Commits on Feb 2, 2022

1. rules/python: Rework built-in coveragepy support …

   [Coveragepy recently gained support for lcov
   output][nedbat/coveragepy#1289], which allows implementing full
   support for python coverage without relying on a downstream fork of
   that project
   
   Coveragepy actually must be invoked twice; One generating a
   `.coverage` database file, the other exporting the data. This means
   that it is incompatible with the old implementation.
   
   The fork of coveragepy previously intended for use with bazel
   circumvented this by just changing how coveragepy works, and never
   outputting that database - just outputting the lcov directly
   instead. If we'd like to use upstream coveragepy, this is of course
   not possible.
   
   The stub_template seems to be written with the idea of supporting
   other coverage tooling in mind, however it still hard-codes arguments
   specific to coveragepy. Instead, we think it makes sense to properly
   support one of them for now, and to rethink a more generic interface
   later - it will probably take specific scripting for each
   implementation of coverage in python anyway.
   
   As such, this patch rewrites the python stub template to fully support
   upstream coveragepy as a coverage tool, and reworks some of the logic
   around invoking python to do so more cleanly.
   
   Additional notes:
   
     - Python coverage will only work with Python 3.7+ with upstream
       coveragepy, since the first release with lcov support does not
       support earlier Python versions - this is unfortunate, but there
       is not much we can do downstream short of forking to resolve
       that. The stub template itself should still work with Python 2.4+.
   
     - Comments in the code claim to use `os.execv` for performance
       reasons. There may be a small overhead to `subprocess.call`, but
       it shouldn't be too impactful, especially considering the overhead
       in logic (written in Python) this involves - if this is indeed for
       performance reasons, this is probably a somewhat premature
       optimization.
   
       A colleauge helped dig through some history, finding
       3bed4af as the source of this -
       if that commit is to believed, this is actually to resolve issues
       with signal handling, however that seems odd as well, since this
       calls arbitrary python applications, which in turn may use
       subprocesses again as well, and therefore break what that commit
       seems to attempt to fix.
   
       It's completely opaque to me why we put so much effort into trying
       to ensure we use `os.execv`. I've replicated the behavior and
       comments assuming it was correct previously, but the patch
       probably shouldn't land as-is - the comment explaining the use of
       `os.execv` is most likely misleading.
   
   ---
   
   [nedbat/coveragepy#1289]: nedbat/coveragepy#1289
   
   Co-authored-by: Bradley Burns <bradley.burns@codethink.co.uk>

   

   TLATER and bradb423 committed Feb 2, 2022
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2. WIP: rules/python: Add PYTHON_COVERAGE_TARGET …

   This resolves bazelbuild#14436 by adding a new environment variable that will
   perform the coverage label resolution inside the python_stub_template
   directly, which resolves the python coverage tool by label rather than
   path.
   
   Currently this resolves the path in the runfiles directory by guessing
   the path the label should resolve to - this of course does not work in
   general, even just defining an alias breaks it. Since labels appear to
   only be resolved in the analysis phase, there does not seem to be an
   easy way around this, however.
   
   This is a draft, showing the behavior I would like - suggestions on
   how to correctly implement this would be appreciated.
   
   Co-authored-by: Bradley Burns <bradley.burns@codethink.co.uk>

   

   TLATER and bradb423 committed Feb 2, 2022
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