fix(backport): Skip callbacks with dead weakrefs while iterating over…

… callbacks

* Backport PR https://github.com/scikit-hep/pyhf/pull/ 2310
* Check refs while processing callbacks in case a callee is de-ref'd during
  the callback process.
* Additionally flush after processing callbacks in case any callback de-ref's
  a callee.
* Note that no additional tests are added for this case as the problem arises from
  and intermediate callback triggering a garbage-collect. It is unclear how to force
  this scenario deterministically in testing.
* Add Jonas Rembser to contributors list.

Co-authored-by: Jonas Rembser <jonas.rembser@cern.ch>

docs/contributors.rst

@@ -32,3 +32,4 @@ Contributors include:
 - Nathan Simpson
 - Beojan Stanislaus
 - Daniel Werner
+- Jonas Rembser

src/pyhf/events.py

@@ -64,12 +64,20 @@ def _flush(self):
         self._callbacks = _callbacks
 
     def __call__(self, *args, **kwargs):
-        for func, arg in self.callbacks:
+        for func, arg in self._callbacks:
             # weakref: needs to be de-ref'd first before calling
             if arg is not None:
-                func()(arg(), *args, **kwargs)
+                arg_ref = arg()
+                if arg_ref is not None:
+                    func()(arg_ref, *args, **kwargs)
             else:
                 func()(*args, **kwargs)
+        # Flush after calling all the callbacks, not before, as callbacks in the
+        # beginning of the iteration might cause new dead arg weakrefs in
+        # callbacks that are iterated over later.
+        # Checking for dead weakrefs in each iteration and flushing at the end
+        # avoids redundant dead weakref checking in subsequent calls.
+        self._flush()
 
     def __iter__(self):
         return iter(self.callbacks)