ci: Add Downstream Protobuf-Java Source Compatibility Test (#3405)

Set up a nightly CI job that tests Protobuf compatibility with
downstream libraries. This step of the CI job is to test source
compatibility.

The nightly job will check the compatibility of these libraries with the
latest in Protobuf-Java 3.x and and 4.x. This job can be manually
invoked with the Protobuf-Java versions as a list.

Future steps may introduce testing for binary compatibility.

Author: lqiu96

.github/workflows/downstream_protobuf_compatibility_check_nightly.yaml

@@ -0,0 +1,50 @@
+on:
+  pull_request:
+    # Runs on PRs targeting main, but will be filtered for Release PRs
+    branches:
+      - 'main'
+  workflow_dispatch:
+    inputs:
+      protobuf_runtime_versions:
+        description: 'Comma separated list of Protobuf-Java versions (i.e. "3.25.x","4.x.y")'
+        required: true
+  schedule:
+    - cron: '0 1 * * *' # Nightly at 1am
+
+name: Downstream Protobuf Compatibility Check Nightly
+jobs:
+  downstream-protobuf-test:
+    # Checks if PR comes from Release-Please branch or if invoked from nightly job
+    if: github.head_ref == 'release-please--branches--main' || github.event_name == 'schedule'
+    runs-on: ubuntu-22.04
+    strategy:
+      fail-fast: false
+      matrix:
+        repo:
+          - google-cloud-java
+          - java-bigtable
+          - java-bigquery
+          - java-bigquerystorage
+          - java-datastore
+          - java-firestore
+          - java-logging
+          - java-logging-logback
+          - java-pubsub
+          - java-pubsublite
+          - java-spanner-jdbc
+          - java-spanner
+          - java-storage
+          - java-storage-nio
+        # Default Protobuf-Java versions to use are specified here. Without this, the nightly workflow won't know
+        # which values to use and would resolve to ''.
+        protobuf-version: ${{ fromJSON(format('[{0}]', inputs.protobuf_runtime_versions || '"3.25.5","4.28.3"')) }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-java@v4
+        with:
+          java-version: 17
+          distribution: temurin
+      - name: Print Protobuf-Java testing version
+        run: echo "Testing with Protobuf-Java v${{ matrix.protobuf-version }}"
+      - name: Perform downstream source compatibility testing
+        run: REPOS_UNDER_TEST="${{ matrix.repo }}" PROTOBUF_RUNTIME_VERSION="${{ matrix.protobuf-version}}" ./.kokoro/nightly/downstream-protobuf-source-compatibility.sh

.kokoro/nightly/downstream-protobuf-source-compatibility.sh

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+#!/bin/bash
+# Copyright 2023 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+set -eo pipefail
+
+# Comma-delimited list of repos to test with the local java-shared-dependencies
+if [ -z "${REPOS_UNDER_TEST}" ]; then
+  echo "REPOS_UNDER_TEST must be set to run downstream-protobuf-source-compatibility.sh"
+  echo "Expects a comma-delimited list: i.e REPOS_UNDER_TEST=\"java-bigtable,java-bigquery\""
+  exit 1
+fi
+
+# Version of Protobuf-Java runtime to compile with
+if [ -z "${PROTOBUF_RUNTIME_VERSION}" ]; then
+  echo "PROTOBUF_RUNTIME_VERSION must be set to run downstream-protobuf-source-compatibility.sh"
+  echo "Expects a single Protobuf-Java runtime version i.e. PROTOBUF_RUNTIME_VERSION=\"4.28.3\""
+  exit 1
+fi
+
+for repo in ${REPOS_UNDER_TEST//,/ }; do # Split on comma
+  # Perform source-compatibility testing on main (latest changes)
+  git clone "https://github.com/googleapis/$repo.git" --depth=1
+  pushd "$repo"
+
+  # Compile the Handwritten Library with the Protobuf-Java version to test source compatibility
+  # Run unit tests to help check for any behavior differences (dependant on coverage)
+  mvn clean test -B -V -ntp \
+      -Dclirr.skip=true \
+      -Denforcer.skip=true \
+      -Dmaven.javadoc.skip=true \
+      -Dprotobuf.version=${PROTOBUF_RUNTIME_VERSION} \
+      -T 1C
+  popd
+done