gh-61215: New mock to wait for multi-threaded events to happen (#16094)

mock: Add `ThreadingMock` class

Add a new class that allows to wait for a call to happen by using
`Event` objects. This mock class can be used to test and validate
expectations of multithreading code.

It uses two attributes for events to distinguish calls with any argument
and calls with specific arguments.

The calls with specific arguments need a lock to prevent two calls in
parallel from creating the same event twice.

The timeout is configured at class and constructor level to allow users
to set a timeout, we considered passing it as an argument to the
function but it could collide with a function parameter. Alternatively
we also considered passing it as positional only but from an API
caller perspective it was unclear what the first number meant on the
function call, think `mock.wait_until_called(1, "arg1", "arg2")`, where
1 is the timeout.

Lastly we also considered adding the new attributes to magic mock
directly rather than having a custom mock class for multi threading
scenarios, but we preferred to have specialised class that can be
composed if necessary. Additionally, having added it to `MagicMock`
directly would have resulted in `AsyncMock` having this logic, which
would not work as expected, since when if user "waits" on a
coroutine does not have the same meaning as waiting on a standard
call.

Co-authored-by: Karthikeyan Singaravelan <tir.karthi@gmail.com>

Lib/unittest/mock.py

@@ -14,6 +14,7 @@ __all__ = (
     'call',
     'create_autospec',
     'AsyncMock',
+    'ThreadingMock',
     'FILTER_DIR',
     'NonCallableMock',
     'NonCallableMagicMock',
@@ -32,6 +33,7 @@ import sys
 import builtins
 import pkgutil
 from asyncio import iscoroutinefunction
+import threading
 from types import CodeType, ModuleType, MethodType
 from unittest.util import safe_repr
 from functools import wraps, partial
@@ -3003,6 +3005,98 @@ class PropertyMock(Mock):
         self(val)
 
 
+_timeout_unset = sentinel.TIMEOUT_UNSET
+
+class ThreadingMixin(Base):
+
+    DEFAULT_TIMEOUT = None
+
+    def _get_child_mock(self, /, **kw):
+        if "timeout" in kw:
+            kw["timeout"] = kw.pop("timeout")
+        elif isinstance(kw.get("parent"), ThreadingMixin):
+            kw["timeout"] = kw["parent"]._mock_wait_timeout
+        elif isinstance(kw.get("_new_parent"), ThreadingMixin):
+            kw["timeout"] = kw["_new_parent"]._mock_wait_timeout
+        return super()._get_child_mock(**kw)
+
+    def __init__(self, *args, timeout=_timeout_unset, **kwargs):
+        super().__init__(*args, **kwargs)
+        if timeout is _timeout_unset:
+            timeout = self.DEFAULT_TIMEOUT
+        self.__dict__["_mock_event"] = threading.Event()  # Event for any call
+        self.__dict__["_mock_calls_events"] = []  # Events for each of the calls
+        self.__dict__["_mock_calls_events_lock"] = threading.Lock()
+        self.__dict__["_mock_wait_timeout"] = timeout
+
+    def reset_mock(self, /, *args, **kwargs):
+        """
+        See :func:`.Mock.reset_mock()`
+        """
+        super().reset_mock(*args, **kwargs)
+        self.__dict__["_mock_event"] = threading.Event()
+        self.__dict__["_mock_calls_events"] = []
+
+    def __get_event(self, expected_args, expected_kwargs):
+        with self._mock_calls_events_lock:
+            for args, kwargs, event in self._mock_calls_events:
+                if (args, kwargs) == (expected_args, expected_kwargs):
+                    return event
+            new_event = threading.Event()
+            self._mock_calls_events.append((expected_args, expected_kwargs, new_event))
+        return new_event
+
+    def _mock_call(self, *args, **kwargs):
+        ret_value = super()._mock_call(*args, **kwargs)
+
+        call_event = self.__get_event(args, kwargs)
+        call_event.set()
+
+        self._mock_event.set()
+
+        return ret_value
+
+    def wait_until_called(self, *, timeout=_timeout_unset):
+        """Wait until the mock object is called.
+
+        `timeout` - time to wait for in seconds, waits forever otherwise.
+        Defaults to the constructor provided timeout.
+        Use None to block undefinetively.
+        """
+        if timeout is _timeout_unset:
+            timeout = self._mock_wait_timeout
+        if not self._mock_event.wait(timeout=timeout):
+            msg = (f"{self._mock_name or 'mock'} was not called before"
+                   f" timeout({timeout}).")
+            raise AssertionError(msg)
+
+    def wait_until_any_call(self, *args, **kwargs):
+        """Wait until the mock object is called with given args.
+
+        Waits for the timeout in seconds provided in the constructor.
+        """
+        event = self.__get_event(args, kwargs)
+        if not event.wait(timeout=self._mock_wait_timeout):
+            expected_string = self._format_mock_call_signature(args, kwargs)
+            raise AssertionError(f'{expected_string} call not found')
+
+
+class ThreadingMock(ThreadingMixin, MagicMixin, Mock):
+    """
+    A mock that can be used to wait until on calls happening
+    in a different thread.
+
+    The constructor can take a `timeout` argument which
+    controls the timeout in seconds for all `wait` calls of the mock.
+
+    You can change the default timeout of all instances via the
+    `ThreadingMock.DEFAULT_TIMEOUT` attribute.
+
+    If no timeout is set, it will block undefinetively.
+    """
+    pass
+
+
 def seal(mock):
     """Disable the automatic generation of child mocks.