it could also prevent that action from taking place.

Preventing an action is not possible as AVA is implemented as a Herald action, and a Herald action can only react to something that has already happened. In my understanding.

Hrm; random idea — maybe we could:

• Ask upstream for them to introduce a "prevent action from taking place" Herald action [that could be used] for global Maniphest Herald rules (similar to the "block push/diff with message" Herald actions that already exist for Differential & Diffusion)
• Modify AVA to do its calculations (& also disable accounts/delete sessions as appropriate) as a Herald condition, rather than a Herald action
• Edit {H293} to use the modified AVA condition, and - as the action - use the new "prevent action from taking place" action implemented by upstream

?

There are probably also other ways that this could be implemented downstream, but some of them might (e.g.) require our downstream fork of Phorge to be aware of AVA… which seems maybe kinda hacky & I don't know if you and/or RelEng would want to maintain that sort of downstream patch :)

More context is in https://we.phorge.it/T16290 which rules out the first bullet point (and I guess the followup ones, assuming it's an ordered list).

Yes I'd love to see this too but the current architecture is pretty much against us. :-/

In T412583#11462191, @Aklapper wrote:

More context is in https://we.phorge.it/T16290 which rules out the first bullet point (and I guess the followup ones, assuming it's an ordered list).

FWIW, maybe I'm missing something but I don't currently see how that task (re [evaluating the possibility of] asynchronous Herald rules) would necessarily block a "prevent action from taking place" Herald action from being added (assuming that async Herald rules end up ever going ahead :p) -- as I noted at https://we.phorge.it/T16290#28596, it seems like there would always be some Herald rules that probably couldn't be async.

In T412583#11462286, @A_smart_kitten wrote:

I don't currently see how that task (re [evaluating the possibility of] asynchronous Herald rules) would necessarily block a "prevent action from taking place" Herald action from being added

Hmm, maybe _I_ misunderstand now. :) In my understanding there is no way to have Herald block a transaction as Herald runs after the transaction has already taken place? (Maybe Herald could try to revert the transaction, though that might also not trivial to implement.)

Basically, I 'stole'/repurposed the idea for a "prevent [Maniphest] action from taking place" Herald action from Diffusion/Differential, which have their own implementations of a similar action ("block push with message" / "block diff with message").
I'm unfamiliar with Phorge code; but it looks like, at the time of those applications calculating the Herald effects for a given event, they check for whether that specific Herald action has been triggered, and block the event from taking place if it has been (Differential code, Diffusion code).