[RFC] On sending forge pull request discussions to the mailing list

Hi folks,
I've thought about Joseph's proposal to send all discussion happening on 
forge to the mailing list*, specifically which bot workflows would need 
to be implemented and what should happen at what time and here's a 
slightly specific description that I plan to implement soon. I'm posting 
this to get feedback as.

- A "PR rollup" means:
   - if the user has force pushed a new version of the patch series, a 
PR version is created
   - if the current PR version has not been posted to the mailing list 
yet, an up to date email patch series is sent
   - messages related to the PR are sent as replies to the cover letter 
of the latest patch series
     - this includes all comments and reviews
     - this includes requests to review
     - excluding those that were already sent to the mailing list as 
emails (the list is tracked on a database table)
     - one message per comment or review

- Messages that must not be sent to the mailing list:
   - all messages by the batrachomyomachia bot
   - messages by the linaro bot (that do not include a <!-- bmm:send --> 
tag in the comment, which is a convention we can ask the linaro bot to 
adopt for comments that are meant to be sent to the mailing list)
   - comments that include a <!-- bmm:nosend --> tag, which is a 
convention to allow users to explicitly exclude certain comments from 
being sent to the mailing list
   - comments that contain the /submit or /preview command, since they 
are meant to trigger an email rather than be included in the email 
themselves

I think it will be enough to implement the following workflows in 
batrachomyomachia:
- when a PR is opened
   - no email is sent
   - user gets a welcome comment with guidance on how to use the bot and 
the available commands (a shorter version than what we currently have!)

- when someone requests a review on a PR by setting the Reviewers list
   - "PR rollup" occurs
- when someone requests a review on a PR by writing a /submit comment
   - "PR rollup" occurs
- when someone posts a review on a PR (hits the Finish review button and 
completes a review, with any review state)
   - "PR rollup" occurs

- When someone writes a comment that is not filtered by the rules above
   - if a patch email has already been sent for the PR, an email is sent 
to the mailing list with the content of the comment, with In-Reply-To 
set to the message id of the last cover letter, so that it is grouped in 
the same thread.
   Note that this is not PR rollup (random comments don't create a new 
PR version and patch posting)
   - if no patch email has been sent for the PR, no email is sent to 
avoid sending messages that are not linked to a patch series. These 
comments will be included in the email when a patch series is eventually 
sent.

- when a PR is closed without being merged and no patch email has been 
sent and no messages would be sent
   - no email is sent as no interaction happened
- when a PR is closed without being merged and at least one patch email 
has been sent or at least one message would be sent
   - a single email is sent to the mailing list notifying about the 
closure, with In-Reply-To set to the message id of the last cover letter
   - the email includes all comments and reviews in sequence

I think this strikes a balance between wanting to keep the mailing list 
up to date with what's happening and overwhelming it with too many emails.

One thing to add: the linaro bot currently updates existing comments so 
it will be good to figure out which messages are a "final state" and 
either make the bot recognize them or mark them with <!-- bmm:send -->.

* https://forge.sourceware.org/forge/batrachomyomachia/issues/15


Cheers
Claudio