ci: move downstream check to independent yaml file (#2016)

* ci: move downstream check to independent yaml file

Fixes #1942

.github/workflows/ci.yaml

@@ -286,38 +286,3 @@ jobs:
       with:
         bom-path: gapic-generator-java-bom/pom.xml
 
-  downstream-compatibility:
-    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
-    steps:
-      - uses: actions/checkout@v3
-      - uses: actions/setup-java@v3
-        with:
-          java-version: 17
-          distribution: temurin
-      - run: mvn -version
-      - name: Install xmllint
-        run: |
-          sudo apt-get update
-          sudo apt-get -y install libxml2-utils
-      - name: Test helper scripts
-        run: ./.kokoro/presubmit/common_test.sh
-      - name: Perform downstream compatibility testing
-        run: REPOS_UNDER_TEST="${{ matrix.repo }}" ./.kokoro/presubmit/downstream-compatibility.sh

.github/workflows/downstream.yaml

@@ -0,0 +1,49 @@
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    # Changes to these directories do not directly affect the downstream libraries
+    paths-ignore:
+      - gapic-generator-java/**
+      - library_generation/**
+      - showcase/**
+      - test/**
+
+name: Downstream Check
+jobs:
+  downstream-compatibility:
+    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
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-java@v3
+        with:
+          java-version: 17
+          distribution: temurin
+      - run: mvn -version
+      - name: Install xmllint
+        run: |
+          sudo apt-get update
+          sudo apt-get -y install libxml2-utils
+      - name: Test helper scripts
+        run: ./.kokoro/presubmit/common_test.sh
+      - name: Perform downstream compatibility testing
+        run: REPOS_UNDER_TEST="${{ matrix.repo }}" ./.kokoro/presubmit/downstream-compatibility.sh