[tahoe-lafs-trac-stream] [Tahoe-LAFS] #3138: Run the test suite concurrency on CircleCI

[Tahoe-LAFS]

#3138: Run the test suite concurrency on CircleCI
--------------------------------+---------------------------
 Reporter:  exarkun             |          Owner:  exarkun
     Type:  defect              |         Status:  new
 Priority:  normal              |      Milestone:  undecided
Component:  dev-infrastructure  |        Version:  n/a
 Keywords:                      |  Launchpad Bug:
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 CircleCI provides runners with two vCPUs.  If a single CircleCI job can
 split its work across two Python processes we can take better advantage of
 the runner configuration and hopefully get a much faster CI run.

 trial supports spreading test execution over multiple local processes with
 `--jobs`.  We can already pass extra arguments to trial through tox thanks
 to #2894.

 There are some problems with coverage collection's interactions with
 `--jobs`.  There is a Twisted ticket to deal with this,
 https://twistedmatrix.com/trac/ticket/9663, but there is also
 https://pypi.org/project/coverage_enable_subprocess/ which basically
 solves the problem externally.

 So, add coverage_enable_subprocess and `--jobs 2` to our CircleCI
 configuration and get a ~50% reduction in CircleCI runtime.


 See #2845 and #2894 for past ideas in this space.  See  for a Twisted
 ticket meant to support this effort.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3138>
Tahoe-LAFS <https://Tahoe-LAFS.org>
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