[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
 Keywords:                      |  Launchpad Bug:
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 We have a buildbot builder that runs tests with
 `PYTHONWARNINGS=default::DeprecationWarning` (you can do the same at home
 by running `tox -e deprecations`). But unfortunately it's not currently
 parsing the output for the `DeprecationWarning: ` lines, so to find out if
 there are any problems, you have to look at the logfile and scan for red
 stderr messages.

 My preferred fix would be for the tox `deprecations` environment to run
 some shell scripts, to `tee` the logfile into a separate file, `grep` for
 the warnings, and dump the results in a new `deprecations.log` file. Then
 the buildbot could add that as a "logfile", and flunk the step unless it's
 empty. Unfortunately it seems that `tox` really wants to encourage cross-
 platform compatibility, by not running the commands in a shell: I tried
 `trial allmydata | tee -a log.out` and got a complaint from trial that `|`
 wasn't a valid test module to run.

 We could change this to `make test-deprecations`, which might provoke a
 tox complaint (since it doesn't remember installing `make`), but might
 still work. Or we could write some python tooling that installs a command
 into the virtualenv's `bin/` that does the same thing.

 I kind of want `tox` to do this, because then it's easier to run at home.
 I think I want it to dump the list of deprecations at the end of the test
 run (unless there are a lot of them, then maybe dumping the count and the
 name of the file would be enough). It could also exit with rc!=0 if there
 were any warnings.

 If we can't get tox to drive this, we could find a way to change the
 buildbot to grep stderr (and not just stdout) for the messages. I think it
 used to do this, but something changed in buildbot in the last few years
 (maybe related to "new-style" build steps, and the emulation code that
 provides support for old-style ones), and now the regexp is only being
 given stdout lines, not stderr.

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