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improve-coverage

Testing & Quality
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Improve code coverage for a Bazel C++ test target or source file

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Source SKILL.md: https://github.com/redpanda-data/redpanda/blob/HEAD/.claude/skills/improve-coverage/SKILL.md

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Improve Code Coverage

Improve test coverage for a Bazel C++ test target or source file path.

Input Resolution

  1. Bazel test target (starts with //, e.g. //src/v/bytes/tests:iobuf_test): use it directly.

  2. Source file path (e.g. src/v/bytes/iobuf.cc): find the test target:

    bazel query "kind('.*_test', rdeps(//src/v/..., //$(dirname $FILE):$(basename $FILE .cc), 1))" 2>/dev/null

    If multiple targets match, pick the one in the closest tests/ directory. If none match, tell the user and stop.

Workflow

Step 1: Baseline Coverage

Run coverage and read the LLM report:

tools/run-cov <target> -r llm

Note the total line coverage percentage — this is the baseline to beat.

Read the report and identify:

  • Uncovered functions — highest impact. One test can cover many lines.
  • Partially covered functions — uncovered branches indicate missing error-path or edge-case tests.
  • Zero-coverage files in scope — may need new test cases entirely.

Step 2: Understand the Code

Read the source files named in the report. Focus on:

  • Uncovered functions: what they do, inputs, preconditions.
  • The existing test file: patterns, fixtures, helpers.

Key codebase patterns:

  • redpanda_cc_gtest targets use GTest (TEST(), TEST_F()). redpanda_cc_btest targets use Seastar Boost test (SEASTAR_THREAD_TEST_CASE). Match whichever the test file uses.
  • Many functions are ss::future<> coroutines — test with SEASTAR_THREAD_TEST_CASE or the Seastar GTest runner.
  • Use ss namespace prefix for Seastar types.
  • Use EXPECT_* for most checks, ASSERT_* only when continuing would crash.

Step 3: Write Tests

Prioritize by impact:

  1. Uncovered functions — call each one. A 20-line function = 20 lines covered.
  2. Uncovered error paths — trigger the uncovered branches in partially covered functions.
  3. Zero-coverage files — add smoke tests if practical.

Rules:

  • Add tests to the existing test file. Don't create new files or BUILD targets unless necessary.
  • Follow existing naming conventions in the file.
  • One logical behavior per test case.
  • Prefer deterministic tests — construct error conditions directly.
  • Do NOT add comments that restate the code.

Step 4: Verify Build

bazel build <target>

Fix compilation errors before proceeding.

Step 5: Verify Coverage

Re-run coverage:

tools/run-cov <target> -r llm

Compare to the baseline. Report:

Coverage: X.Y% → A.B% (+N.N%)
New lines covered: <count>

If coverage did not improve, investigate:

  • Build errors preventing the new tests from running?
  • Code compiled out by preprocessor guards?
  • Template instantiations not triggered by test types?

Step 6: Summary

Report:

  • Which functions/paths are now covered
  • What test cases were added
  • Before/after coverage numbers
  • Remaining significant gaps and suggestions