ci: resolve Gradle dependencies through the runners' Maven mirror

.github/actions/maven-mirror/action.yml

@@ -0,0 +1,42 @@
+name: Maven mirror
+description: >
+  Points Gradle at the runners' Maven mirror (Maven Central, the Gradle Plugin
+  Portal and PaperMC) by installing a Gradle init script. Uses the runner's
+  MAVEN_MIRROR_URL, which only the self-hosted runners set. Without it, or when
+  the mirror does not answer, the action warns and changes nothing, so Gradle
+  resolves from upstream exactly as before.
+
+outputs:
+  init-script:
+    description: >
+      Path of the installed init script, empty when the mirror is not used.
+      Pass it to Docker builds as the BuildKit secret `gradle_init`.
+    value: ${{ steps.install.outputs.init-script }}
+
+runs:
+  using: composite
+  steps:
+    - name: Install Maven mirror init script
+      id: install
+      shell: bash
+      env:
+        TEMPLATE: ${{ github.action_path }}/grounds-maven-mirror.init.gradle.kts
+      run: |
+        set -euo pipefail
+        url="${MAVEN_MIRROR_URL:-}"
+        url="${url%/}"
+        if [ -z "$url" ]; then
+          echo "::warning::MAVEN_MIRROR_URL is not set on this runner, resolving from upstream"
+          exit 0
+        fi
+        probe="$url/releases/org/jetbrains/kotlin/kotlin-stdlib/maven-metadata.xml"
+        status="$(curl -s -o /dev/null -m 3 -w '%{http_code}' "$probe" || true)"
+        if [ "$status" != "200" ]; then
+          echo "::warning::Maven mirror $url unreachable (HTTP ${status:-none}), resolving from upstream"
+          exit 0
+        fi
+        script="$HOME/.gradle/init.d/grounds-maven-mirror.init.gradle.kts"
+        mkdir -p "$(dirname "$script")"
+        sed "s|@MIRROR_URL@|$url|" "$TEMPLATE" > "$script"
+        echo "init-script=$script" >> "$GITHUB_OUTPUT"
+        echo "Using Maven mirror $url"

.github/actions/maven-mirror/grounds-maven-mirror.init.gradle.kts

@@ -0,0 +1,55 @@
+// Points the public upstream repositories at the CI runners' Maven mirror.
+// Installed by the groundsgg/.github maven-mirror action on self-hosted runners only.
+// GitHub Packages, mavenLocal() and every other repository stay as the build declares them.
+//
+// The mirror is plain HTTP inside the cluster, so only the rewritten repositories get
+// isAllowInsecureProtocol.
+
+val mirror = "@MIRROR_URL@".trimEnd('/')
+val pluginPortal = "https://plugins.gradle.org/m2"
+val rewrites = mapOf(
+    "https://repo.maven.apache.org/maven2" to "$mirror/releases",
+    "https://repo1.maven.org/maven2" to "$mirror/releases",
+    pluginPortal to "$mirror/gradle-plugins",
+    "https://repo.papermc.io/repository/maven-public" to "$mirror/papermc",
+)
+
+fun RepositoryHandler.useMirror() {
+    whenObjectAdded {
+        val repo = this as? MavenArtifactRepository ?: return@whenObjectAdded
+        val upstream = repo.url.toString().trimEnd('/')
+        val target = rewrites[upstream] ?: return@whenObjectAdded
+        if (upstream == pluginPortal) {
+            // gradlePluginPortal() is a repository Gradle builds itself; swap it for a plain
+            // maven repository rather than mutating it. Adding here keeps its place in the list.
+            remove(repo)
+            maven {
+                name = "groundsMavenMirrorGradlePlugins"
+                url = uri(target)
+                isAllowInsecureProtocol = true
+            }
+        } else {
+            repo.url = uri(target)
+            repo.isAllowInsecureProtocol = true
+        }
+    }
+}
+
+println("Using Maven mirror $mirror")
+
+beforeSettings {
+    buildscript.repositories.useMirror()
+    pluginManagement.repositories.useMirror()
+    dependencyResolutionManagement.repositories.useMirror()
+}
+
+settingsEvaluated {
+    // Without any pluginManagement repositories Gradle falls back to an implicit plugin portal
+    // that never enters this list; declare it so it gets swapped for the mirror.
+    if (pluginManagement.repositories.isEmpty()) pluginManagement.repositories.gradlePluginPortal()
+}
+
+allprojects {
+    buildscript.repositories.useMirror()
+    repositories.useMirror()
+}

.github/workflows/docker-gradle-build-push.yml

@@ -80,6 +80,14 @@ jobs:
             echo "github.token=${{ secrets.PACKAGES_TOKEN || secrets.GITHUB_TOKEN }}"
           } > ~/.gradle/gradle.properties
 
+      # Installs the init script on the runner, for the `./gradlew -q properties`
+      # below, and hands the same file to the image builds as the BuildKit secret
+      # `gradle_init`. A Dockerfile opts in by mounting it on its Gradle RUN:
+      #   --mount=type=secret,id=gradle_init,target=/root/.gradle/init.d/grounds-maven-mirror.init.gradle.kts,required=false
+      - name: Use the Maven mirror
+        id: mirror
+        uses: groundsgg/.github/.github/actions/maven-mirror@main
+
       - name: Read Gradle project version
         id: gradle_version
         run: |
@@ -186,6 +194,7 @@ jobs:
             GITHUB_USER=${{ github.actor }}
           secrets: |
             github_token=${{ secrets.PACKAGES_TOKEN || secrets.GITHUB_TOKEN }}
+          secret-files: ${{ steps.mirror.outputs.init-script && format('gradle_init={0}', steps.mirror.outputs.init-script) || '' }}
 
       - name: Build and push image
         if: github.event_name != 'pull_request'
@@ -203,6 +212,7 @@ jobs:
             GITHUB_USER=${{ github.actor }}
           secrets: |
             github_token=${{ secrets.PACKAGES_TOKEN || secrets.GITHUB_TOKEN }}
+          secret-files: ${{ steps.mirror.outputs.init-script && format('gradle_init={0}', steps.mirror.outputs.init-script) || '' }}
 
       - name: Generate SBOM
         if: github.event_name != 'pull_request'

.github/workflows/gradle-ci.yml

@@ -68,6 +68,9 @@ jobs:
         with:
           cache-disabled: ${{ runner.environment != 'github-hosted' }}
 
+      - name: Use the Maven mirror
+        uses: groundsgg/.github/.github/actions/maven-mirror@main
+
       - name: Configure GitHub Packages credentials
         run: |
           mkdir -p ~/.gradle

.github/workflows/gradle-publish.yml

@@ -68,6 +68,9 @@ jobs:
         with:
           cache-disabled: ${{ runner.environment != 'github-hosted' }}
 
+      - name: Use the Maven mirror
+        uses: groundsgg/.github/.github/actions/maven-mirror@main
+
       - name: Configure GitHub Packages credentials for resolution
         run: |
           mkdir -p ~/.gradle