fix(flags): only install HyperCacheFlagProvider in US

[sampennington]

Problem

Local feature-flag evaluation in our server-side posthoganalytics SDK returns None instead of True/False in EU pods (works in US). Customer-facing example: posthoganalytics.feature_enabled("persons-on-events-v2-reads-enabled", ..., only_evaluate_locally=True) finds no flag definition.

apps.py keys HyperCacheFlagProvider by team_id=2 (no env override is set anywhere), assuming "local team 2 == PostHog company project". That holds in US Postgres, not in EU. The provider also returns a truthy dict for an empty flags list, so the SDK accepts the EU cache as authoritative and never falls back to its API path.

Changes

Gate provider install on settings.CLOUD_DEPLOYMENT == "US". Outside US, leave flag_definition_cache_provider unset so the SDK polls posthoganalytics.host (us.i.posthog.com) directly — the cross-region behavior that worked before #50737. Gate lives in a small get_default_flag_definition_cache_provider() helper in sdk_cache_provider.py so it's testable without invoking AppConfig.ready().

How did you test this code?

I'm an agent (PostHog Code). Added TestGetDefaultFlagDefinitionCacheProvider parameterized over US / EU / DEV / E2E / LOCAL / unset, with and without POSTHOG_SELF_TEAM_ID override (8 cases). Existing tests untouched. Did not run the full pytest suite — sandbox lacked the flox venv. Verified ruff check clean and smoke-tested the helper against all 8 cases.

Publish to changelog?

no

🤖 Agent context

• Authored by PostHog Code (Claude). Human review required.
• Diagnosis: apps.py:79-88, sdk_cache_provider.py:43-68, and posthoganalytics 7.13.0::client.py::_load_feature_flags (lines 1253-1296 — the truthy-payload-accepted-as-authoritative path).
• Deviation from the originating prompt: the gate was extracted into a small factory rather than placed inline in apps.py, because AppConfig.ready() forces posthoganalytics.disabled=True in TEST mode and runs many other side effects, making a true regression test fragile. The factory captures the same semantic gate and is directly testable.
• Reviewer should sanity-check that EU Postgres team 2 isn't the company project (the fix doesn't depend on which team it is, only that it's not the company project — but worth confirming).
• Follow-ups (not in this PR): metric/log when get_flag_definitions() returns empty flags; CLOUD_DEPLOYMENT in {"US"} allowlist constant for future US-WEST style splits.

[greptile-apps]

Prompt To Fix All With AI

This is a comment left during a code review.
Path: posthog/feature_flags/test_sdk_cache_provider.py
Line: 227-250

Comment:
**Non-parameterized US and EU override tests**

The four test methods in this class test the same function with varying `(CLOUD_DEPLOYMENT, POSTHOG_SELF_TEAM_ID, expected_result)` combinations. Per the project's preference for parameterised tests, these could be consolidated into a single `@parameterized.expand` case list, eliminating the repeated `setUp`/`tearDown` env-isolation boilerplate and making it trivial to add new regions or env-var combinations:

```python
@parameterized.expand([
    ("us_default",         "US",   None,  2),
    ("us_env_override",    "US",   "42",  42),
    ("eu_no_env",          "EU",   None,  None),
    ("eu_env_override",    "EU",   "42",  None),
    ("dev",                "DEV",  None,  None),
    ("e2e",                "E2E",  None,  None),
    ("none",               None,   None,  None),
])
@patch.dict(os.environ, {}, clear=False)
def test_provider(self, _name, deployment, env_team_id, expected_team_id):
    if env_team_id is not None:
        os.environ["POSTHOG_SELF_TEAM_ID"] = env_team_id
    else:
        os.environ.pop("POSTHOG_SELF_TEAM_ID", None)
    with override_settings(CLOUD_DEPLOYMENT=deployment):
        provider = get_default_flag_definition_cache_provider()
    if expected_team_id is None:
        assert provider is None
    else:
        assert isinstance(provider, HyperCacheFlagProvider)
        assert provider._team_id == expected_team_id
```

This covers all seven current cases in one place and makes the separate `setUp`/`tearDown` unnecessary.

How can I resolve this? If you propose a fix, please make it concise.

_{Reviews (1): Last reviewed commit: "fix(flags): only install HyperCacheFlagP..." | Re-trigger Greptile}