Fix adding failed emitters on observer schedule (#872)

* Moved `_add_emitter` call post emitter start so it isn't called if the emitter fails to start

* Added test

src/watchdog/observers/api.py

@@ -298,9 +298,9 @@ def schedule(self, event_handler, path, recursive=False):
                 emitter = self._emitter_class(event_queue=self.event_queue,
                                               watch=watch,
                                               timeout=self.timeout)
-                self._add_emitter(emitter)
                 if self.is_alive():
                     emitter.start()
+                self._add_emitter(emitter)
             self._watches.add(watch)
         return watch
 

tests/test_observer.py

@@ -140,3 +140,22 @@ def mocked_start():
     # Re-schduling the watch should work
     observer.schedule(None, '')
     assert len(observer.emitters) == 1
+
+
+def test_schedule_failure_should_not_prevent_future_schedules(monkeypatch, observer):
+    observer.start()
+
+    # Make the emitter fail on start(), and subsequently the observer to fail on schedule()
+    def mocked_start(emitter):
+        raise OSError()
+    monkeypatch.setattr(EventEmitter, "start", mocked_start)
+
+    with pytest.raises(OSError):
+        observer.schedule(None, '')
+    # The emitter should not be in the list
+    emitters = observer.emitters
+    assert len(emitters) == 0
+
+    # Re-schduling the watch should work
+    observer.schedule(None, '')
+    assert len(observer.emitters) == 1