Add git submodule regression check

.buildkite/pipeline.yml

@@ -614,3 +614,11 @@ steps:
       - "build/packages/eosio_highsierra.rb"
       - "build/packages/eosio.rb"
     timeout: 60
+
+  - command: |
+        echo "+++ :microscope: Running git submodule regression check" && \
+        ./scripts/submodule_check.sh
+    label: "Git submodule regression check"
+    agents:
+      queue: "automation-large-builder-fleet"
+    timeout: 240

scripts/submodule_check.sh

@@ -0,0 +1,35 @@
+#!/bin/bash
+
+REPO_DIR=`mktemp -d`
+git clone "$BUILDKITE_REPO" "$REPO_DIR"
+git submodule update --init --recursive
+cd "$REPO_DIR"
+
+declare -A PR_MAP
+declare -A BASE_MAP
+
+echo "getting submodule info for $BUILDKITE_BRANCH"
+git checkout "$BUILDKITE_BRANCH" &> /dev/null
+git submodule update --init &> /dev/null
+while read -r a b; do
+  PR_MAP[$a]=$b
+done < <(git submodule --quiet foreach --recursive 'echo $path `git log -1 --format=%ct`')
+
+echo "getting submodule info for $BUILDKITE_PULL_REQUEST_BASE_BRANCH"
+git checkout "$BUILDKITE_PULL_REQUEST_BASE_BRANCH" &> /dev/null
+git submodule update --init &> /dev/null
+while read -r a b; do
+  BASE_MAP[$a]=$b
+done < <(git submodule --quiet foreach --recursive 'echo $path `git log -1 --format=%ct`')
+
+for k in "${!BASE_MAP[@]}"; do
+  base_ts=${BASE_MAP[$k]}
+  pr_ts=${PR_MAP[$k]}
+  echo "submodule $k"
+  echo "  timestamp on $BUILDKITE_BRANCH: $pr_ts"
+  echo "  timestamp on $BUILDKITE_PULL_REQUEST_BASE_BRANCH: $base_ts"
+  if (( $pr_ts < $base_ts)); then
+    echo "ERROR: $k is older on $BUILDKITE_BRANCH than $BUILDKITE_PULL_REQUEST_BASE_BRANCH"
+    exit 1
+  fi
+done