Add check for release notes update. (#582)

All PRs merging to `main` must either modify `release_build_files/readme.md` or contain the label `skip-release-notes`.

Author: jonsimantov

.github/workflows/checks.yml

@@ -9,6 +9,7 @@ env:
   triggerLabelQuick: "tests-requested: quick"
   statusLabelInProgress: "tests: in-progress"
   statusLabelFailed: "tests: failed"
+  skipReleaseNotesLabel: "skip-release-notes"
 
 jobs:
   file_format_check:
@@ -92,3 +93,33 @@ jobs:
         run: |
           set -e
           bash scripts/gha/check_copyright.sh -g
+
+  release_notes_check:
+    # Check that the readme was updated, unless the PR has a specific label set (env.skipReleaseNotesLabel).
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        # Skip this if the PR has the skipReleaseNotes label or if it's a merge to other than main.
+        if: ${{!contains(github.event.pull_request.labels.*.name, env.skipReleaseNotesLabel) && (github.event.pull_request.base.ref == 'main')}}
+        with:
+          fetch-depth: 0
+          submodules: false
+      - name: Check whether release notes have been updated
+        # Skip this if the PR has the skipReleaseNotes label or if it's a merge to other than main.
+        if: ${{!contains(github.event.pull_request.labels.*.name, env.skipReleaseNotesLabel) && (github.event.pull_request.base.ref == 'main')}}
+        run: |
+          set -e
+          # Filename to check.
+          README_FILE=release_build_files/readme.md
+          # Determine the github merge base - same logic as integration_tests.yml
+          # "git merge-base main branch_name" will give the common ancestor of both branches.
+          MERGE_BASE=$(git merge-base origin/${{github.event.pull_request.head.ref}} origin/${{github.event.pull_request.base.ref}} || true)
+          # If MERGE_BASE can't be determined, ignore this check, something odd is going on.
+          if [[ -n "${MERGE_BASE}" ]]; then
+            DIFF_RESULT=$(git diff --name-only "origin/${{github.event.pull_request.head.ref}}..${MERGE_BASE}" -- "${README_FILE}")
+            if [[ "${DIFF_RESULT}" != "${README_FILE}" ]]; then
+              echo "::error ::Please update release notes (${README_FILE}) or add '${{env.skipReleaseNotesLabel}}' label."
+              exit 1
+            fi
+          fi
+