Fix event dispatch when handlers modify subscriptions

src/prompt_toolkit/utils.py

@@ -71,7 +71,7 @@ def __init__(
 
     def __call__(self) -> None:
         "Fire event."
-        for handler in self._handlers:
+        for handler in self._handlers[:]:
             handler(self.sender)
 
     def fire(self) -> None:

tests/test_event_mutation.py

@@ -0,0 +1,110 @@
+import pytest
+
+from prompt_toolkit.buffer import Buffer
+from prompt_toolkit.utils import Event
+
+
+@pytest.mark.parametrize("fire_method", ["fire", "__call__"])
+def test_self_removing_handler_does_not_skip_next(fire_method):
+    sender = object()
+    event = Event(sender)
+    calls = []
+
+    def once(value):
+        calls.append(("once", value))
+        event.remove_handler(once)
+
+    def persistent(value):
+        calls.append(("persistent", value))
+
+    event += once
+    event += persistent
+    getattr(event, fire_method)()
+    getattr(event, fire_method)()
+    assert calls == [("once", sender), ("persistent", sender), ("persistent", sender)]
+
+
+def test_handlers_added_during_dispatch_wait_until_next_event():
+    event = Event(None)
+    calls = []
+
+    def added(sender):
+        calls.append("added")
+
+    def initial(sender):
+        calls.append("initial")
+        event.remove_handler(initial)
+        event.add_handler(added)
+
+    event += initial
+    event.fire()
+    assert calls == ["initial"]
+    event.fire()
+    assert calls == ["initial", "added"]
+
+
+def test_removing_later_handler_applies_to_next_dispatch():
+    event = Event(None)
+    calls = []
+
+    def first(sender):
+        calls.append("first")
+        event.remove_handler(second)
+
+    def second(sender):
+        calls.append("second")
+
+    event += first
+    event += second
+    event.fire()
+    event.fire()
+    assert calls == ["first", "second", "first"]
+
+
+def test_reentrant_event_uses_updated_handler_list():
+    event = Event(None)
+    calls = []
+
+    def first(sender):
+        calls.append("first")
+        event.remove_handler(first)
+        event.fire()
+
+    def second(sender):
+        calls.append("second")
+
+    event += first
+    event += second
+    event.fire()
+    assert calls == ["first", "second", "second"]
+
+
+def test_buffer_text_change_reaches_persistent_listener():
+    buffer = Buffer()
+    changes = []
+
+    def once(sender):
+        sender.on_text_changed.remove_handler(once)
+
+    def record(sender):
+        changes.append(sender.text)
+
+    buffer.on_text_changed += once
+    buffer.on_text_changed += record
+    buffer.text = "first"
+    buffer.text = "second"
+    assert changes == ["first", "second"]
+
+
+def test_handler_exceptions_still_stop_dispatch():
+    event = Event(None)
+    calls = []
+
+    def fail(sender):
+        raise ValueError("handler failed")
+
+    event += fail
+    event += calls.append
+    with pytest.raises(ValueError, match="handler failed"):
+        event.fire()
+    assert calls == []