fix: thread safety issues

openfeature/_event_support.py

@@ -61,6 +61,13 @@ def add_client_handler(
         handlers = _client_handlers[client][event]
         handlers.append(handler)
 
+    # outside the lock intentionally: the immediate-fire status check acquires the registry lock, so calling it
+    # under _client_lock risks lock-order inversion against run_handlers_for_provider (registry lock → _client_lock).
+    # As a consequence, a narrow double-fire is possible: if dispatch_event(client's event) runs concurrently, it
+    # sets the matching provider status (enabling the immediate fire below) and then re-runs every handler for this
+    # client. If _run_immediate_handler lands after that status set but before dispatch snapshots the handler list,
+    # the handler fires twice — once here, once from dispatch. Only happens when the registered event matches the event
+    # being dispatched; otherwise the immediate fire is a no-op.
     _run_immediate_handler(client, event, handler)
 
 
@@ -78,6 +85,7 @@ def add_global_handler(event: ProviderEvent, handler: EventHandler) -> None:
 
     from openfeature.api import get_client  # noqa: PLC0415
 
+    # See comment in add_client_handler for why this runs outside the lock.
     _run_immediate_handler(get_client(), event, handler)
 
 
@@ -133,7 +141,6 @@ def _run_handler(handler: EventHandler, details: EventDetails) -> None:
 
 
 def clear() -> None:
-    with _global_lock:
+    with _global_lock, _client_lock:
         _global_handlers.clear()
-    with _client_lock:
         _client_handlers.clear()

openfeature/client.py

@@ -1,4 +1,5 @@
 import logging
+import threading
 import typing
 from collections.abc import Awaitable, Mapping, Sequence
 from dataclasses import dataclass
@@ -86,6 +87,7 @@ def __init__(
         self.version = version
         self.context = context or EvaluationContext()
         self.hooks = hooks or []
+        self._hooks_lock = threading.RLock()
 
     @property
     def provider(self) -> FeatureProvider:
@@ -98,7 +100,10 @@ def get_metadata(self) -> ClientMetadata:
         return ClientMetadata(domain=self.domain)
 
     def add_hooks(self, hooks: list[Hook]) -> None:
-        self.hooks = self.hooks + hooks
+        # Guards the read-concat-store against a lost update; this practically never races under the default 5ms GIL
+        # switch interval, but is essential under a no-GIL build.
+        with self._hooks_lock:
+            self.hooks = self.hooks + hooks
 
     def get_boolean_value(
         self,
@@ -468,8 +473,9 @@ def _establish_hooks_and_provider(
 
     def _assert_provider_status(
         self,
+        provider: FeatureProvider,
     ) -> OpenFeatureError | None:
-        status = self.get_provider_status()
+        status = provider_registry.get_provider_status(provider)
         if status == ProviderStatus.NOT_READY:
             return ProviderNotReadyError()
         if status == ProviderStatus.FATAL:
@@ -589,7 +595,7 @@ async def evaluate_flag_details_async(
         )
 
         try:
-            if provider_err := self._assert_provider_status():
+            if provider_err := self._assert_provider_status(provider):
                 error_hooks(
                     flag_type,
                     provider_err,
@@ -765,7 +771,7 @@ def evaluate_flag_details(
         )
 
         try:
-            if provider_err := self._assert_provider_status():
+            if provider_err := self._assert_provider_status(provider):
                 error_hooks(
                     flag_type,
                     provider_err,

openfeature/hook/__init__.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import threading
 import typing
 from collections.abc import Mapping, MutableMapping, Sequence
 from datetime import datetime
@@ -24,6 +25,7 @@
 ]
 
 _hooks: list[Hook] = []
+_hooks_lock = threading.RLock()
 
 
 # https://openfeature.dev/specification/sections/hooks/#requirement-461
@@ -151,14 +153,21 @@ def supports_flag_value_type(self, flag_type: FlagType) -> bool:
         return True
 
 
+# while the lock guarantees safety, even without it there was never a loss within 50.000 runs (with the default GIL
+# switch interval of 5ms). only when the switch interval was significantly shortened to 0.1 microseconds, losses were
+# observed without locks every now and then. with a no-GIL python, the lock would be essential
+
+
 def add_hooks(hooks: list[Hook]) -> None:
-    global _hooks
-    _hooks = _hooks + hooks
+    with _hooks_lock:
+        global _hooks
+        _hooks = _hooks + hooks
 
 
 def clear_hooks() -> None:
-    global _hooks
-    _hooks = []
+    with _hooks_lock:
+        global _hooks
+        _hooks = []
 
 
 def get_hooks() -> list[Hook]:

openfeature/provider/__init__.py

@@ -261,5 +261,6 @@ def emit_provider_stale(self, details: ProviderEventDetails) -> None:
         self.emit(ProviderEvent.PROVIDER_STALE, details)
 
     def emit(self, event: ProviderEvent, details: ProviderEventDetails) -> None:
-        if hasattr(self, "_on_emit"):
-            self._on_emit(self, event, details)
+        on_emit = getattr(self, "_on_emit", None)
+        if on_emit is not None:
+            on_emit(self, event, details)

openfeature/transaction_context/__init__.py

@@ -1,3 +1,5 @@
+import threading
+
 from openfeature.evaluation_context import EvaluationContext
 from openfeature.transaction_context.context_var_transaction_context_propagator import (
     ContextVarsTransactionContextPropagator,
@@ -21,25 +23,28 @@
 _evaluation_transaction_context_propagator: TransactionContextPropagator = (
     NoOpTransactionContextPropagator()
 )
+_propagator_lock = threading.RLock()
 
 
 def set_transaction_context_propagator(
     transaction_context_propagator: TransactionContextPropagator,
 ) -> None:
     global _evaluation_transaction_context_propagator
-    _evaluation_transaction_context_propagator = transaction_context_propagator
+    with _propagator_lock:
+        _evaluation_transaction_context_propagator = transaction_context_propagator
 
 
 def clear_transaction_context_propagator() -> None:
     set_transaction_context_propagator(NoOpTransactionContextPropagator())
 
 
 def get_transaction_context() -> EvaluationContext:
-    return _evaluation_transaction_context_propagator.get_transaction_context()
+    with _propagator_lock:
+        propagator = _evaluation_transaction_context_propagator
+        return propagator.get_transaction_context()
 
 
 def set_transaction_context(evaluation_context: EvaluationContext) -> None:
-    global _evaluation_transaction_context_propagator
-    _evaluation_transaction_context_propagator.set_transaction_context(
-        evaluation_context
-    )
+    with _propagator_lock:
+        propagator = _evaluation_transaction_context_propagator
+        propagator.set_transaction_context(evaluation_context)

tests/test_client.py

@@ -4,11 +4,12 @@
 import types
 import uuid
 from concurrent.futures import ThreadPoolExecutor
-from unittest.mock import MagicMock
+from unittest.mock import MagicMock, Mock
 
 import pytest
 
 from openfeature import _event_support, api
+from openfeature import client as client_module
 from openfeature.api import (
     add_hooks,
     clear_hooks,
@@ -20,7 +21,7 @@
 from openfeature.client import OpenFeatureClient, _typecheck_flag_value
 from openfeature.evaluation_context import EvaluationContext
 from openfeature.event import EventDetails, ProviderEvent, ProviderEventDetails
-from openfeature.exception import ErrorCode, OpenFeatureError
+from openfeature.exception import ErrorCode, OpenFeatureError, ProviderFatalError
 from openfeature.flag_evaluation import FlagResolutionDetails, FlagType, Reason
 from openfeature.hook import Hook
 from openfeature.provider import FeatureProvider, ProviderStatus
@@ -291,9 +292,10 @@ def test_provider_should_return_error_status_if_failed():
 async def test_should_shortcircuit_if_provider_is_not_ready(
     no_op_provider_client, monkeypatch
 ):
-    # Given
     monkeypatch.setattr(
-        no_op_provider_client, "get_provider_status", lambda: ProviderStatus.NOT_READY
+        provider_registry,
+        "get_provider_status",
+        lambda provider: ProviderStatus.NOT_READY,
     )
     spy_hook = MagicMock(spec=Hook)
     no_op_provider_client.add_hooks([spy_hook])
@@ -321,9 +323,10 @@ async def test_should_shortcircuit_if_provider_is_not_ready(
 async def test_should_shortcircuit_if_provider_is_in_irrecoverable_error_state(
     no_op_provider_client, monkeypatch
 ):
-    # Given
     monkeypatch.setattr(
-        no_op_provider_client, "get_provider_status", lambda: ProviderStatus.FATAL
+        provider_registry,
+        "get_provider_status",
+        lambda provider: ProviderStatus.FATAL,
     )
     spy_hook = MagicMock(spec=Hook)
     no_op_provider_client.add_hooks([spy_hook])
@@ -768,3 +771,32 @@ def test_should_noop_if_provider_does_not_support_tracking(monkeypatch):
     set_provider(provider)
     client = get_client()
     client.track(tracking_event_name="test")
+
+
+def test_assert_provider_status_uses_passed_provider_not_current_registry_state():
+    fatal_provider = NoOpProvider()
+    ready_provider = NoOpProvider()
+
+    registry_mock = Mock()
+    registry_mock.get_provider_status.side_effect = lambda p: (
+        ProviderStatus.FATAL if p is fatal_provider else ProviderStatus.READY
+    )
+    registry_mock.get_provider.return_value = ready_provider
+
+    original = client_module.provider_registry
+    client_module.provider_registry = registry_mock
+    try:
+        c = OpenFeatureClient(domain=None, version=None)
+        assert c.provider is ready_provider, (
+            "test setup: self.provider should resolve via the patched registry"
+        )
+
+        err = c._assert_provider_status(fatal_provider)
+        assert isinstance(err, ProviderFatalError), (
+            "status check used self.provider (READY) instead of the captured "
+            "fatal_provider — TOCTOU regression"
+        )
+        registry_mock.get_provider_status.assert_any_call(fatal_provider)
+        assert c._assert_provider_status(ready_provider) is None
+    finally:
+        client_module.provider_registry = original