Add coverage support for fuzzing_regression_test

[zhenyudg]

Adds support for bazel coverage to gather coverage on fuzz targets replayed on the corpus.

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[zhenyudg]

@googlebot I signed it!

[stefanbucur]

Apologies for the delay!

[stefanbucur]

I think I need to understand this better: Why do we need to add these dependencies? Are these going to be built even in non-coverage mode?

(It would help if you could point me to some documentation discussing this.)

[zhenyudg]

collect_cc_coverage runs the gcov/llvm-cov commands to gather coverage.

lcov_merger merges different coverage output files into a single output file for consumption.

I found these dependencies from the implementation of the native cc_test rule.

They will be built in non-coverage mode. Native rules with coverage support use late-bound attributes to avoid building these dependencies unnecessarily, but that's not yet an option in Starlark.

[stefanbucur]

Unfortunately I'm reluctant to accept this change as-is. I understand this is a Starlark limitation, but the workaround of including these dependencies on all builds is inadequate IMO.

Let me do some additional research to better understand all of our alternatives. If coverage support is not mature enough yet in Bazel, I would postpone the addition of this feature for now.

[fmeum]

This should be fixed by bazelbuild/bazel#13983. Until then, I don't know of a better solution.

[fmeum]

The underlying issue is now a release blocker for Bazel 5, so it's likely that lcov_merger will be provided by default in a way that it would be built only with bazel coverage. The same does not apply to collect_cc_coverage, but that is merely a shell script and thus causes no compilation overhead.

[fmeum]

My fix will not be accepted, so this will very likely not be possible with Bazel 5.

[fmeum]

@zhenyudg I haven't forgotten about this. If you want to know whether there has been progress in getting the underlying issue resolved in Bazel, you can track bazelbuild/bazel#8736.

[fmeum]

@zhenyudg While working on the upstream fix, I may have found a workaround for Bazel 4+: Use an alias as the default for _lcov_merger and in that alias, use a select on a config_setting for collect_code_coverage to switch between the real lcov_merger target and a dummy target (e.g., an empty executable file). Let me know if you need help with this.

[fmeum]

@zhenyudg Are you still planning to work on this PR? I have a concrete need for this feature now and would thus like to pick it up soon.

[zhenyudg]

@fmeum Pardon the radio silence on the MR. I'm on leave until October 3, and I would be more than happy if you can pick this up.

[zhenyudg]

Superseded by #212