Ensure automatically provisioned environment is torn down

docs/changelog/3600.bugfix.rst

@@ -0,0 +1,4 @@
+Previously, when tox ran in an automatically provisioned environment, it could hang waiting for a PEP 517 build backend
+if used in conjunction with the ``--installpkg`` option. This has been fixed by properly tearing down the automatically
+provisioned environment after the tests.
+- by :user:`vytas7`

src/tox/provision.py

@@ -146,10 +146,14 @@ def run_provision(name: str, state: State) -> int:
     logging.info("will run in a automatically provisioned python environment under %s", env_python)
     try:
         tox_env.setup()
+        args: list[str] = [str(env_python), "-m", "tox"]
+        args.extend(state.args)
+        outcome = tox_env.execute(
+            cmd=args, stdin=StdinSource.user_only(), show=True, run_id="provision", cwd=Path.cwd()
+        )
+        return cast("int", outcome.exit_code)
     except Skip as exception:
         msg = f"cannot provision tox environment {tox_env.conf['env_name']} because {exception}"
         raise HandledError(msg) from exception
-    args: list[str] = [str(env_python), "-m", "tox"]
-    args.extend(state.args)
-    outcome = tox_env.execute(cmd=args, stdin=StdinSource.user_only(), show=True, run_id="provision", cwd=Path.cwd())
-    return cast("int", outcome.exit_code)
+    finally:
+        tox_env.teardown()

src/tox/tox_env/package.py

@@ -97,7 +97,8 @@ def mark_active_run_env(self, run_env: RunToxEnv) -> None:
         self._envs.add(run_env.conf.name)
 
     def teardown_env(self, conf: EnvConfigSet) -> None:
-        self._envs.remove(conf.name)
+        if conf.name in self._envs:
+            self._envs.remove(conf.name)
         if len(self._envs) == 0:
             self._teardown()