Allow the user to set PIP_CONSTRAINT and it affect build-system.requires

cibuildwheel/macos.py

@@ -409,7 +409,9 @@ def build(options: Options, tmp_path: Path) -> None:
                                 config.version
                             )
                         )
-                        build_env["PIP_CONSTRAINT"] = constraint_path.as_uri()
+                        build_env["PIP_CONSTRAINT"] = (
+                            constraint_path.as_uri() + " " + build_env.get("PIP_CONSTRAINT", "")
+                        )
                     build_env["VIRTUALENV_PIP"] = get_pip_version(env)
                     call(
                         "python",

cibuildwheel/windows.py

@@ -455,7 +455,9 @@ def build(options: Options, tmp_path: Path) -> None:
                             tmp_file.write_bytes(constraints_path.read_bytes())
                             constraints_path = tmp_file
 
-                        build_env["PIP_CONSTRAINT"] = str(constraints_path)
+                        build_env["PIP_CONSTRAINT"] = (
+                            str(constraints_path) + " " + build_env.get("PIP_CONSTRAINT", "")
+                        )
                         build_env["VIRTUALENV_PIP"] = get_pip_version(env)
                         call(
                             "python",

docs/options.md

@@ -674,7 +674,7 @@ Platform-specific environment variables are also available:<br/>
     cibuildwheel always defines the environment variable `CIBUILDWHEEL=1`. This can be useful for [building wheels with optional extensions](faq.md#building-packages-with-optional-c-extensions).
 
 !!! note
-    To do its work, cibuildwheel internally sets the options `PIP_CONSTRAINT`, `VIRTUALENV_PIP`, `DIST_EXTRA_CONFIG`, `SETUPTOOLS_EXT_SUFFIX`, `PIP_DISABLE_PIP_VERSION_CHECK`, `PIP_ROOT_USER_ACTION`. Your assignments to these options might be overridden.
+    To do its work, cibuildwheel internally sets the options `VIRTUALENV_PIP`, `DIST_EXTRA_CONFIG`, `SETUPTOOLS_EXT_SUFFIX`, `PIP_DISABLE_PIP_VERSION_CHECK`, `PIP_ROOT_USER_ACTION`. Your assignments to these options might be overridden.
 
 ### `CIBW_ENVIRONMENT_PASS_LINUX` {: #environment-pass}
 > Set environment variables on the host to pass-through to the container.

test/test_environment.py

@@ -93,3 +93,40 @@ def test_overridden_path(tmp_path, capfd):
     assert len(os.listdir(output_dir)) == 0
     captured = capfd.readouterr()
     assert "python available on PATH doesn't match our installed instance" in captured.err
+
+
+@pytest.mark.parametrize("build_frontend", ["pip", "build"])
+def test_overridden_pip_constraint(tmp_path, build_frontend):
+    project_dir = tmp_path / "project"
+
+    project = test_projects.new_c_project(
+        setup_py_add=textwrap.dedent(
+            """
+            import pytz
+            assert pytz.__version__ == "2022.4"
+        """
+        )
+    )
+    project.files["pyproject.toml"] = textwrap.dedent(
+        """
+        [build-system]
+        requires = ["setuptools", "pytz"]
+        build-backend = "setuptools.build_meta"
+    """
+    )
+    project.generate(project_dir)
+
+    constraints_file = tmp_path / "constraints.txt"
+    constraints_file.write_text("pytz==2022.4")
+
+    actual_wheels = utils.cibuildwheel_run(
+        project_dir,
+        add_env={
+            "CIBW_BUILD": "cp312-*",
+            "CIBW_BUILD_FRONTEND": build_frontend,
+            "CIBW_ENVIRONMENT": f"PIP_CONSTRAINT={constraints_file}",
+        },
+    )
+
+    expected_wheels = [w for w in utils.expected_wheels("spam", "0.1.0") if "cp312" in w]
+    assert set(actual_wheels) == set(expected_wheels)