[issue43262] "_weakrefset.py", line 56: IndexError: pop from empty list

[Jinghao Shi]

New submission from Jinghao Shi <jhshi89 at gmail.com>:

One of our asyncio based application sporadically crash due to this stack trace:

```
File ".../lib/python3.8/asyncio/runners.py", line 48, in run
    loop.run_until_complete(loop.shutdown_asyncgens())
  File ".../lib/python3.8/asyncio/base_events.py", line 595, in run_until_complete
    future = tasks.ensure_future(future, loop=self)
  File ".../lib/python3.8/asyncio/tasks.py", line 669, in ensure_future
    task = loop.create_task(coro_or_future)
  File ".../lib/python3.8/asyncio/base_events.py", line 431, in create_task
    task = tasks.Task(coro, loop=self, name=name)
  File ".../lib/python3.8/_weakrefset.py", line 83, in add
    self._commit_removals()
  File ".../lib/python3.8/_weakrefset.py", line 56, in _commit_removals
    discard(l.pop())
IndexError: pop from empty list
```

We do not have a reliable repro. From reading the source code, it appears we should never try to pop from empty list:

```
 def _commit_removals(self):
        l = self._pending_removals
        discard = self.data.discard
        while l:
            discard(l.pop()) # <-- HERE
```

----------
components: asyncio
messages: 387288
nosy: asvetlov, jhshi89, yselivanov
priority: normal
severity: normal
status: open
title: "_weakrefset.py", line 56: IndexError: pop from empty list
type: crash
versions: Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue43262>
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