Add missing exception objects

changelog.rst

@@ -8,7 +8,9 @@ Changelog
 
 2021-0x-x • `full history <https://github.com/gorakhargosh/watchdog/compare/v2.0.1...master>`__
 
-- Thanks to our beloved contributors: @
+- [mac] Add missing exception objects (`#766 <https://github.com/gorakhargosh/watchdog/pull/766>`_)
+
+- Thanks to our beloved contributors: @CCP-Aporia
 
 
 2.0.1

src/watchdog_fsevents.c

@@ -634,7 +634,10 @@ watchdog_add_watch(PyObject *self, PyObject *args)
                                           &python_callback, &paths_to_watch));
 
     /* Watch must not already be scheduled. */
-    G_RETURN_NULL_IF(PyDict_Contains(watch_to_stream, watch) == 1);
+    if(PyDict_Contains(watch_to_stream, watch) == 1) {
+        PyErr_Format(PyExc_RuntimeError, "Cannot add watch %S - it is already scheduled", watch);
+        return NULL;
+    }
 
     /* Create an instance of the callback information structure. */
     stream_callback_info_ref = PyMem_New(StreamCallbackInfo, 1);
@@ -689,6 +692,9 @@ watchdog_add_watch(PyObject *self, PyObject *args)
     {
         FSEventStreamInvalidate(stream_ref);
         FSEventStreamRelease(stream_ref);
+        // There's no documentation on _why_ this might fail - "it ought to always succeed". But if it fails the
+        // documentation says to "fall back to performing recursive scans of the directories [...] as appropriate".
+        PyErr_SetString(PyExc_SystemError, "Cannot start fsevents stream. Use a kqueue or polling observer instead.");
         return NULL;
     }
 

tests/test_fsevents.py

@@ -79,6 +79,27 @@ def test_coalesced_event_check(event, expectation):
     assert event.is_coalesced == expectation
 
 
+def test_add_watch_twice(observer):
+    """ Adding the same watch twice used to result in a null pointer return without an exception.
+
+    See https://github.com/gorakhargosh/watchdog/issues/765
+    """
+
+    a = p("a")
+    mkdir(a)
+    h = FileSystemEventHandler()
+    w = ObservedWatch(a, recursive=False)
+
+    def callback(path, inodes, flags, ids):
+        pass
+
+    _fsevents.add_watch(h, w, callback, [w.path])
+    with pytest.raises(RuntimeError):
+        _fsevents.add_watch(h, w, callback, [w.path])
+    _fsevents.remove_watch(w)
+    rmdir(a)
+
+
 def test_remove_watch_twice():
     """
 ValueError: PyCapsule_GetPointer called with invalid PyCapsule object