bun test --parallel: do not let an import-only worker mark an executed function as uncovered

[robobun]

Problem

• bun test --coverage --parallel reports a fully executed function as partially covered when another worker imports the module without calling the function. The repro in bun test --coverage --parallel does not merge line coverage across workers #39930 shows subject.ts | 100.00 | 80.00 | 5 under --parallel=2 and 100% serially. Line 5 is a blank line.
• The merge in merge_coverage_fragments (src/runtime/cli/test/parallel/aggregate.rs:164) unions the per-worker DA: line sets. A worker that never executed the function marks the function's whole line range, blank lines included, as executable with zero hits (src/sourcemap_jsc/CodeCoverage.rs:601). A worker that executed it has real per-line data and omits those lines. The union keeps DA:5,0.

Fix

• The merge now counts, per line, how many fragments list it. A zero-hit line survives the merge only when every fragment that covers the file lists it. A line with hits anywhere always survives.
• This matches the serial reporter: when the function executes, the over-approximated range is replaced by the real per-line data. When no worker executes the function, all fragments agree and the range is still reported as uncovered.
• The text table, the lcov DA/LF/LH records, and the threshold fractions all read the same filtered line set.
• Verified: test/cli/test/coverage.test.ts (new test, stock bun fails it 10 of 10 runs). Also the rest of coverage.test.ts and test/cli/test/parallel.test.ts (one pre-existing timing failure, also fails on main in this environment).
• Fixes bun test --coverage --parallel does not merge line coverage across workers #39930

Background

• With --parallel, each worker process renders its own LCOV report and streams it to the coordinator over IPC. The coordinator merges the fragments after the run.
• Line coverage comes from JSC's control flow profiler. Basic blocks only exist for compiled code, so an uncalled function has no per-line data. Bun intentionally reports its whole line range as uncovered in that case.
• The fragments carry no per-function FN:/FNDA: records, so the merge cannot attribute lines to functions. The presence count is the signal that a zero-hit line is real: a worker with finer data for the same file omits over-approximated lines.

Notes

Per-worker lcov fragments for the repro:

Worker that executes the function:

DA:1,20
DA:3,25
DA:4,30
DA:6,13

Worker that only imports:

DA:1,20
DA:3,0
DA:4,0
DA:5,0
DA:6,1

Only completely empty lines diverge between the two shapes. A line with any content gets marked by basic-block byte iteration in both cases, so a real uncovered line inside an executed function is present in every fragment and the rule keeps it. The same holds for the sourcemapped path, where the fill covers original lines that have no mappings.

The FN/FNDA gap also makes % Funcs take the per-worker max instead of a union. That is a separate, pre-existing limitation, noted in the doc comment on merge_coverage_fragments and unchanged here.

[robobun]

^{Updated 3:38 AM PT - Aug 21st, 2026}

✅ @autofix-ci[bot], your commit d6f580002282b59a919d18330f9ddf161909d517 passed in Build #102566! 🎉

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Walkthrough

Parallel coverage aggregation now merges line hits across worker-generated file fragments. LCOV and text reports use the merged data, and a regression test verifies that import-only fragments do not add falsely executable blank lines.

Changes

Parallel coverage aggregation

Layer / File(s)	Summary
Merge line coverage across fragments src/runtime/cli/test/parallel/aggregate.rs	FileCoverage tracks fragment counts and per-line fragment coverage. Merged lines sum hits, sort entries, and filter inconsistent zero-hit lines.
Use merged lines in reports src/runtime/cli/test/parallel/aggregate.rs, test/cli/test/coverage.test.ts	LCOV output, coverage fractions, and uncovered-line ranges use merged lines. The parallel regression test validates four covered executable lines, excludes the false blank line, and expects successful 100% coverage.

Suggested reviewers: jarred-sumner, alii

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly describes the main coverage-merging fix for import-only workers.
Description check	✅ Passed	The description explains the problem, fix, implementation context, regression test, and verification results.

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In `@test/cli/test/coverage.test.ts`:
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