test_provision_plugin_runner: testenv depends on plugin-defined runner

A testenv depends on a runner supplied via a plugin that tox needs to
provision.

Added the plugin to demo_pkg_inline since it seemed like the easiest place.

Author: masenf

tests/demo_pkg_inline/build.py

@@ -14,11 +14,38 @@
 version = "1.0.0"
 dist_info = "{}-{}.dist-info".format(name, version)
 logic = "{}/__init__.py".format(name)
+plugin = "{}/example_plugin.py".format(name)
+entry_points = "{}/entry_points.txt".format(dist_info)
 metadata = "{}/METADATA".format(dist_info)
 wheel = "{}/WHEEL".format(dist_info)
 record = "{}/RECORD".format(dist_info)
 content = {
     logic: "def do():\n    print('greetings from {}')".format(name),
+    plugin: dedent(
+        """
+        try:
+            from tox.plugin import impl
+            from tox.tox_env.python.virtual_env.runner import VirtualEnvRunner
+            from tox.tox_env.register import ToxEnvRegister
+        except ImportError:
+            pass
+        else:
+            class ExampleVirtualEnvRunner(VirtualEnvRunner):
+                @staticmethod
+                def id() -> str:
+                    return "example"
+            @impl
+            def tox_register_tox_env(register: ToxEnvRegister) -> None:
+                register.add_run_env(ExampleVirtualEnvRunner)
+    """,
+    ),
+    entry_points: dedent(
+        """
+        [tox]
+        example = {}.example_plugin""".format(
+            name,
+        ),
+    ),
     metadata: """
         Metadata-Version: 2.1
         Name: {}

tests/test_provision.py

@@ -187,3 +187,18 @@ def test_provision_no_recreate_json(tox_project: ToxProjectCreator) -> None:
     with (project.path / "out.json").open() as file_handler:
         requires = json.load(file_handler)
     assert requires == {"minversion": None, "requires": ["p", "tox"]}
+
+
+@pytest.mark.integration()
+@pytest.mark.usefixtures("_pypi_index_self")
+def test_provision_plugin_runner(tox_project: ToxProjectCreator, tmp_path: Path) -> None:
+    log = tmp_path / "out.log"
+    proj = tox_project({"tox.ini": "[tox]\nrequires=demo-pkg-inline\n[testenv]\nrunner=example"})
+    result_first = proj.run("r", "-e", "py", "--result-json", str(log))
+    result_first.assert_success()
+    prov_msg = (
+        f"ROOT: will run in automatically provisioned tox, host {sys.executable} is missing"
+        f" [requires (has)]: demo-pkg-inline"
+    )
+    print(result_first.out)
+    assert prov_msg in result_first.out