6a2b42f650
CultMath.Tests is on xunit.v3. Stryker's default vstest runner cannot capture coverage against xunit.v3's test host and hangs retrying test sessions indefinitely (observed: 2096 mutants queued, zero completed after ~9 minutes of retries). Stryker's Microsoft.Testing.Platform runner (-t mtp, still preview) works correctly against xunit.v3 and completed the same 1351-mutant run in ~4.5 minutes. Pin it in config so a plain `dotnet stryker` works without a CLI flag. Add docs/mutation-testing.md: how to run it, the --since form for a single cut, the triage rules, a known coverage-capture caveat in the mtp runner (confirmed by hand-mutation against static readonly fields), and the packages still needing their own config. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
86 lines
3.6 KiB
Markdown
86 lines
3.6 KiB
Markdown
# Mutation Testing
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CultLib measures its C# test suites with Stryker.NET, pinned as a repo-local
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tool in `dotnet-tools.json`. Coverage is added project by project; a project
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with no `stryker-config.json` next to its test project has not been scoped
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yet (see "Follow-ups" below for what's outstanding).
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## Running it
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Restore the tool once per checkout:
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```powershell
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dotnet tool restore
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```
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Run a full mutation pass from a covered test project's directory, for
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example CultMath:
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```powershell
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cd packages/cultmath/tests/CultMath.Tests
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dotnet stryker -t mtp
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```
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`-t mtp` selects Stryker's Microsoft.Testing.Platform runner. It is required
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for test projects on `xunit.v3` (CultMath.Tests is one): Stryker's default
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`vstest` runner's coverage capture does not work against xunit.v3's native
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test host and the run stalls retrying test sessions indefinitely. Projects
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still on `xunit` v2 with `xunit.runner.visualstudio` can use the default
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`vstest` runner.
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For a single cut, scope the run to what changed instead of the whole
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project:
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```powershell
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dotnet stryker -t mtp --since:<base-commit-or-branch>
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```
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Reports land in `StrykerOutput/` next to the test project (git-ignored).
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Open the `reports/mutation-report.html` file for a browsable view, or read
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`reports/mutation-report.json` for a scriptable one.
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## Triage rules
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The mutation score is not a gate (`break` threshold is 0). Every surviving
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mutant is a finding about the tests, not a build failure, and gets triaged
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into one of:
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- **missing behavioural test** — name the test that should exist and what
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branch, edge case, or component it would exercise.
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- **degenerate fixture** — the code path is exercised, but the fixture's
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values make two different mutants produce the same observable result (a
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zero component that erases a `+`/`-` distinction, an already-normalized
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input that erases a `*`/`/` distinction, all-equal or all-same-sign inputs
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that erase a boundary or `&&`/`||` distinction). Fix the fixture, don't
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invent a new one that only kills that one mutant.
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- **equivalent** — the mutant cannot change behaviour. Float-threshold
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boundary flips (`>` to `>=`) are equivalent by default. A reasoned
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equivalence claim (for example, `>>` and `>>>` on an already-unsigned
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operand) needs its one-line reason recorded, not assumed.
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Do not write a hand-rolled mutation suite or anchor test as a substitute.
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Where the tool can't reach (native/shader/editor code), the defence is a
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behavioural test at the layer the rule is decided, not a committed mutation
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harness.
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## Known tool caveat
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Stryker 5.0.0's `mtp` runner is documented as preview. Its per-test coverage
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capture was found to report false "Survived" verdicts specifically for
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mutations inside `static readonly` field initializers that a test does
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cover (confirmed by hand-mutating `bool3.@false` and `bool4.@true` and
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running the full suite directly with `dotnet test`, both of which fail
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against the mutation that Stryker reported as surviving). Treat a survivor
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inside a `static readonly` field initializer as unproven until confirmed
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with a direct `dotnet test` run against the same mutation; do not triage it
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as a missing test without that check.
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## Follow-ups
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CultMath.Tests is the only project currently scoped. `GameCult.Caching`,
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`GameCult.Geometry`, `GameCult.Mesh` (+ `.Quic`, `.Quic.Native`),
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`GameCult.Networking` (+ `.WebSockets`), and the non-.NET runtimes
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(`cultcache-rs/py/ts`, `cultmesh-*`, `cultnet-*`) each need their own
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`stryker-config.json` (or ecosystem-equivalent tool: `cargo-mutants` for
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Rust, `StrykerJS` for TypeScript, `mutmut` for Python) and a baseline pass.
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None of that is wired into CI yet.
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