[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
   Resolution:          |   Keywords:  coverage subprocess
Launchpad Bug:          |
------------------------+---------------------------------
Description changed by daira:

Old description:

> 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`.

New description:

 Currently, coverage reports underestimate covered code in some places
 (this particularly affects the code for starting 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`.

--

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2297#comment:1>
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