Handle escaped underscores in usernames

Underscores may need to be escaped due to being embedded in markdown.

Changelog: fixed

app/services/quick_actions/interpret_service.rb

@@ -77,7 +77,10 @@ module QuickActions
       # want to also handle bare usernames. The ReferenceExtractor also has
       # different behaviour, and will return all group members for groups named
       # using a user-style reference, which is not in scope here.
+      #
+      # nb: underscores may be passed in escaped to protect them from markdown rendering
       args        = params.split(/\s|,/).select(&:present?).uniq - ['and']
+      args.map! { _1.gsub(/\\_/, '_') }
       usernames   = (args - ['me']).map { _1.delete_prefix('@') }
       found       = User.by_username(usernames).to_a.select { can?(:read_user, _1) }
       found_names = found.map(&:username).to_set

spec/services/quick_actions/interpret_service_spec.rb

@@ -671,6 +671,19 @@ RSpec.describe QuickActions::InterpretService do
     end
 
     shared_examples 'assign command' do
+      it 'assigns to users with escaped underscores' do
+        user = create(:user)
+        base = user.username
+        user.update!(username: "#{base}_")
+        issuable.project.add_developer(user)
+
+        cmd = "/assign @#{base}\\_"
+
+        _, updates, _ = service.execute(cmd, issuable)
+
+        expect(updates).to eq(assignee_ids: [user.id])
+      end
+
       it 'assigns to a single user' do
         _, updates, _ = service.execute(content, issuable)