NO-JIRA: [Python] Fix exception in IOHandler.on_selectable_expired/update

[Jmennius]

This occasionally happens after my laptop wakes up from overnight sleep.

IOHandler.on_selectable_expired() is invoked with selectable that has
_terminated=True and _transport=None so when IOHandler.update()
is called it crashes when trying to handle the exception
(since transport is None).

Fix this by checking if transport attribute is set and that selectable is not
terminated before invoking IOHandler.update().

PROTON Jira issue: https://issues.apache.org/jira/browse/PROTON-2776

[Jmennius]

Here is a traceback:

Traceback (most recent call last):
  File "/path/.venv/lib/python3.7/site-packages/proton/_handlers.py", line 1293, in update
    capacity = transport.capacity()
AttributeError: 'NoneType' object has no attribute 'capacity'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/path/my-project.py", line 285, in run
    self.container.run()
  File "/path/.venv/lib/python3.7/site-packages/proton/_reactor.py", line 197, in run
    while self.process():
  File "/path/.venv/lib/python3.7/site-packages/proton/_reactor.py", line 262, in process
    event.dispatch(self._global_handler)
  File "/path/.venv/lib/python3.7/site-packages/proton/_events.py", line 158, in dispatch
    _dispatch(handler, type.method, self)
  File "/path/.venv/lib/python3.7/site-packages/proton/_events.py", line 131, in _dispatch
    handler.on_unhandled(method, *args)
  File "/path/.venv/lib/python3.7/site-packages/proton/_reactor.py", line 905, in on_unhandled
    event.dispatch(self.base)
  File "/path/.venv/lib/python3.7/site-packages/proton/_events.py", line 158, in dispatch
    _dispatch(handler, type.method, self)
  File "/path/.venv/lib/python3.7/site-packages/proton/_events.py", line 129, in _dispatch
    m(*args)
  File "/path/.venv/lib/python3.7/site-packages/proton/_handlers.py", line 1237, in on_selectable_expired
    self.update(t, s, r.now)
  File "/path/.venv/lib/python3.7/site-packages/proton/_handlers.py", line 1296, in update
    if transport.closed:
AttributeError: 'NoneType' object has no attribute 'closed'

[codecov-commenter]

Codecov Report

Merging #364 (4d22a80) into main (a920192) will increase coverage by 20.11%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             main     #364       +/-   ##
===========================================
+ Coverage   68.24%   88.36%   +20.11%     
===========================================
  Files         367       47      -320     
  Lines       73285     2397    -70888     
===========================================
- Hits        50011     2118    -47893     
+ Misses      23274      279    -22995     

Impacted Files	Coverage Δ
python/proton/_handlers.py
python/tests/proton_tests/__init__.py
cpp/src/ssl_options_impl.hpp
python/proton/utils.py
c/src/core/object/object.c
cpp/include/proton/endpoint.hpp
c/tests/object_test.cpp
python/proton/_selectable.py
c/src/reactor/reactor.c
cpp/include/proton/reconnect_options.hpp
... and 310 more

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[Jmennius]

@astitcher What do you think?
Should I open an issue?

[itechbear]

Could we get this PR merged? I ran into the bug recently. It causes qpid proton to be pretty unusable.

[Jmennius]

I've opened a Jira issue for this https://issues.apache.org/jira/browse/PROTON-2776.
I can work on the PR if ncessary.