Make EntityUnleashEvent cancellable

[Machine-Maker]

Closes #4780

One thing to note. Should it be possible to cancel the event if the leashed entity dies? I tested if the player dies and you do stay leashed to it, but if the leashed entity dies, maybe it should unleash regardless of event cancellation.

[stale]

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[Owen1212055]

No.

[Machine-Maker]

Rebased after the NMS repackage

[kennytv]

You definitely have to still call Vanilla's unleash method when a) the holder disconnected (otherwise the server will freak out trying to unleash the entity for eternity), or b) the leashed entity dies (for internal distance tracking purposes). Im pretty sure both of these should be covered in EntityInsentient#eA(), tho Bukkit's lacking unleash reasons didn't really help looking at that 😛 so there might even be more cases, but not sure

[Machine-Maker]

Hmm, if you can't cancel it in several situations, how should this be handled? Just document in the event that cancelling in those situations will not actually do anything?

[kennytv]

Any "good" suggestions I'd have would require slightly breaking the event, I can't really think of anything but document what cases aren't cancellable and that setting cancel to true won't have any effect there, yeah.

[Machine-Maker]

Ok, made it uncancellable in those situations I think.

[Machine-Maker]

Rebased for 1.17.1