """Path of Python executable used to invoke PEP 517 hooks."""
+        return self._python_executable
 
     @property
     def build_system_requires(self) -> Set[str]:
@@ -323,7 +313,7 @@ def build_system_requires(self) -> Set[str]:
         ``build-system.requires`` field or the default build dependencies
         if ``pyproject.toml`` is missing or ``build-system`` is undefined.
         """
-        return set(self._build_system['requires'])
+        return self._requires
 
     def get_requires_for_build(self, distribution: str, config_settings: Optional[ConfigSettingsType] = None) -> Set[str]:
         """

src/build/__main__.py

@@ -1,6 +1,5 @@
 # SPDX-License-Identifier: MIT
 
-
 import argparse
 import contextlib
 import os
@@ -18,7 +17,7 @@
 import build
 
 from build import BuildBackendException, BuildException, ConfigSettingsType, PathType, ProjectBuilder
-from build.env import IsolatedEnvBuilder
+from build.env import IsolatedEnvManager
 
 
 _COLORS = {
@@ -85,7 +84,7 @@ def log(message: str) -> None:
         print('{bold}* {}{reset}'.format(message, **_STYLES))
 
 
-class _IsolatedEnvBuilder(IsolatedEnvBuilder):
+class _IsolatedEnvManager(IsolatedEnvManager):
     @staticmethod
     def log(message: str) -> None:
         print('{bold}* {}{reset}'.format(message, **_STYLES))
@@ -96,25 +95,25 @@ def _format_dep_chain(dep_chain: Sequence[str]) -> str:
 
 
 def _build_in_isolated_env(
-    builder: ProjectBuilder, outdir: PathType, distribution: str, config_settings: Optional[ConfigSettingsType]
+    srcdir: PathType, outdir: PathType, distribution: str, config_settings: Optional[ConfigSettingsType]
 ) -> str:
-    with _IsolatedEnvBuilder() as env:
-        builder.python_executable = env.executable
-        builder.scripts_dir = env.scripts_dir
+    with _IsolatedEnvManager() as env:
+        builder = _ProjectBuilder.from_isolated_env(env, srcdir)
         # first install the build dependencies
-        env.install(builder.build_system_requires)
+        env.install_packages(builder.build_system_requires)
         # then get the extra required dependencies from the backend (which was installed in the call above :P)
-        env.install(builder.get_requires_for_build(distribution))
+        env.install_packages(builder.get_requires_for_build(distribution))
         return builder.build(distribution, outdir, config_settings or {})
 
 
 def _build_in_current_env(
-    builder: ProjectBuilder,
+    srcdir: PathType,
     outdir: PathType,
     distribution: str,
     config_settings: Optional[ConfigSettingsType],
     skip_dependency_check: bool = False,
 ) -> str:
+    builder = _ProjectBuilder(srcdir)
     if not skip_dependency_check:
         missing = builder.check_dependencies(distribution)
         if missing:
@@ -127,16 +126,16 @@ def _build_in_current_env(
 
 def _build(
     isolation: bool,
-    builder: ProjectBuilder,
+    srcdir: PathType,
     outdir: PathType,
     distribution: str,
     config_settings: Optional[ConfigSettingsType],
     skip_dependency_check: bool,
 ) -> str:
     if isolation:
-        return _build_in_isolated_env(builder, outdir, distribution, config_settings)
+        return _build_in_isolated_env(srcdir, outdir, distribution, config_settings)
     else:
-        return _build_in_current_env(builder, outdir, distribution, config_settings, skip_dependency_check)
+        return _build_in_current_env(srcdir, outdir, distribution, config_settings, skip_dependency_check)
 
 
 @contextlib.contextmanager
@@ -194,9 +193,8 @@ def build_package(
     :param skip_dependency_check: Do not perform the dependency check
     """
     built: List[str] = []
-    builder = _ProjectBuilder(srcdir)
     for distribution in distributions:
-        out = _build(isolation, builder, outdir, distribution, config_settings, skip_dependency_check)
+        out = _build(isolation, srcdir, outdir, distribution, config_settings, skip_dependency_check)
         built.append(os.path.basename(out))
     return built
 
@@ -222,8 +220,7 @@ def build_package_via_sdist(
     if 'sdist' in distributions:
         raise ValueError('Only binary distributions are allowed but sdist was specified')
 
-    builder = _ProjectBuilder(srcdir)
-    sdist = _build(isolation, builder, outdir, 'sdist', config_settings, skip_dependency_check)
+    sdist = _build(isolation, srcdir, outdir, 'sdist', config_settings, skip_dependency_check)
 
     sdist_name = os.path.basename(sdist)
     sdist_out = tempfile.mkdtemp(prefix='build-via-sdist-')
@@ -232,11 +229,17 @@ def build_package_via_sdist(
     with tarfile.open(sdist) as t:
         t.extractall(sdist_out)
         try:
-            builder = _ProjectBuilder(os.path.join(sdist_out, sdist_name[: -len('.tar.gz')]))
             if distributions:
-                builder.log(f'Building {_natural_language_list(distributions)} from sdist')
+                _ProjectBuilder.log(f'Building {_natural_language_list(distributions)} from sdist')
             for distribution in distributions:
-                out = _build(isolation, builder, outdir, distribution, config_settings, skip_dependency_check)
+                out = _build(
+                    isolation,
+                    os.path.join(sdist_out, sdist_name[: -len('.tar.gz')]),
+                    outdir,
+                    distribution,
+                    config_settings,
+                    skip_dependency_check,
+                )
                 built.append(os.path.basename(out))
         finally:
             shutil.rmtree(sdist_out, ignore_errors=True)