improve-coverage

name	improve-coverage
description	Use when you need to raise main-source line coverage in the bazel-diff repo, write tests for an under-covered Kotlin file, fix a CI failure on the 90% coverage gate, or pick the highest-leverage files to test next. Triggers on requests like "the coverage gate is failing, fix it", "write tests for X", "we need more coverage", or "what should I test to get to 90%".

Improving coverage to clear the 90% gate

bazel-diff enforces a 90% main-source line-coverage gate on every PR (see coverage-status for the inspection side). When the gate fails or you want to raise the bar, the workflow is: pick the worst-covered files, write small focused unit tests, re-run the gate locally before pushing.

1. Pick the right files to target

Run make coverage (or check the latest CI artifact) and look at the top of the sorted table. Prioritise files by uncovered-lines-per-test-effort, not by lowest percentage:

• Highest-leverage: small files at 0% (e.g. enum classes, value objects, single-method utilities) — one short unit test usually moves the needle without much code.
• Highest absolute gain: large files with moderate coverage (e.g. BazelQueryService.kt at 303 lines / 93%) — closing a small percentage gap covers many lines.
• Lowest leverage: tiny files at 50–80% where the remaining branches are error paths needing fault injection or refactors.

A worked example: the PR that added the gate (#356) raised coverage from 88.76% → 90.37% by adding five small files of tests — BazelTargetTypeTest, VersionProviderTest, BazelDiffTest, StderrLoggerTest, BazelTargetTest — for a total of 26 newly-covered lines.

2. Write the test

Existing tests follow a consistent shape:

• Live under cli/src/test/kotlin/... mirroring the main source path.
• Use JUnit 4 (@Test, @Before, @After, org.junit.Assert.assertThrows).
• Use assertk for assertions (assertk.assertThat, with explicit imports for each assertion like assertk.assertions.isEqualTo). Forgetting import assertk.assertions.contains on a String.contains assertion produces a confusing receiver-mismatch error — import every assertion you use.
• Use mockito-kotlin when mocking is required (existing examples: cli/src/test/kotlin/com/bazel_diff/bazel/BazelClientTest.kt).
• Use koin for DI-test setup (existing pattern: cli/src/test/kotlin/com/bazel_diff/interactor/CalculateImpactedTargetsInteractorIssue335Test.kt).

Tiny files (enums, value objects, small command classes) usually only need a few targeted tests. Looking at the bytes via assertThat(BazelTargetType.entries).hasSize(N).containsExactlyInAnyOrder(...) is enough to cover an enum's declaration lines.

3. Register the test target in cli/BUILD

Every test needs its own kt_jvm_test entry:

kt_jvm_test(
    name = "BazelTargetTypeTest",
    test_class = "com.bazel_diff.bazel.BazelTargetTypeTest",
    runtime_deps = [":cli-test-lib"],
)

The :cli-test-lib glob picks up the new test source automatically; the explicit kt_jvm_test rule is what makes it executable via bazel test //cli:<name>.

4. Verify locally before pushing

bazel test //cli:<YourNewTest>        # one-off run of the new test
make coverage                          # full gate

The local number may be lower than CI's because //cli:E2ETest often fails or is excluded on dev machines (JDK-env sandbox issues). If you've added tests for a file that's also exercised by E2E (e.g. BazelQueryService.kt), the CI delta will be smaller than the local delta — count only the lines that weren't already covered by E2E.

5. Things that don't work / aren't worth attempting

• Main.kt — calls exitProcess(...) which kills the JVM; can't be tested in-process without a SecurityManager hack or refactor. Stays at 0%; the threshold tolerates it.
• throw IllegalArgumentException(...) branches in resource-loading code like VersionProvider.kt — the production code resolves the classloader from this::class.java, with no injection seam to swap it for one missing the resource. Refactor or skip.
• else -> BazelTargetType.UNKNOWN branches — only reachable when Bazel's Build.Target.Discriminator adds a new enum value the production code doesn't recognise. Triggering today would require a hand-forged proto with a reserved discriminator number, which the protobuf builder rejects.

When the gate fails on a flake, not on a coverage drop

If the CI threshold step fails with error: LCOV report not found at 'bazel-out/_coverage/_coverage_report.dat', that's an infrastructure issue, not a coverage regression. The fix that landed in PR #356 was to propagate USE_BAZEL_VERSION to the threshold step so bazelisk run doesn't start a different bazel server. If you see a similar mismatch resurface, check that env propagation first.