Add DbWriter mutation testing for #1669

Author: Widthdom

.github/workflows/mutation-testing.yml

@@ -0,0 +1,35 @@
+name: Mutation testing
+
+on:
+  schedule:
+    - cron: "17 3 * * 1"
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  dbwriter:
+    name: DbWriter mutation testing
+    runs-on: ubuntu-latest
+    timeout-minutes: 60
+
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+
+      - name: Set up .NET
+        uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: |
+            8.0.x
+            9.0.x
+
+      - name: Restore
+        run: dotnet restore CodeIndex.sln --locked-mode
+
+      - name: Install Stryker.NET
+        run: dotnet tool install --global dotnet-stryker
+
+      - name: Run DbWriter mutation tests
+        run: dotnet stryker --config-file stryker-config.json

DEVELOPER_GUIDE.md

@@ -39,6 +39,13 @@ failures from pass-on-retry flakes. For test suite structure, shared helpers,
 state-isolation rules, timeout diagnostics, and test-writing conventions, see
 [TESTING_GUIDE.md](TESTING_GUIDE.md).
 
+The weekly `Mutation testing` workflow runs Stryker.NET against
+`src/CodeIndex/Database/DbWriter.cs` using `stryker-config.json`. Keep this
+scope focused on transaction, savepoint, rollback, and batch-write behavior
+unless the runtime budget is intentionally expanded. The mutation score gates
+are high 75, low 70, and break 65 so changes that weaken rollback or savepoint
+coverage fail outside the regular PR test path.
+
 ## CI / Artifact Distribution
 
 Query commands accept `--read-only` (alias `--immutable`) to open an existing

changelog.d/unreleased/1669.internal.md

@@ -0,0 +1,17 @@
+---
+category: internal
+issues:
+  - 1669
+affected:
+  - stryker-config.json
+  - .github/workflows/mutation-testing.yml
+  - DEVELOPER_GUIDE.md
+---
+
+## English
+
+- **Added weekly DbWriter mutation testing (#1669)** — Stryker.NET now runs against the DbWriter transaction and rollback surface on a scheduled workflow with documented score gates.
+
+## 日本語
+
+- **DbWriter の週次 mutation testing を追加しました (#1669)** — Stryker.NET が DbWriter の transaction / rollback 周辺を定期 workflow で検査し、score gate も文書化されました。

stryker-config.json

@@ -0,0 +1,22 @@
+{
+  "stryker-config": {
+    "solution": "CodeIndex.sln",
+    "project": "src/CodeIndex/CodeIndex.csproj",
+    "test-projects": [
+      "tests/CodeIndex.Tests/CodeIndex.Tests.csproj"
+    ],
+    "mutate": [
+      "src/CodeIndex/Database/DbWriter.cs"
+    ],
+    "thresholds": {
+      "high": 75,
+      "low": 70,
+      "break": 65
+    },
+    "reporters": [
+      "progress",
+      "cleartext",
+      "html"
+    ]
+  }
+}