Add a script to cancel previously started builds for the same review

Fixes #278

scripts/cancel_previous_build.py

@@ -0,0 +1,33 @@
+#!/usr/bin/env python3
+# Copyright 2020 Google LLC
+#
+# Licensed under the the Apache License v2.0 with LLVM Exceptions (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://llvm.org/LICENSE.txt
+#
+# 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.
+
+import os
+import requests
+
+def main(review_number, api_key):
+    buildkite = requests.Session()
+    buildkite.headers.update({'Authorization': 'Bearer {}'.format(api_key)})
+    api = lambda query: 'https://api.buildkite.com/v2' + query
+
+    builds = buildkite.get(api('/organizations/llvm-project/builds?state[]=scheduled&state[]=running')).json()
+    for build in builds:
+        if build['message'] == review_number:
+            pipeline = build['pipeline']['slug']
+            buildkite.put(api('/organizations/llvm-project/pipelines/{}/builds/{}/cancel'.format(pipeline, build['number'])))
+
+if __name__ == '__main__':
+    review_number = 'D{}'.format(os.getenv('ph_buildable_revision'))
+    api_key = # TODO where do we get that? The agents on the service queue need to have the proper access (read_builds and write_builds)
+    main(review_number, api_key)