add safety check before running checkout

Author: chrisirhc

tools/lib/git.sh

@@ -90,3 +90,13 @@ git_base_commit() {
 git_changed_files() {
     git diff --name-only --diff-filter=d "$@"
 }
+
+# Note that this assumes ONE submodule. If we ever have more than one,
+# we'll need to generalize this.
+submodule_is_clean()
+{
+    # first character of every line status indicates the status of the
+    # submodule. If there is a space, it means the submodule is clean
+    # and initiated.
+    git submodule status | grep -q "^ "
+}

tools/setup-vendor-flutter

@@ -1,6 +1,19 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
+this_dir=${BASH_SOURCE[0]%/*}
+
+# shellcheck source=tools/lib/git.sh
+. "${this_dir}"/lib/git.sh
+
+
 # One-time setup to let git submodule + flutter SDK know that we're
 # using the main channel (main branch).
+# We should do a quick check that there's no changes.
+if ! submodule_is_clean; then
+    echo >&2 "There are changes in the submodule or it is not initialized."
+    echo >&2 "Please run 'git submodule update --init' then run this script again."
+    exit 1
+fi
+
 git -C vendor/flutter checkout -B main HEAD