ci: backports: check if a backport PR has a valid issue

This is an automated check for the Backports project to
require one or more `Fixes #<issue>` items in the body
of the pull request.

Fixes #46164

Signed-off-by: Christopher Friedt <cfriedt@fb.com>

.github/workflows/backport_issue_check.yml

@@ -0,0 +1,30 @@
+name: Backport Issue Check
+
+on:
+  pull_request_target:
+    branches:
+      - v*-branch
+
+jobs:
+  backport:
+    name: Backport Issue Check
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check out source code
+        uses: actions/checkout@v2
+
+      - name: Install Python dependencies
+        run: |
+          sudo pip3 install -U setuptools wheel pip
+          pip3 install -U pygithub
+
+      - name: Run backport issue checker
+        env:
+          GITHUB_TOKEN: ${{ secrets.ZB_GITHUB_TOKEN }}
+        run: |
+          ./scripts/release/list_backports.py \
+            -o ${{ github.event.repository.owner.login }} \
+            -r ${{ github.event.repository.name }} \
+            -b ${{ github.event.pull_request.base.ref }} \
+            -p ${{ github.event.pull_request.number }}

scripts/release/list_backports.py

@@ -254,8 +254,7 @@ class Backport(object):
                 self._pulls_without_an_issue.append(p)
                 continue
 
-            # FIXME: when we have upgrade to python3.9+, use "issue_map | issues_for_this_pr"
+            issue_map = issue_map | issues_for_this_pr
-            issue_map = {**issue_map, **issues_for_this_pr}
 
         issues = list(issue_map.values())