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module: disable the compile cache while V8 coverage is collected - #67120
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V8 cannot collect block coverage for functions that are deserialized from the code cache, so it only reports how many times each of them ran. With a warm compile cache, branches that never ran inside those functions are reported as covered, and tools that merge reports from several processes can report functions that ran as never run. The documentation recommended turning the cache off by hand. Do it automatically when coverage collection starts with the process, through NODE_V8_COVERAGE or --experimental-test-coverage. The compile cache tests stop propagating NODE_V8_COVERAGE to the processes they spawn, so that they keep working when the test suite itself runs with coverage. Signed-off-by: Shelley Vohr <shelley.vohr@gmail.com>
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Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## main #67120 +/- ##
==========================================
- Coverage 90.66% 90.66% -0.01%
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Files 797 797
Lines 278791 278796 +5
Branches 53620 53630 +10
==========================================
- Hits 252768 252761 -7
- Misses 16376 16380 +4
- Partials 9647 9655 +8
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With a warm compile cache, V8 coverage is wrong in both directions. V8 can't collect block coverage for functions deserialized from the code cache and only reports a call count for them, so a branch that never ran inside one reads as covered. Tools that merge reports from several processes, including the test runner, can also drop a function that did run.
On main with
node --test --experimental-test-coverageandNODE_COMPILE_CACHEset:ifthat is never taken, linesThe docs recommended turning the cache off by hand for this. This does it automatically when coverage starts with the process, through
NODE_V8_COVERAGEor--experimental-test-coverage.module.enableCompileCache()then returnsDISABLEDwith a message. Coverage started later, e.g. through the inspector, behaves as before, and the docs now say so.19 compile cache tests now clear
NODE_V8_COVERAGEfor the processes they spawn, since they would otherwise fail in the coverage jobs.Disclosure: the code, tests and this description were written by Claude Code, directed and reviewed by @codebytere.