🏗 CircleCI bash file optimizations (ampproject#33734)

* Move graceful halt logic into separate file
* Store a copy of the merge commit info file in the workspace

.circleci/compute_merge_commit.sh

@@ -40,6 +40,11 @@ MERGE_BRANCH="refs/pull/$PR_NUMBER/merge"
 echo $(GREEN "Computing merge SHA of $MERGE_BRANCH...")
 CIRCLE_MERGE_SHA="$(git ls-remote https://github.com/ampproject/amphtml.git "$MERGE_BRANCH" | awk '{print $1}')"
 
+# TODO(danielrozenberg): after #33708 is submitted, remove local directory copy and only store one in the workspace.
 echo "$CIRCLE_MERGE_SHA" > .CIRCLECI_MERGE_COMMIT
-echo $(GREEN "Stored merge SHA $CIRCLE_MERGE_SHA in .CIRCLECI_MERGE_COMMIT.")
+
+mkdir -p /tmp/workspace
+cp .CIRCLECI_MERGE_COMMIT /tmp/workspace/.CIRCLECI_MERGE_COMMIT
+
+echo $(GREEN "Stored merge SHA $CIRCLE_MERGE_SHA in /tmp/workspace/.CIRCLECI_MERGE_COMMIT.")
 exit 0

.circleci/maybe_gracefully_halt.sh

@@ -0,0 +1,28 @@
+#!/bin/bash
+#
+# Copyright 2021 The AMP HTML Authors. All Rights Reserved.
+#
+# 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.
+
+# Script used by AMP's CI builds to gracefully halt unnecessary jobs, before
+# executing time-consuming steps such as checking out the code or installing
+# dependencies.
+
+set -e
+
+GREEN() { echo -e "\n\033[0;32m$1\033[0m"; }
+
+if ls /tmp/restored-workspace/.CI_GRACEFULLY_HALT_* 1>/dev/null 2>&1; then
+  echo $(GREEN "Gracefully halting this job.")
+  circleci-agent step halt
+fi