Read Code Coverage Metrics

This skill fetches and processes code coverage data for a given Gerrit CL and patchset. It uses the live Findit API to retrieve coverage percentages and line-by-line details, records coverage metrics for all modified source files, and extracts the code blocks corresponding to uncovered lines.

When to Use This Skill

Activate this skill when:

Establishing a baseline coverage state for a CL before attempting fixes.
Verifying if coverage has improved after applying changes and re-running try jobs.
You need to know not just the percentage, but which specific lines and code blocks are untested.

Inputs (Passed directly in prompt)

When invoked as a subagent, the parent agent MUST explicitly pass:

cl_url: The full Gerrit CL URL to fetch coverage for (e.g., https://chromium-review.googlesource.com/c/chromium/src/+/7916168).
artifact_path: The destination file path to store the JSON metrics artifact (e.g., scratch/original_metrics.json).
target_files: Optional list of specific target file paths to filter coverage for (e.g., ["chrome/browser/.../CommerceBottomSheetContentMediator.java"]). If omitted, records metrics across all modified source files.

Workflow

Extract CL Details & Run Coverage Metrics Script: Use tools/code_coverage/parse_gerrit_url.py to extract host, project, change, and patchset from cl_url:
bash
```
vpython3 tools/code_coverage/parse_gerrit_url.py <cl_url>
```
Execute fetch_coverage_metrics.py passing the extracted parameters and optional --files filter:
bash
```
vpython3 tools/code_coverage/fetch_coverage_metrics.py --host <host> \
  --project <project> --change <change> --patchset <patchset> \
  --output <artifact_path> --files <target_files>
```

Verify Coverage Artifact: Confirm the script generated the artifact at <artifact_path>. The artifact maps modified source files to their coverage metrics and uncovered lines:

json

{
  "chrome/browser/.../CommerceBottomSheetContentMediator.java": {
    "metrics": {
      "absolute_coverage": 80.0,
      "incremental_coverage": 65.0,
      "absolute_unit_tests_coverage": 80.0,
      "incremental_unit_tests_coverage": 65.0
    },
    "low_coverage_type": [
      "incremental_coverage",
      "incremental_unit_tests_coverage"
    ],
    "uncovered_lines": {
      "77": "    mBottomSheetContent.setConfirmBoxVisible(false);",
      "78": "    mBottomSheetContent.setConfirmButtonEnabled(false);"
    }
  }
}

Output

The skill stores the detailed coverage JSON artifact directly at <artifact_path> (scratch/original_metrics.json, scratch/control_metrics.json, or scratch/fix_metrics.json). It should return a summary message indicating the coverage metrics and unexecuted line counts across modified source files.

Important Considerations

Authentication: If querying private CLs, pass --auth-token or set LUCI_AUTH_TOKEN in the environment.
Error Handling: If the script exits with non-zero status, check stderr for API or decoding failures.