Sync "6.0/stage" with "master" via GitHub Actions

.github/scripts/install-hub.sh

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+#!/bin/bash -eux
+
+curl -fsSL https://github.com/github/hub/raw/master/script/get | bash -s v2.14.2

.github/scripts/sync-with-master.sh

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+#!/bin/bash -eux
+
+#
+# This variable must be passed into this script.
+#
+[[ -n "${BRANCH}" ]] || exit 1
+
+#
+# We need these config parameters set in order to do the git-merge.
+#
+git config user.name "${GITHUB_ACTOR}"
+git config user.email "${GITHUB_ACTOR}@users.noreply.github.com"
+
+#
+# We need the full git repository history in order to do the git-merge.
+#
+git fetch --unshallow
+
+#
+# In order to open a pull request, we need to push to a remote branch.
+# To avoid conflicting with existing remote branches, we use branches
+# within the "sync-with-master" namespace.
+#
+git checkout -b "sync-with-master/${BRANCH}" "origin/${BRANCH}"
+git merge -Xtheirs origin/master
+git push -f origin "sync-with-master/${BRANCH}"
+
+#
+# Opening a pull request may fail if there already exists a pull request
+# for the branch; e.g. if a previous pull request was previously made,
+# but not yet merged by the time we run this "sync" script again. Thus,
+# rather than causing the automation to report a failure in this case,
+# we swallow the error and report success.
+#
+# Additionally, as along as the git branch was properly updated (via the
+# "git push" above), the existing PR will have been updated as well, so
+# the "hub" command is unnecessary (hence ignoring the error).
+#
+git log -1 --format=%B | hub pull-request -F - -b "${BRANCH}" || true

.github/workflows/sync-with-master.yml

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+on:
+  push:
+    branches:
+      - master
+  schedule:
+    - cron:  '0 0 * * *'
+
+jobs:
+  sync:
+    strategy:
+      matrix:
+        branch:
+          - 6.0/stage
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@v2
+      - run: ./.github/scripts/install-hub.sh
+      - run: ./.github/scripts/sync-with-master.sh
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          BRANCH: ${{ matrix.branch }}