[Minor] Dkim_signing: allow custom routine for getting dkim domain

lualib/lua_dkim_tools.lua

@@ -204,6 +204,31 @@ local function prepare_dkim_signing(N, task, settings)
     dkim_domain = get_dkim_domain('use_domain_sign_inbound')
     lua_util.debugm(N, task, 'inbound: use domain(%s) for signature: %s',
       settings.use_domain_sign_inbound, dkim_domain)
+  elseif settings.use_domain_custom then
+    if type(settings.use_domain_custom) == 'string' then
+      -- Load custom function
+      local loadstring = loadstring or load
+      local ret, res_or_err = pcall(loadstring(settings.use_domain_custom))
+      if ret then
+        if type(res_or_err) == 'function' then
+          settings.use_domain_custom = res_or_err
+          dkim_domain = settings.use_domain_custom(task)
+          lua_util.debugm(N, task, 'use custom domain for signing: %s',
+              dkim_domain)
+        else
+          logger.errx(task, 'cannot load dkim domain custom script: invalid type: %s, expected function',
+              type(res_or_err))
+          settings.use_domain_custom = nil
+        end
+      else
+        logger.errx(task, 'cannot load dkim domain custom script: %s', res_or_err)
+        settings.use_domain_custom = nil
+      end
+    else
+      dkim_domain = settings.use_domain_custom(task)
+      lua_util.debugm(N, task, 'use custom domain for signing: %s',
+          dkim_domain)
+    end
   else
     dkim_domain = get_dkim_domain('use_domain')
     lua_util.debugm(N, task, 'use domain(%s) for signature: %s',