The repository push email can be very expensive to generate, especially
with syntax-highlighted diffs. Instead of generating the email for each
recipient, generate one email object and reset the Message-Id and To
headers for each recipient. (Cloning would also be expensive in the case
of large emails, although probably not as bad as generating from
scratch.)
Rails' "url_helpers" method creates an anonymous Module (which a bunch
of methods) on every call. By caching the output of this method in a
dedicated method we can shave off about 10 seconds of loading time for
an issue with around 200 comments.
A few things to note:
- The IncomingEmail feature is now enabled even without a
correctly-formatted sub-address
- Message-ID for new thread mail are kept the same so that subsequent
notifications to this thread are grouped in the thread by the email
service that receives the notification
(i.e. In-Reply-To of the answer == Message-ID of the first thread message)
- To maximize our chance to be able to retrieve the reply key, we look
for it in the In-Reply-To header and the References header
- The pattern for the fallback reply message id is "reply-[key]@[gitlab_host]"
- Improve docs thanks to Axil
