[FIX] digest: do not write on default digest if not necessary

When a portal user is created (e.g. through the `auth_signup` module),
an unnecessary write is done on the `write_date` of the default digest.
This `write` is unnecessary since a portal user is never subscribed to
the default digest.

In case of a high signup frequency, it can cause concurrent transaction
errors.

We avoid writing if no internal user is being created.

closes odoo/odoo#162563

X-original-commit: bbc427e837b08187f351febfde4339262e660944
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/digest/models/res_users.py

@@ -12,7 +12,8 @@ class ResUsers(models.Model):
         users = super(ResUsers, self).create(vals_list)
         default_digest_emails = self.env['ir.config_parameter'].sudo().get_param('digest.default_digest_emails')
         default_digest_id = self.env['ir.config_parameter'].sudo().get_param('digest.default_digest_id')
-        if default_digest_emails and default_digest_id:
+        users_to_subscribe = users.filtered_domain([('share', '=', False)])
+        if default_digest_emails and default_digest_id and users_to_subscribe:
             digest = self.env['digest.digest'].sudo().browse(int(default_digest_id)).exists()
-            digest.user_ids |= users.filtered_domain([('share', '=', False)])
+            digest.user_ids |= users_to_subscribe
         return users