LcovParser does not follow spec for FN: line

[alexeagle]

Description of the bug:

Bazel's coverage data handling follows the LCOV specification, which defines a descriptor of a function with FN. You can find with man geninfo or in the hosted manpage

                     FN:<line number of function start>,[<line number of function end>,]<function
                     name>

              The 'end' line number is optional, and is generated only if the compiler/toolchain
              version is recent enough to generate the data

However Bazel's implementation in LcovParser.java does not permit the 'end line number'. It's grammar is

FN:<line number of function start>,<function name>

As a result, coverage gathered with a tool like coverage.py cannot be parsed by Bazel.

Which category does this issue belong to?

Core

What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

see https://github.com/aspect-build/rules_py/actions/runs/13018843537/job/36314576810 which has a very simple usage of coverage

During bazel coverage we see a warning:

Jan 28, 2025 8:20:41 PM com.google.devtools.coverageoutputgenerator.LcovParser parseFNLine
WARNING: Tracefile contains invalid FN line FN:1,2,welcome

this produces a corrupted file that lcov and genhtml cannot read, since a dangling reference to the missing function ends up in the merged lcov data:

genhtml: ERROR: unknown function 'welcome'
	(use "genhtml --ignore-errors mismatch ..." to bypass this error)

Which operating system are you running Bazel on?

macos or linux

What is the output of bazel info release?

7.4.1

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse HEAD ?

If this is a regression, please try to identify the Bazel commit where the bug was introduced with bazelisk --bisect.

No response

Have you found anything relevant by searching the web?

No response

Any other information, logs, or outputs that you want to share?

No response