DetailedList - no smooth scroll and crash after 30s - macOS

Expected Behavior

Fluent scrolling, no jitter, no crash

Current Behavior

1. jitter when scrolling up and down.
2. app crashes after 30-60s of scrolling with this error:

/Users/Jonas/.local/share/virtualenvs/toga-Z1tvi9lv/bin/python -m detailedlist
Fatal Python error: deallocating None

Current thread 0x00007fffb2b9b380 (most recent call first):
  File "/Users/Jonas/Documents/Programming/PyBee/toga/src/cocoa/toga_cocoa/widgets/internal/refresh.py", line 172 in scrollWheel_
  File "/Users/Jonas/Documents/Programming/PyBee/rubicon-objc/rubicon/objc/runtime.py", line 1032 in _objc_method
  File "/Users/Jonas/Documents/Programming/PyBee/rubicon-objc/rubicon/objc/runtime.py", line 829 in __call__
  File "/Users/Jonas/Documents/Programming/PyBee/rubicon-objc/rubicon/objc/runtime.py", line 907 in __call__
  File "/Users/Jonas/Documents/Programming/PyBee/rubicon-objc/rubicon/objc/eventloop.py", line 621 in start
  File "/Users/Jonas/Documents/Programming/PyBee/rubicon-objc/rubicon/objc/eventloop.py", line 381 in run
  File "/Users/Jonas/Documents/Programming/PyBee/rubicon-objc/rubicon/objc/eventloop.py", line 425 in run_forever
  File "/Users/Jonas/Documents/Programming/PyBee/toga/src/cocoa/toga_cocoa/app.py", line 171 in main_loop
  File "/Users/Jonas/Documents/Programming/PyBee/toga/src/core/toga/app.py", line 169 in main_loop
  File "/Users/Jonas/Documents/Programming/PyBee/toga/examples/detailedlist/detailedlist/__main__.py", line 4 in <module>
  File "/usr/local/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/runpy.py", line 85 in _run_code
  File "/usr/local/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/runpy.py", line 193 in _run_module_as_main

Process finished with exit code 134 (interrupted by signal 6: SIGABRT)

Steps to reproduce

1. run the detailedlist.py example
2. scroll up and down for 30-60s.

Your Environment

• Python Version (3.6.5)

• Operating System and Version (select from the following and list the specific version number; if your OS is not listed, list that as well)

  • macOS - version: 10.13.5 (High Sierra)
  • Linux - distro: - version:
  • Windows - version:
  • Other - name: - version:

• Toga Target (the type of app you are trying to generate)

  • android
  • cocoa
  • django
  • gtk
  • iOS
  • tvOS
  • watchOS
  • winforms
  • win32
  • Other (please specify)

[dgelessus]

Turns out that the root cause of the crash is a bug in ctypes itself, which I’ve reported now (bpo-36880). Whenever a ctypes callback with restype py_object returns None, the refcount of None is decremented once too much.

In this example, the bug is triggered a few times for each scroll operation. If this happens often enough, None’s refcount decreases to zero - this is never supposed to happen, so Python crashes.

The code that triggers the bug is in rubicon-objc here. In this case we can probably work around the bug by returning an object other than None from the callback - I’ll try to work on a patch for Rubicon soon, so that we don’t have to wait for the bug to be fixed in CPython.

[freakboy3742]

Thanks to the awesome debugging work by @lschumm and @dgelessus, this bug has now been squashed; Rubicon 0.3.1 includes the code necessary to work around this problem.