[tahoe-lafs-trac-stream] [Tahoe-LAFS] #2297: improve precision of coverage reports by including coverage by subprocesses

[Tahoe-LAFS]

#2297: improve precision of coverage reports by including coverage by subprocesses
---------------------------------+---------------------------
 Reporter:  daira                |          Owner:
     Type:  defect               |         Status:  new
 Priority:  normal               |      Milestone:  undecided
Component:  code                 |        Version:  1.10.0
 Keywords:  coverage subprocess  |  Launchpad Bug:
---------------------------------+---------------------------
 Currently, coverage reports underestimate covered code in some places
 (this particularly affects the code for running nodes) because only Python
 code directly executed by `coverage run` is measured.

 `coverage` has a feature to measure the code running in multiple processes
 and then combine the measurements:

 http://nedbatchelder.com/code/coverage/subprocess.html
 http://nedbatchelder.com/code/coverage/cmd.html#cmd-combining

 To make this work, we need to:

 * Run this early on when starting Tahoe processes (e.g. in
 [source:src/allmydata/__init__.py]):
 {{{
 try:
     from coverage import process_startup
     process_startup()
 except ImportError:
     pass
 }}}

 * Set the `COVERAGE_PROCESS_START` environment variable (which will be
 inherited by subprocesses) when running `coverage` from [source:setup.py]
 and the [source:Makefile].

 * Pass the `-p` option to the `coverage` command line.

 * Ensure that the `.coverage.*` files are being generated in the same
 directory.

 * After `coverage run`, execute `coverage combine`.

--
Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2297>
Tahoe-LAFS <https://Tahoe-LAFS.org>
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