Add optional SonarQube Cloud scan

.github/workflows/ci.yml

@@ -37,6 +37,20 @@ on:
         description: 'Check that scaladoc builds'
         type: boolean
         default: true
+      sonar:
+        description: >-
+          Run a SonarQube Cloud scan. Requires a SONAR_TOKEN secret and an existing project on
+          SonarQube Cloud with Automatic Analysis turned off, otherwise the two conflict.
+        type: boolean
+        default: false
+      sonar_project_key:
+        description: 'SonarQube project key. Defaults to <owner>_<repo>.'
+        type: string
+        default: ''
+      sonar_args:
+        description: 'Extra -D arguments for the Sonar scanner'
+        type: string
+        default: ''
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
@@ -142,3 +156,31 @@ jobs:
       - name: scaladoc ${{ matrix.scala }}
         if: inputs.doc_check
         run: sbt "++${{ matrix.scala }}; Compile/doc"
+
+      # Scanned once, on the first Scala version only: SonarQube tracks one analysis per branch, so
+      # running it per matrix leg would have the legs overwrite each other.
+      - name: sonar settings
+        id: sonar
+        if: inputs.sonar && matrix.scala == fromJSON(inputs.scala_versions)[0]
+        env:
+          KEY: ${{ inputs.sonar_project_key }}
+        run: |
+          if [[ -z "$KEY" ]]; then
+            KEY="${GITHUB_REPOSITORY/\//_}"
+          fi
+          echo "key=$KEY" >> "$GITHUB_OUTPUT"
+          # scoverage writes one report per module; Sonar takes a comma separated list
+          reports=$(find . -path '*/scoverage-report/scoverage.xml' | paste -sd, -)
+          echo "reports=$reports" >> "$GITHUB_OUTPUT"
+
+      - name: sonar scan
+        if: inputs.sonar && matrix.scala == fromJSON(inputs.scala_versions)[0]
+        uses: SonarSource/sonarqube-scan-action@v8.2
+        env:
+          SONAR_TOKEN: ${{ secrets.SONAR_TOKEN }}
+        with:
+          args: >
+            -Dsonar.organization=${{ github.repository_owner }}
+            -Dsonar.projectKey=${{ steps.sonar.outputs.key }}
+            -Dsonar.scala.coverage.reportPaths=${{ steps.sonar.outputs.reports }}
+            ${{ inputs.sonar_args }}

README.md

@@ -38,6 +38,9 @@ so `secrets: inherit` is needed for `GITHUB_TOKEN`.
 | `version_policy_check` | `true` | requires [sbt-version-policy](https://github.com/scalacenter/sbt-version-policy/) |
 | `scalafmt_check` | `true` | |
 | `doc_check` | `true` | runs `Compile/doc` |
+| `sonar` | `false` | run a SonarQube Cloud scan, see below |
+| `sonar_project_key` | `<owner>_<repo>` | |
+| `sonar_args` | `''` | extra `-D` arguments for the scanner |
 
 Example for a project without `sbt-version-policy` and on a different Scala set:
 
@@ -69,6 +72,29 @@ Binary compatibility, formatting and scaladoc run as **explicit sbt tasks**, not
 `check` alias. An alias can be stubbed out (`addCommandAlias("check", "show version")`), which makes
 the gate silently guarantee nothing.
 
+The workflow checks out with `fetch-depth: 0`. Without tags sbt-dynver reports the version as
+`0.0.0`, `versionPolicyCheck` then has no previous version to compare against, and the binary
+compatibility check passes without checking anything.
+
+### SonarQube Cloud
+
+Off by default. Enable with `sonar: true`, which scans once, on the first Scala version, importing
+scoverage's per-module reports. Configuration is passed as scanner arguments, so no per-repo
+`sonar-project.properties` is needed.
+
+Three prerequisites, all outside this repo:
+
+1. A `SONAR_TOKEN` organization secret. It does not exist yet, so `sonar: true` will fail until it is
+   added.
+2. The project must already exist on SonarQube Cloud — the scanner reports to a project, it does not
+   create one.
+3. **Automatic Analysis must be turned off** for that project. It and CI-based scanning are mutually
+   exclusive, and Automatic Analysis wins, so the scan will be rejected while it is on.
+
+Note that the SonarQube Cloud GitHub App creates a check suite on every commit even in repositories
+it never analyses, which leaves a check permanently queued and reporting no result. Repositories not
+being analysed should have the app removed rather than left in that state.
+
 ## Scala Release workflow (v3, v4, v5)
 
 ### Setup