test: verify Maven cache restoration and local .m2 overwrite in CI

.github/workflows/ci.yaml

@@ -20,6 +20,7 @@ on:
   push:
     branches:
     - main
+    - test-ci-cache-overwrite
   pull_request:
 name: ci
 jobs:
@@ -440,4 +441,104 @@ jobs:
       uses: googleapis/java-cloud-bom/tests/validate-bom@377c8d1fac6b1521dc52a10f4d02e5d371a0de67 # v26.79.0
       with:
         bom-path: gapic-libraries-bom/pom.xml
+  test-cache-overwrite:
+    name: verify-cache-overwrite
+    needs: bulk-filter
+    if: ${{ needs.bulk-filter.outputs.runnable == 'true' }}
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+      with:
+        persist-credentials: false
+    - uses: actions/setup-java@c1e323688fd81a25caa38c78aa6df2d33d3e20d9 # v4.8.0
+      with:
+        distribution: temurin
+        java-version: 17
+        cache: maven
+    - name: 1. Inspect Initial Restored .m2 Cache State
+      run: |
+        echo "============================================================"
+        echo "Checking initial state of ~/.m2/repository from cache..."
+        echo "============================================================"
+        if [ -d "$HOME/.m2/repository" ]; then
+          echo "Cache Restored! ~/.m2/repository directory size:"
+          du -sh "$HOME/.m2/repository" || true
+          echo ""
+          echo "Total number of files in restored cache:"
+          find "$HOME/.m2/repository" -type f | wc -l
+          echo ""
+          echo "All cached JAR and POM artifacts in ~/.m2/repository:"
+          find "$HOME/.m2/repository" \( -name "*.jar" -o -name "*.pom" \) | sed "s|$HOME/.m2/repository/||" | sort
+        else
+          echo "No existing ~/.m2/repository found (first cold run for this cache key)."
+        fi
+    - name: 2. Build and Install Baseline Module to ~/.m2 & Assert File Absence
+      run: |
+        echo "============================================================"
+        echo "Building and installing baseline java-common-protos to ~/.m2..."
+        echo "============================================================"
+        mvn install -Pquick-build -DskipTests=true -pl java-common-protos/proto-google-common-protos -am -B -ntp -T 1C
+        ARTIFACT_DIR="$HOME/.m2/repository/com/google/api/grpc/proto-google-common-protos"
+        BASELINE_JAR=$(find "$ARTIFACT_DIR" -name "proto-google-common-protos-*.jar" ! -name "*-sources.jar" ! -name "*-tests.jar" | head -n 1)
+        echo "Baseline JAR Path: $BASELINE_JAR"
+
+        BASELINE_SHA=$(sha256sum "$BASELINE_JAR" | awk '{print $1}')
+        echo "Baseline JAR SHA-256: $BASELINE_SHA"
+        echo "BASELINE_SHA=$BASELINE_SHA" >> "$GITHUB_ENV"
+        echo "BASELINE_JAR=$BASELINE_JAR" >> "$GITHUB_ENV"
+
+        echo "Verifying CacheTestMarker class DOES NOT exist in baseline JAR..."
+        if jar tf "$BASELINE_JAR" | grep -q "CacheTestMarker"; then
+          echo "ERROR: CacheTestMarker already exists in baseline JAR before code changes!"
+          exit 1
+        else
+          echo "ASSERTION PASSED: CacheTestMarker is not in the baseline JAR."
+        fi
+    - name: 3. Simulate Code Change in Module & Re-install
+      run: |
+        echo "============================================================"
+        echo "Modifying source file in java-common-protos to create a new version..."
+        echo "============================================================"
+        MARKER_FILE="java-common-protos/proto-google-common-protos/src/main/java/com/google/type/CacheTestMarker.java"
+        mkdir -p "$(dirname "$MARKER_FILE")"
+        TIMESTAMP=$(date +%s)
+        echo "package com.google.type;" > "$MARKER_FILE"
+        echo "public class CacheTestMarker {" >> "$MARKER_FILE"
+        echo "  public static final String TIMESTAMP = \"$TIMESTAMP\";" >> "$MARKER_FILE"
+        echo "  public static final String VERSION_MARKER = \"TEST_CACHE_OVERWRITE_VERIFIED_$TIMESTAMP\";" >> "$MARKER_FILE"
+        echo "}" >> "$MARKER_FILE"
+        echo "Created source marker:"
+        cat "$MARKER_FILE"
+
+        echo "Re-installing modified module with mvn install..."
+        mvn install -Pquick-build -DskipTests=true -pl java-common-protos/proto-google-common-protos -B -ntp -T 1C
+    - name: 4. Verify & Assert Overwrite, SHA Change, and New Class in ~/.m2
+      run: |
+        echo "============================================================"
+        echo "Verifying newly built version physically overwrote ~/.m2..."
+        echo "============================================================"
+        ARTIFACT_DIR="$HOME/.m2/repository/com/google/api/grpc/proto-google-common-protos"
+        NEW_JAR=$(find "$ARTIFACT_DIR" -name "proto-google-common-protos-*.jar" ! -name "*-sources.jar" ! -name "*-tests.jar" | head -n 1)
+        echo "New JAR Path: $NEW_JAR"
+
+        NEW_SHA=$(sha256sum "$NEW_JAR" | awk '{print $1}')
+        echo "Baseline SHA-256: $BASELINE_SHA"
+        echo "New JAR  SHA-256: $NEW_SHA"
+
+        if [ "$BASELINE_SHA" = "$NEW_SHA" ]; then
+          echo "ERROR: SHA-256 did not change! The artifact was not overwritten."
+          exit 1
+        fi
+        echo "ASSERTION PASSED: SHA-256 changed ($BASELINE_SHA -> $NEW_SHA), proving file was recompiled and overwritten."
+
+        echo "Verifying CacheTestMarker class NOW EXISTS inside the updated ~/.m2 jar..."
+        if jar tf "$NEW_JAR" | grep -q "CacheTestMarker"; then
+          jar tf "$NEW_JAR" | grep "CacheTestMarker"
+          echo "ASSERTION PASSED: CacheTestMarker.class is now present in the newly installed JAR."
+        else
+          echo "ERROR: CacheTestMarker class not found in installed JAR!"
+          exit 1
+        fi
+
+        echo "SUCCESS: All assertions passed! Local modules overwrite cached artifacts in ~/.m2/repository upon rebuild."