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

[martinez-hub]

Summary

Fixes #574 — the intermittent MissingConfigException: In 'hydra/config': Could not find 'hydra/sweeper/basic' failures in tests/test_launch/test_validation.py that surface as flaky, Python-version-correlated CI failures.

Root cause

The clean_store fixtures (in tests/conftest.py and tests/test_store.py) snapshot and restore Hydra's global ConfigStore. Hydra registers its plugin-provided configs (e.g. hydra/sweeper/basic) into that store lazily, upon first plugin discovery (triggered by the first compose/initialize).

If a fixture snapshots the store before discovery has occurred and then restores that snapshot on teardown, it strips those plugin configs from the global store — permanently, since Plugins caches discovery and won't re-register them. Any later test that composes the Hydra config (e.g. with return_hydra_config=True) then fails to find hydra/sweeper/basic.

This is order-dependent, which is exactly why it manifested as flaky failures that correlated with the Python version (the set/order of collected tests differs across versions — e.g. *py310* tests are excluded on 3.9).

This can be reproduced deterministically:

from copy import deepcopy
from hydra import compose, initialize
from hydra.core.config_store import ConfigStore

cs = ConfigStore.instance()
prev = deepcopy(cs.repo)                     # snapshot BEFORE discovery
with initialize(config_path=None, version_base="1.3"):  # triggers discovery
    pass
cs.repo = prev                               # restore -> strips hydra/sweeper/basic
cs.store(name="cfg", node={"a": 1})
with initialize(config_path=None, version_base="1.3"):
    compose(config_name="cfg", overrides=[], return_hydra_config=True)
# MissingConfigException: Could not find 'hydra/sweeper/basic'

Fix

Force Hydra plugin discovery (Plugins.instance().discover()) before snapshotting in both clean_store fixtures, so the snapshot includes the plugin configs and the restore preserves them.

This is a test-infrastructure fix — there is no user-facing change, so no changelog entry is added. (Note: the approach floated earlier in #574 of clearing Singleton._instances inside launch/hydra_main was intentionally avoided — Hydra's ConfigStore is itself stored in Singleton._instances, so clearing it would wipe user-registered configs, and it does not address the actual mechanism above.)

Regression test

tests/test_launch/test_global_state_isolation.py reproduces the triggering order deterministically in an isolated pytester subprocess, using the real clean_store fixture: the first test snapshots via clean_store and then triggers discovery; the second test then requires hydra/sweeper/basic. Verified to fail without this fix and pass with it.

Verification

• Regression test: red before the fix, green after.
• Full test suite passes on Python 3.9 + hydra-core 1.3.3 (the configuration that was failing in CI): 1605 passed.
• ruff format --check and ruff check clean on all changed files.