When provisioning ignore other env configurations

When running provision, ignore other environments, since their configuration
might not be valid until running in the provisioned environment.

Explicitly instantiate the env config for the provision environment without a
base so that any directives set in `testenv` will not leak through into the
provision environment.

Fix #2862

docs/changelog/2862.bugfix.rst

@@ -0,0 +1,4 @@
+The provision environment (``.tox``) will never inherit from ``testenv``.
+During provisioning, other test environments are not processed, allowing the
+use of keys and values that may be registered by later tox version or
+provisioned plugins - by :user:`masenf`.

src/tox/config/main.py

@@ -147,6 +147,7 @@ def get_env(
         item: str,
         package: bool = False,
         loaders: Sequence[Loader[Any]] | None = None,
+        base: list[str] | None = None,
     ) -> EnvConfigSet:
         """
         Return the configuration for a given tox environment (will create if not exist yet).
@@ -157,9 +158,11 @@ def get_env(
         :return: the tox environments config
         """
         section, base_test, base_pkg = self._src.get_tox_env_section(item)
+        if base is None:
+            base = base_pkg if package else base_test
         conf_set = self.get_section_config(
             section,
-            base=base_pkg if package else base_test,
+            base=base,
             of_type=EnvConfigSet,
             for_env=item,
             loaders=loaders,

src/tox/session/env_select.py

@@ -241,13 +241,29 @@ def _finalize_config(self) -> None:
         self._state.conf.core.mark_finalized()
 
     def _build_run_env(self, name: str) -> RunToxEnv | None:
-        if self._provision is not None and self._provision[0] is False and name == self._provision[1]:
+        provision_on = provision_tox_env = provision_loader = None
+        if self._provision is not None:
+            provision_on, provision_tox_env, provision_loader = self._provision
+        if not provision_on and name == provision_tox_env:
+            # not a provision run, so don't include provision env (.tox) as a run env
             return None
-        env_conf = self._state.conf.get_env(name, package=False)
-        if self._provision is not None and self._provision[1] == name:
-            env_conf.loaders.insert(0, self._provision[2])
-        desc = "the tox execute used to evaluate this environment"
-        env_conf.add_config(keys="runner", desc=desc, of_type=str, default=self._state.conf.options.default_runner)
+        if provision_on and name != provision_tox_env:
+            # for provision run, don't consider any environment other than provision env
+            return None
+        if name == provision_tox_env:
+            # explicitly load the provision env without any base to avoid inheritance
+            env_conf = self._state.conf.get_env(name, package=False, base=[])
+            if provision_loader is not None:
+                env_conf.loaders.insert(0, provision_loader)
+        else:
+            env_conf = self._state.conf.get_env(name, package=False)
+        runner_key_desc = "the tox execute used to evaluate this environment"
+        env_conf.add_config(
+            keys="runner",
+            desc=runner_key_desc,
+            of_type=str,
+            default=self._state.conf.options.default_runner,
+        )
         runner = REGISTER.runner(cast(str, env_conf["runner"]))
         journal = self._journal.get_env_journal(name)
         args = ToxEnvCreateArgs(env_conf, self._state.conf.core, self._state.conf.options, journal, self._log_handler)