API: Introduce `#find_group!` which also check access permission

lib/api/groups.rb

@@ -82,7 +82,7 @@ module API
                         :lfs_enabled, :request_access_enabled
       end
       put ':id' do
-        group = find_group(params[:id])
+        group = find_group!(params[:id])
         authorize! :admin_group, group
 
         if ::Groups::UpdateService.new(group, current_user, declared_params(include_missing: false)).execute
@@ -96,13 +96,13 @@ module API
         success Entities::GroupDetail
       end
       get ":id" do
-        group = find_group(params[:id])
+        group = find_group!(params[:id])
         present group, with: Entities::GroupDetail
       end
 
       desc 'Remove a group.'
       delete ":id" do
-        group = find_group(params[:id])
+        group = find_group!(params[:id])
         authorize! :admin_group, group
         DestroyGroupService.new(group, current_user).execute
       end
@@ -111,7 +111,7 @@ module API
         success Entities::Project
       end
       get ":id/projects" do
-        group = find_group(params[:id])
+        group = find_group!(params[:id])
         projects = GroupProjectsFinder.new(group).execute(current_user)
         projects = paginate projects
         present projects, with: Entities::Project, user: current_user

lib/api/helpers.rb

@@ -105,7 +105,15 @@ module API
     end
 
     def find_group(id)
-      group = Group.find_by(path: id) || Group.find_by(id: id)
+      if id =~ /^\d+$/
+        Group.find_by(id: id)
+      else
+        Group.find_by(path: id)
+      end
+    end
+
+    def find_group!(id)
+      group = find_group(id)
 
       if can?(current_user, :read_group, group)
         group

lib/api/helpers/members_helpers.rb

@@ -2,7 +2,7 @@ module API
   module Helpers
     module MembersHelpers
       def find_source(source_type, id)
-        public_send("find_#{source_type}", id)
+        public_send("find_#{source_type}!", id)
       end
 
       def authorize_admin_source!(source_type, source)

lib/api/issues.rb

@@ -68,7 +68,7 @@ module API
       #   GET /groups/:id/issues?milestone=1.0.0
       #   GET /groups/:id/issues?milestone=1.0.0&state=closed
       get ":id/issues" do
-        group = find_group(params[:id])
+        group = find_group!(params[:id])
 
         params[:state] ||= 'opened'
         params[:group_id] = group.id