build.script: use build environment for execution

With this change, Poetry now creates an ephemeral build environment
with all requirements specified under `build-system.requires` when a
build script is specified. Otherwise, project environment is reused.

src/poetry/console/commands/build.py

@@ -1,8 +1,15 @@
 from __future__ import annotations
 
+from typing import TYPE_CHECKING
+
 from cleo.helpers import option
 
 from poetry.console.commands.env_command import EnvCommand
+from poetry.utils.env import build_environment
+
+
+if TYPE_CHECKING:
+    from poetry.utils.env import Env
 
 
 class BuildCommand(EnvCommand):
@@ -23,11 +30,13 @@ class BuildCommand(EnvCommand):
     def handle(self) -> None:
         from poetry.core.masonry.builder import Builder
 
-        fmt = self.option("format") or "all"
-        package = self.poetry.package
-        self.line(
-            f"Building <c1>{package.pretty_name}</c1> (<c2>{package.version}</c2>)"
-        )
+        env: Env
+        with build_environment(poetry=self.poetry, env=self.env, io=self.io) as env:
+            fmt = self.option("format") or "all"
+            package = self.poetry.package
+            self.line(
+                f"Building <c1>{package.pretty_name}</c1> (<c2>{package.version}</c2>)"
+            )
 
-        builder = Builder(self.poetry)
-        builder.build(fmt, executable=self.env.python)
+            builder = Builder(self.poetry)
+            builder.build(fmt, executable=env.python)  # type: ignore[attr-defined]

src/poetry/masonry/builders/editable.py

@@ -15,14 +15,15 @@
 
 from poetry.utils._compat import WINDOWS
 from poetry.utils._compat import decode
+from poetry.utils.env import build_environment
 from poetry.utils.helpers import is_dir_writable
 from poetry.utils.pip import pip_install
 
 
 if TYPE_CHECKING:
     from cleo.io.io import IO
-    from poetry.core.poetry import Poetry
 
+    from poetry.poetry import Poetry
     from poetry.utils.env import Env
 
 SCRIPT_TEMPLATE = """\
@@ -75,8 +76,12 @@ def build(self) -> None:
         self._add_dist_info(added_files)
 
     def _run_build_script(self, build_script: Path) -> None:
-        self._debug(f"  - Executing build script: <b>{build_script}</b>")
-        self._env.run("python", str(self._path.joinpath(build_script)), call=True)
+        env: Env
+        with build_environment(poetry=self._poetry, env=self._env, io=self._io) as env:
+            self._debug(f"  - Executing build script: <b>{build_script}</b>")
+            env.run(  # type: ignore[attr-defined]
+                "python", str(self._path.joinpath(build_script)), call=True
+            )
 
     def _setup_build(self) -> None:
         builder = SdistBuilder(self._poetry)

src/poetry/utils/env.py

@@ -1852,6 +1852,46 @@ def ephemeral_environment(
         yield VirtualEnv(venv_dir, venv_dir)
 
 
+@contextmanager
+def build_environment(
+    poetry: Poetry, env: Env | None = None, io: IO | None = None
+) -> ContextManager[Env]:
+    """
+    If a build script is specified for the project, there could be additional build
+    time dependencies, eg: cython, setuptools etc. In these cases, we create an
+    ephemeral build environment with all requirements specified under
+    `build-system.requires` and return this. Otherwise, the given default project
+    environment is returned.
+    """
+    if not env or poetry.package.build_script:
+        with ephemeral_environment(executable=env.python if env else None) as venv:
+            overwrite = io and io.output.is_decorated() and not io.is_debug()
+            if io:
+                requires = map(
+                    lambda r: f"<c1>{r}</c1>", poetry.pyproject.build_system.requires
+                )
+                if not overwrite:
+                    io.write_line("")
+
+                io.overwrite(
+                    "<b>Preparing</b> build environment with build-system requirements"
+                    f" {', '.join(requires)}"
+                )
+            venv.run_pip(
+                "install",
+                "--disable-pip-version-check",
+                "--ignore-installed",
+                *poetry.pyproject.build_system.requires,
+            )
+
+            if overwrite:
+                io.write_line("")
+
+            yield venv
+    else:
+        yield env
+
+
 class MockEnv(NullEnv):
     def __init__(
         self,

tests/fixtures/extended_project_without_setup/pyproject.toml

@@ -27,3 +27,7 @@ generate-setup-file = false
 # Requirements
 [tool.poetry.dependencies]
 python = "~2.7 || ^3.4"
+
+[build-system]
+requires = ["poetry-core", "cython"]
+build-backend = "poetry.core.masonry.api"

tests/masonry/builders/test_editable_builder.py

@@ -229,9 +229,13 @@ def test_builder_installs_proper_files_when_packages_configured(
 
 
 def test_builder_should_execute_build_scripts(
-    extended_without_setup_poetry: Poetry, tmp_dir: str
+    mocker: MockerFixture, extended_without_setup_poetry: Poetry, tmp_dir: str
 ):
     env = MockEnv(path=Path(tmp_dir) / "foo")
+    mocker.patch(
+        "poetry.masonry.builders.editable.build_environment"
+    ).return_value.__enter__.return_value = env
+
     builder = EditableBuilder(extended_without_setup_poetry, env, NullIO())
 
     builder.build()

tests/utils/test_env.py

@@ -25,9 +25,11 @@
 from poetry.utils.env import EnvManager
 from poetry.utils.env import GenericEnv
 from poetry.utils.env import InvalidCurrentPythonVersionError
+from poetry.utils.env import MockEnv
 from poetry.utils.env import NoCompatiblePythonVersionFound
 from poetry.utils.env import SystemEnv
 from poetry.utils.env import VirtualEnv
+from poetry.utils.env import build_environment
 
 
 if TYPE_CHECKING:
@@ -1270,3 +1272,51 @@ def test_generate_env_name_ignores_case_for_case_insensitive_fs(tmp_dir: str):
         assert venv_name1 == venv_name2
     else:
         assert venv_name1 != venv_name2
+
+
+@pytest.fixture()
+def extended_without_setup_poetry() -> Poetry:
+    poetry = Factory().create_poetry(
+        Path(__file__).parent.parent / "fixtures" / "extended_project_without_setup"
+    )
+
+    return poetry
+
+
+def test_build_environment_called_build_script_specified(
+    mocker: MockerFixture, extended_without_setup_poetry: Poetry, tmp_dir: str
+):
+    project_env = MockEnv(path=Path(tmp_dir) / "project")
+    ephemeral_env = MockEnv(path=Path(tmp_dir) / "ephemeral")
+
+    mocker.patch(
+        "poetry.utils.env.ephemeral_environment"
+    ).return_value.__enter__.return_value = ephemeral_env
+
+    with build_environment(extended_without_setup_poetry, project_env) as env:
+        assert env == ephemeral_env
+        assert env.executed == [
+            [
+                "python",
+                env.pip_embedded,
+                "install",
+                "--disable-pip-version-check",
+                "--ignore-installed",
+                *extended_without_setup_poetry.pyproject.build_system.requires,
+            ]
+        ]
+
+
+def test_build_environment_not_called_without_build_script_specified(
+    mocker: MockerFixture, poetry: Poetry, tmp_dir: str
+):
+    project_env = MockEnv(path=Path(tmp_dir) / "project")
+    ephemeral_env = MockEnv(path=Path(tmp_dir) / "ephemeral")
+
+    mocker.patch(
+        "poetry.utils.env.ephemeral_environment"
+    ).return_value.__enter__.return_value = ephemeral_env
+
+    with build_environment(poetry, project_env) as env:
+        assert env == project_env
+        assert not env.executed