Pass CA and client TLS certificates to build subprocesses (#13063)

The _PIP_STANDALONE_CERT environment variable hack is no longer required
as pip doesn't run a zip archive of itself to provision build dependencies
these days (which due to a CPython bug would leave behind temporary certifi
files).

Some people do depend on this private envvar in the wild, so the removal
has been called out in the news entry.

news/5502.bugfix.rst

@@ -0,0 +1,2 @@
+Configured TLS server and client certificates are now used while installing build dependencies.
+Consequently, the private ``_PIP_STANDALONE_CERT`` environment variable is no longer used.

src/pip/_internal/build_env.py

@@ -246,6 +246,8 @@ def _install_requirements(
             # target from config file or env var should be ignored
             "--target",
             "",
+            "--cert",
+            finder.custom_cert or where(),
         ]
         if logger.getEffectiveLevel() <= logging.DEBUG:
             args.append("-vv")
@@ -272,19 +274,19 @@ def _install_requirements(
 
         for host in finder.trusted_hosts:
             args.extend(["--trusted-host", host])
+        if finder.client_cert:
+            args.extend(["--client-cert", finder.client_cert])
         if finder.allow_all_prereleases:
             args.append("--pre")
         if finder.prefer_binary:
             args.append("--prefer-binary")
         args.append("--")
         args.extend(requirements)
-        extra_environ = {"_PIP_STANDALONE_CERT": where()}
         with open_spinner(f"Installing {kind}") as spinner:
             call_subprocess(
                 args,
                 command_desc=f"pip subprocess to install {kind}",
                 spinner=spinner,
-                extra_environ=extra_environ,
             )
 
 

src/pip/_internal/index/package_finder.py

@@ -666,6 +666,20 @@ def trusted_hosts(self) -> Iterable[str]:
         for host_port in self._link_collector.session.pip_trusted_origins:
             yield build_netloc(*host_port)
 
+    @property
+    def custom_cert(self) -> Optional[str]:
+        # session.verify is either a boolean (use default bundle/no SSL
+        # verification) or a string path to a custom CA bundle to use. We only
+        # care about the latter.
+        verify = self._link_collector.session.verify
+        return verify if isinstance(verify, str) else None
+
+    @property
+    def client_cert(self) -> Optional[str]:
+        cert = self._link_collector.session.cert
+        assert not isinstance(cert, tuple), "pip only supports PEM client certs"
+        return cert
+
     @property
     def allow_all_prereleases(self) -> bool:
         return self._candidate_prefs.allow_all_prereleases

src/pip/_vendor/requests/certs.py

@@ -11,14 +11,7 @@
 environment, you can change the definition of where() to return a separately
 packaged CA bundle.
 """
-
-import os
-
-if "_PIP_STANDALONE_CERT" not in os.environ:
-    from pip._vendor.certifi import where
-else:
-    def where():
-        return os.environ["_PIP_STANDALONE_CERT"]
+from pip._vendor.certifi import where
 
 if __name__ == "__main__":
     print(where())

tests/functional/test_install.py

@@ -2361,6 +2361,38 @@ def test_install_sends_client_cert(
         assert environ["SSL_CLIENT_CERT"]
 
 
+def test_install_sends_certs_for_pep518_deps(
+    script: PipTestEnvironment,
+    cert_factory: CertFactory,
+    data: TestData,
+    common_wheels: Path,
+) -> None:
+    cert_path = cert_factory()
+    ctx = ssl.SSLContext(ssl.PROTOCOL_SSLv23)
+    ctx.load_cert_chain(cert_path, cert_path)
+    ctx.load_verify_locations(cafile=cert_path)
+    ctx.verify_mode = ssl.CERT_REQUIRED
+
+    setuptools_pkg = next(common_wheels.glob("setuptools*")).name
+    server = make_mock_server(ssl_context=ctx)
+    server.mock.side_effect = [
+        package_page({setuptools_pkg: f"/files/{setuptools_pkg}"}),
+        file_response(common_wheels / setuptools_pkg),
+    ]
+    url = f"https://{server.host}:{server.port}/simple"
+
+    args = ["install", str(data.packages / "pep517_setup_and_pyproject")]
+    args.extend(["--index-url", url])
+    args.extend(["--cert", cert_path, "--client-cert", cert_path])
+
+    with server_running(server):
+        script.pip(*args)
+
+    for call_args in server.mock.call_args_list:
+        environ, _ = call_args.args
+        assert environ.get("SSL_CLIENT_CERT", "")
+
+
 def test_install_skip_work_dir_pkg(script: PipTestEnvironment, data: TestData) -> None:
     """
     Test that install of a package in working directory

tools/vendoring/patches/requests.patch

@@ -84,26 +84,6 @@ index 8fbcd656..094e2046 100644
 
  try:
      from urllib3.contrib import pyopenssl
-diff --git a/src/pip/_vendor/requests/certs.py b/src/pip/_vendor/requests/certs.py
-index be422c3e..3daf06f6 100644
---- a/src/pip/_vendor/requests/certs.py
-+++ b/src/pip/_vendor/requests/certs.py
-@@ -11,7 +11,14 @@ If you are packaging Requests, e.g., for a Linux distribution or a managed
- environment, you can change the definition of where() to return a separately
- packaged CA bundle.
- """
--from certifi import where
-+
-+import os
-+
-+if "_PIP_STANDALONE_CERT" not in os.environ:
-+    from certifi import where
-+else:
-+    def where():
-+        return os.environ["_PIP_STANDALONE_CERT"]
-
- if __name__ == "__main__":
-     print(where())
 diff --git a/src/pip/_vendor/requests/__init__.py b/src/pip/_vendor/requests/__init__.py
 index 9d4e72c60..04230fc8d 100644
 --- a/src/pip/_vendor/requests/__init__.py