Pass --proxy to build subprocesses

Similar to --cert and --client-cert, the --proxy flag was not passed
down to the isolated build environment. This was simply an oversight.

I opted to store the original proxy string in a new attribute on the
session as digging into the .proxies dictionary felt janky, and so did
passing the proxy string to the finder as an argument.

Co-authored-by: lcmartin <[email protected]>

news/6018.bugfix.rst

@@ -0,0 +1 @@
+The ``--proxy`` command-line option is now respected while installing build dependencies.

src/pip/_internal/build_env.py

@@ -272,6 +272,8 @@ def _install_requirements(
         for link in finder.find_links:
             args.extend(["--find-links", link])
 
+        if finder.proxy:
+            args.extend(["--proxy", finder.proxy])
         for host in finder.trusted_hosts:
             args.extend(["--trusted-host", host])
         if finder.client_cert:

src/pip/_internal/cli/index_command.py

@@ -123,6 +123,7 @@ def _build_session(
                 "https": options.proxy,
             }
             session.trust_env = False
+            session.pip_proxy = options.proxy
 
         # Determine if we can prompt the user for authentication or not
         session.auth.prompting = not options.no_input

src/pip/_internal/index/package_finder.py

@@ -661,6 +661,10 @@ def find_links(self) -> List[str]:
     def index_urls(self) -> List[str]:
         return self.search_scope.index_urls
 
+    @property
+    def proxy(self) -> Optional[str]:
+        return self._link_collector.session.pip_proxy
+
     @property
     def trusted_hosts(self) -> Iterable[str]:
         for host_port in self._link_collector.session.pip_trusted_origins:

src/pip/_internal/network/session.py

@@ -339,6 +339,7 @@ def __init__(
         # Namespace the attribute with "pip_" just in case to prevent
         # possible conflicts with the base class.
         self.pip_trusted_origins: List[Tuple[str, Optional[int]]] = []
+        self.pip_proxy = None
 
         # Attach our User Agent to the request
         self.headers["User-Agent"] = user_agent()

tests/functional/test_proxy.py

@@ -90,3 +90,19 @@ def test_proxy_does_not_override_netrc(
             "simple",
         )
         script.assert_installed(simple="3.0")
+
+
+@pytest.mark.network
+def test_build_deps_use_proxy_from_cli(
+    script: PipTestEnvironment, capfd: pytest.CaptureFixture[str], data: TestData
+) -> None:
+    args = ["wheel", "-v", str(data.packages / "pep517_setup_and_pyproject")]
+    args.extend(["--proxy", "http://127.0.0.1:9000"])
+
+    with proxy.Proxy(port=9000, num_acceptors=1, plugins=[AccessLogPlugin]):
+        result = script.pip(*args)
+
+    wheel_path = script.scratch / "pep517_setup_and_pyproject-1.0-py3-none-any.whl"
+    result.did_create(wheel_path)
+    access_log, _ = capfd.readouterr()
+    assert "CONNECT" in access_log, "setuptools was not fetched using proxy"