[9.0] Add PR upgrade tests from merge base to PR HEAD (#128823)

.buildkite/pipelines/pull-request/pr-upgrade.yml

@@ -0,0 +1,6 @@
+steps:
+  - label: pr-upgrade
+    command: ".buildkite/scripts/run-pr-upgrade-tests.sh"
+    agents:
+      image: "docker.elastic.co/ci-agent-images/eck-region/buildkite-agent:1.5"
+      memory: "4G"

.buildkite/scripts/run-pr-upgrade-tests.sh

@@ -0,0 +1,45 @@
+#!/bin/bash
+
+#
+# Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+# or more contributor license agreements. Licensed under the "Elastic License
+# 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+# Public License v 1"; you may not use this file except in compliance with, at
+# your election, the "Elastic License 2.0", the "GNU Affero General Public
+# License v3.0 only", or the "Server Side Public License, v 1".
+#
+
+set -euo pipefail
+
+if [[ -z "$BUILDKITE_PULL_REQUEST_BASE_BRANCH" ]]; then
+   echo "Not a pull request, skipping PR upgrade tests."
+   exit 0
+fi
+
+# Identify the merge base of the current commit (branch) and the base branch of the pull request.
+# PR upgrade tests are run from the merge base to the current commit.
+BASE_COMMIT=$(git merge-base $BUILDKITE_PULL_REQUEST_BASE_BRANCH $BUILDKITE_COMMIT)
+
+VERSION=$(sed -n 's/^elasticsearch[[:space:]]*=[[:space:]]*\(.*\)/\1/p' build-tools-internal/version.properties)
+
+echo "Running PR upgrade tests from $BUILDKITE_PULL_REQUEST_BASE_BRANCH [$BASE_COMMIT] to $BUILDKITE_BRANCH [$BUILDKITE_COMMIT]."
+
+cat <<EOF | buildkite-agent pipeline upload
+steps:
+    - label: pr-upgrade $BUILDKITE_PULL_REQUEST_BASE_BRANCH -> $BUILDKITE_BRANCH
+      command: .ci/scripts/run-gradle.sh -Dbwc.checkout.align=true -Dorg.elasticsearch.build.cache.push=true -Dignore.tests.seed -Dscan.capture-file-fingerprints -Dtests.bwc.main.version=${VERSION}-SNAPSHOT -Dtests.bwc.refspec.main=${BASE_COMMIT} bcUpgradeTest -Dtests.jvm.argline="-Des.serverless_transport=true"
+      timeout_in_minutes: 300
+      agents:
+        provider: gcp
+        image: family/elasticsearch-ubuntu-2004
+        machineType: n1-standard-32
+        buildDirectory: /dev/shm/bk
+        preemptible: true
+      retry:
+        automatic:
+          - exit_status: "-1"
+            limit: 3
+            signal_reason: none
+          - signal_reason: agent_stop
+            limit: 3
+EOF