On Wed, Nov 16, 2016 at 09:32:24AM -0500, Alex Morcos via bitcoin-dev wrote:
> I think we are misunderstanding the effect of this change.
> It's still "OK" for a 50k re-org to happen.
> We're just saying that if it does, we will now have potentially introduced
> a hard fork between new client and old clients if the reorg contains
> earlier signaling for the most recent ISM soft fork and then blocks which
> do not conform to that soft fork before the block height encoded activation.
> 
> I think the argument is this doesn't substantially add to the confusion or
> usability of the system as its likely that old software won't even handle
> 50k block reorgs cleanly anyway and there will clearly have to be human
> coordination at the time of the event.  In the unlikely event that the new
> chain does cause such a hard fork, that coordination can result in everyone
> upgrading to software that supports the new rules anyway.
> 
> So no, I don't think we should add a checkpoint.  I think we should all
> just agree to a hard fork that only has a very very slim chance of any
> practical effect.

So, conceptually, another way to deal with this is to hardcode a blockhash
where we allow blocks in a chain ending with that blockhash to _not_ follow
BIP65, up until that blockhash, and any blockchain without that blockhash must
respect BIP65 for all blocks in the chain.

This is a softfork: we've only added rules that made otherwise valid chains
invalid, and at the same time we are still accepting large reorgs (albeit under
stricter rules than before).

I'd suggest we call this a exemption hash - we've exempted a particular
blockchains from a soft-forked rule that we would otherwise enforce.

-- 
https://petertodd.org 'peter'[:-1]@petertodd.org