[tahoe-lafs-trac-stream] [Tahoe-LAFS] #2757: improve deprecation-warning builder

[Tahoe-LAFS]

#2757: improve deprecation-warning builder
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     Reporter:  warner              |      Owner:
         Type:  task                |     Status:  new
     Priority:  minor               |  Milestone:  undecided
    Component:  dev-infrastructure  |    Version:  1.10.2
   Resolution:                      |   Keywords:
Launchpad Bug:                      |
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Comment (by warner):

 Done, in [14c513db8], by adding a (python) helper script in
 `misc/build_helpers/run-deprecations.py` that gets run by `tox -e
 deprecations`. The script counts !DeprecationWarning lines in stderr and
 emits a count (to stdout) at the end, along with a copy of all the
 warnings. If there were any warnings, tox exits with an error.

 Alas the `subprocess` module doesn't make it possible to display stdout
 *and* stderr interspersed, while still capturing stderr, so in this mode,
 you can't tell which test is provoking the warning. Run trial without tox
 (and with `PYTHONWARNINGS=default::DeprecationWarning`) to see this, maybe
 also with `|tee out.log 2>&1`.

 The buildbot has been configured to look for the `_trial_temp/stderr.log`
 that this produces, and to scan for the warning count (which is added to
 the buildstep status).

 I also added `tox -e upcoming-deprecations`, which installs the latest
 Twisted and Foolscap (from their Github repos). This should give us some
 advance warning about things we're doing that have become deprecated in
 the upcoming releases of those projects.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2757#comment:1>
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