From 049894a72af98be949df63d3e28616773b14edfa Mon Sep 17 00:00:00 2001
From: Sylwester Lachiewicz <slachiewicz@apache.org>
Date: Wed, 5 Aug 2026 09:51:33 +0200
Subject: [PATCH] [MINOR] Persist the Maven build cache between CI runs

.mvn/extensions.xml enables maven-build-cache-extension, but the cache
never survives a CI run: actions/setup-java's `cache: maven` persists
~/.m2/repository only, while the extension writes to ~/.m2/build-cache.

The effect is visible in any recent build log - ten modules, ten
"Local build was not found by checksum" lines, zero restores - so every
run rebuilds and retests all ten modules from scratch, including for
pull requests that touch only the website or a single module.

Restore ~/.m2/build-cache before the build and save it afterwards. Only
main writes an entry; pull requests restore from main's rather than each
branch consuming the repository-wide 10 GB cache budget.

Source Build Check is deliberately left alone - it passes
-Dmaven.build.cache.enabled=false and skips `cache: maven` on purpose,
to prove the source release builds without prebuilt artifacts.
---
 .github/workflows/mvn-ci-build.yml | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/.github/workflows/mvn-ci-build.yml b/.github/workflows/mvn-ci-build.yml
index 92e478940..eb5ae7bcb 100644
--- a/.github/workflows/mvn-ci-build.yml
+++ b/.github/workflows/mvn-ci-build.yml
@@ -43,5 +43,25 @@ jobs:
           distribution: 'temurin'
           cache: maven
 
+      # `cache: maven` above only persists ~/.m2/repository. The build cache
+      # extension configured in .mvn/extensions.xml writes to ~/.m2/build-cache,
+      # so without this it starts empty on every run and never restores a module.
+      # Only `main` writes an entry, so pull requests warm up from `main` instead
+      # of each branch filling the repository-wide cache budget.
+      - name: Restore Maven build cache
+        uses: actions/cache/restore@v6
+        with:
+          path: ~/.m2/build-cache
+          key: maven-build-cache-${{ runner.os }}-${{ github.sha }}
+          restore-keys: |
+            maven-build-cache-${{ runner.os }}-
+
       - name: Build all module with Maven
         run: ./mvnw clean install -ntp -B
+
+      - name: Save Maven build cache
+        if: github.ref == 'refs/heads/main' && success()
+        uses: actions/cache/save@v6
+        with:
+          path: ~/.m2/build-cache
+          key: maven-build-cache-${{ runner.os }}-${{ github.sha }}