intermittant failures / race-condition on immutable.download.fetcher

[meejah]

While trying to prepare 1.19.0, some builder exhibited an intermittent failure relating to the _shares member of SegmentFetcher not being available.

This can only happen due to some code that *claims* to be "helping the GC" and appears to be a race condition.

Investigate.

[itamarst]

This was supposed to be fixed by ​https://github.com/tahoe-lafs/tahoe-lafs/pull/1348 but it's still happening:

[ERROR]
Traceback (most recent call last):
  File "/home/itamarst/devel/tahoe-lafs/venv311/lib/python3.11/site-packages/foolscap/eventual.py", line 26, in _turn
    cb(*args, **kwargs)
  File "/home/itamarst/devel/tahoe-lafs/src/allmydata/immutable/downloader/node.py", line 259, in got_shares
    self._active_segment.add_shares(shares)
  File "/home/itamarst/devel/tahoe-lafs/src/allmydata/immutable/downloader/fetcher.py", line 82, in add_shares
    self._shares.extend(shares)
builtins.AttributeError: 'SegmentFetcher' object has no attribute '_shares'

allmydata.test.test_system.HTTPSystemTest.test_upload_and_download_convergent

[itamarst]

I can reproduce fairly consistently with trial -u allmydata.test.test_system.HTTPSystemTest.test_upload_and_download_convergent using the branch in ​https://github.com/tahoe-lafs/tahoe-lafs/pull/1354