Fix flaky global Hydra state leaking between tests (#574)

tests/conftest.py

@@ -13,6 +13,7 @@
 import hypothesis.strategies as st
 import pytest
 from hydra.core.config_store import ConfigStore
+from hydra.core.plugins import Plugins
 from hypothesis import settings
 from omegaconf import DictConfig, ListConfig
 
@@ -68,6 +69,14 @@ def cleandir() -> Iterable[str]:
 @pytest.fixture()
 def clean_store() -> Iterable[dict]:
     """Provides access to configstore repo and restores state after test"""
+    # Hydra registers its plugin-provided configs (e.g. `hydra/sweeper/basic`)
+    # into the global ConfigStore lazily, upon first plugin discovery. We force
+    # discovery here so that the snapshot below includes those configs;
+    # otherwise restoring the snapshot would strip them from the global store
+    # and break later tests that compose the Hydra config (e.g. with
+    # `return_hydra_config=True`).
+    # See: https://github.com/mit-ll-responsible-ai/hydra-zen/issues/574
+    Plugins.instance().discover()
     prev_state = deepcopy(_store.repo)
     zen_prev_state = (store._internal_repo.copy(), store._queue.copy())
     yield _store.repo

tests/test_launch/test_global_state_isolation.py

@@ -0,0 +1,69 @@
+# Copyright (c) 2026 Massachusetts Institute of Technology
+# SPDX-License-Identifier: MIT
+import os
+import textwrap
+
+import tests
+
+# `pytester` is enabled via `pytest_plugins = "pytester"` in tests/conftest.py
+
+_REPO_ROOT = os.path.dirname(os.path.dirname(os.path.abspath(tests.__file__)))
+
+
+def test_clean_store_preserves_hydra_plugin_configs(pytester):
+    """Regression test for https://github.com/mit-ll-responsible-ai/hydra-zen/issues/574
+
+    The `clean_store` fixture snapshots and restores Hydra's global ConfigStore.
+    Hydra registers its plugin-provided configs (e.g. ``hydra/sweeper/basic``)
+    into that store lazily, upon first plugin discovery. If the fixture snapshots
+    the store *before* discovery occurs and then restores that snapshot, it
+    strips those plugin configs from the global store -- breaking any later test
+    that composes the Hydra config (e.g. with ``return_hydra_config=True``) with
+    ``MissingConfigException: Could not find 'hydra/sweeper/basic'``.
+
+    This bug is order-dependent (hence the flakiness seen across Python versions
+    in CI). Here we reproduce the triggering order deterministically in an
+    isolated pytest subprocess that uses the *real* ``clean_store`` fixture: the
+    first test snapshots via ``clean_store`` and then triggers discovery; the
+    second test then requires ``hydra/sweeper/basic``.
+    """
+    # Make the project's real `clean_store` fixture available in the subprocess.
+    pytester.makeconftest(
+        f"""
+        import sys
+        sys.path.insert(0, {_REPO_ROOT!r})
+        from tests.conftest import clean_store  # noqa: F401
+        """
+    )
+    pytester.makepyfile(
+        textwrap.dedent(
+            """
+            import pytest
+            from hydra import compose, initialize
+            from hydra.core.config_store import ConfigStore
+
+
+            @pytest.mark.usefixtures("clean_store")
+            def test_snapshot_then_trigger_discovery():
+                # clean_store snapshots the ConfigStore here; this compose then
+                # triggers Hydra's lazy plugin discovery.
+                cs = ConfigStore.instance()
+                cs.store(name="tmp", node={"a": 1})
+                with initialize(config_path=None, version_base="1.3"):
+                    compose(config_name="tmp", overrides=[])
+
+
+            def test_requires_hydra_sweeper_basic():
+                # Fails with MissingConfigException if the previous test's
+                # clean_store teardown stripped hydra/sweeper/basic.
+                cs = ConfigStore.instance()
+                cs.store(name="tmp2", node={"a": 1})
+                with initialize(config_path=None, version_base="1.3"):
+                    compose(
+                        config_name="tmp2", overrides=[], return_hydra_config=True
+                    )
+            """
+        )
+    )
+    result = pytester.runpytest_subprocess("-p", "no:randomly")
+    result.assert_outcomes(passed=2)

tests/test_store.py

@@ -14,6 +14,7 @@
 import pytest
 from hydra.conf import HydraConf
 from hydra.core.config_store import ConfigStore
+from hydra.core.plugins import Plugins
 from hypothesis import assume, given, note, settings
 from omegaconf import DictConfig, ListConfig
 
@@ -35,6 +36,11 @@
 @contextmanager
 def clean_store():
     """Provides access to configstore repo and restores state after test"""
+    # Force Hydra plugin discovery before snapshotting so the snapshot includes
+    # plugin-provided configs (e.g. `hydra/sweeper/basic`); otherwise restoring
+    # it would strip them from the global store. See:
+    # https://github.com/mit-ll-responsible-ai/hydra-zen/issues/574
+    Plugins.instance().discover()
     prev_state = deepcopy(cs.repo)
     zen_prev_state = (default_store._internal_repo.copy(), default_store._queue.copy())
     try: