Hide users, communities, posts, comments from blocked instance

[Nutomic]

This is a problem on ds9.lemmy.ml, which is blocked by enterprise.lemmy.ml. They used to federate before we put DS9 on Enterprise's blocklist. So now users on both instances still see content from each other on their instance and can interact with it. But these interactions are blocked (like they should be). We need to find a way to prevent them from being displayed.

This should be relatively easy on the blocking side (Enterprise in this case), because we know about the block. But its much more tricky on the blocked side (DS9), because all it knows is that its outgoing activities arent being delivered (which might also happen if the other instance is down). And we might not want to tell a malicious instance that we blocked it.

This was reported here, with some more details

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[dessalines]

And we might not want to tell a malicious instance that we blocked it.

This might be kinda difficult. Any rejection or removal of a post / comment to an instance that's blocked us would let us know that they have. And of course they could just visit that instance and see that their posts aren't showing up.

IMO this might not be an issue, because on the blocking side, they can just remove those communities and posts manually as they see fit. And on the blocked side, their posts to that blocked community are going into a black hole.

[Nutomic]

Then maybe its a matter of error handling. Right now, if a user tries to send something to a user/community on a blocked instance, it will look locally as if everything worked fine. But nothing will show up on the remote instance. So Lemmy should be able to show these sending errors to the user. So this is closely related to #1205.

[dessalines]

Complicating both is the async. There is a possibility that if the async returns a special type of error on a community that doesn't live on our instance, then we could delete the item, but unfortunately that would be a little bit after we already show that the post got created locally.

[Nutomic]

Maybe this is not necessary. If there is any problematic content from an instance which was blocked, then admins can remove that manually.

[dessalines]

I agree, it shouldn't really be necessary, and admins can do that cleanup only if they want to.