Make event triggering robust to (un)registration.

Event callbacks can register or unregister new callbacks for the same event
while executing, and the previous triggering implementation allowed for event
callbacks to be inadvertently skipped.

The fix is to make a copy of the list of callbacks before executing any of
them. With this change, the resulting semantics are simple: any callbacks
registered before triggering are executed, and any new callbacks registered
are only visible at the next triggering of the event.

Note that this could potentially break existing callers who expected
newly-appended callbacks were immediately executed.

Fixes  #9447. Originally based on a patch by @marksandler2.

docs/source/whatsnew/pr/incompat-event-triggering.rst

@@ -0,0 +1,7 b''
+Update IPython event triggering to ensure callback registration and
+unregistration only affects the set of callbacks the *next* time that event is
+triggered. See :ghissue:`9447` and :ghpull:`9453`.
+
+This is a change to the existing semantics, wherein one callback registering a
+second callback when triggered for an event would previously be invoked for
+that same event.

IPython/core/events.py

@@ -69,7 +69,7 b' class EventManager(object):'
         Any additional arguments are passed to all callbacks registered for this
         event. Exceptions raised by callbacks are caught, and a message printed.
         """
-        for func in self.callbacks[event]:
+        for func in self.callbacks[event][:]:
             try:
                 func(*args, **kwargs)
             except Exception:

IPython/core/tests/test_events.py

@@ -30,3 +30,25 b' class CallbackTests(unittest.TestCase):'
         self.em.register('ping_received', cb)
         with tt.AssertPrints("Error in callback"):
             self.em.trigger('ping_received')
+
+    def test_unregister_during_callback(self):
+        invoked = [False] * 3
+        
+        def func1(*_):
+            invoked[0] = True
+            self.em.unregister('ping_received', func1)
+            self.em.register('ping_received', func3)
+
+        def func2(*_):
+            invoked[1] = True
+            self.em.unregister('ping_received', func2)
+
+        def func3(*_):
+            invoked[2] = True
+            
+        self.em.register('ping_received', func1)
+        self.em.register('ping_received', func2)
+
+        self.em.trigger('ping_received')
+        self.assertEqual([True, True, False], invoked)
+        self.assertEqual([func3], self.em.callbacks['ping_received'])