As a first impression, I think this proposal is intellectually interesting, but crufty and hackish and should never actually be deployed. Writing code for Bitcoin in a future in which we have deployed a few generalized softforks this way sounds terrifying.

Instead of this:

    CTransaction GetTransaction(CBlock block, unsigned int index) {
        return block->vtx[index];
    }

We might have this:

    CTransaction GetTransaction(CBlock block, unsigned int index) {
        if (!IsBIP102sBlock(block)) {
            return block->vtx[index];
        } else {
            if (!IsOtherGeneralizedSoftforkBlock(block)) {
                // hooray! only one generalized softfork level to deal with!
                return LookupBlock(GetGSHashFromCoinbase(block->vtx[0].vin[0].scriptSig))->vtx[index];
           } else {
               throw NotImplementedError; // I'm too lazy to write pseudocode this complicated just to argue a point
        }
    }

It might be possible to make that a bit simpler with recursion, or by doing subsequent generalized softforks in a way that doesn't have multi-levels-deep block-within-a-block-within-a-block stuff. Still: ugh.




On Dec 29, 2015, at 9:46 PM, joe2015--- via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

> Below is a proof-of-concept implementation of BIP102 as a softfork:
> 
> https://github.com/ZoomT/bitcoin/tree/2015_2mb_blocksize
> https://github.com/jgarzik/bitcoin/compare/2015_2mb_blocksize...ZoomT:2015_2mb_blocksize?diff=split&name=2015_2mb_blocksize
> 
> BIP102 is normally a hardfork.  The softfork version (unofficial
> codename BIP102s) uses the idea described here:
> http://lists.linuxfoundation.org/pipermail/bitcoin-dev/2015-December/012073.html
> 
> The basic idea is that post-fork blocks are constructed in such a way
> they can be mapped to valid blocks under the pre-fork rules.  BIP102s
> is a softfork in the sense that post-fork miners are still creating a
> valid chain under the old rules, albeit indirectly.
> 
> From the POV of non-upgraded clients, BIP102s circumvents the
> block-size limit by moving transaction validation data "outside" of
> the block.  This is a similar trick used by Segregated Witness and
> Extension Blocks (both softfork proposals).
> 
> From the POV of upgraded clients, the block layout is unchanged,
> except:
> - A larger 2MB block-size limit (=BIP102);
> - The header Merkle root has a new (backwards compatible)
>  interpretation;
> - The coinbase encodes the Merkle root of the remaining txs.
> Aside from this, blocks maintain their original format, i.e. a block
> header followed by a vector of transactions.  This keeps the
> implementation simple, and is distinct from SW and EB.
> 
> Since BIP102s is a softfork it means that:
> - A miner majority (e.g. 75%, 95%) force miner consensus (100%).  This
>  is not true for a hardfork.
> - Fraud risk is significantly reduced (6-conf unlikely depending on
>  activation threshold).
> This should address some of the concerns with deploying a block-size
> increase using a hardfork.
> 
> Notes:
> 
> - The same basic idea could be adapted to any of the other proposals
>  (BIP101, 2-4-8, BIP202, etc.).
> - I used Jeff Garzik's BIP102 implementation which is incomplete (?).
>  The activation logic is left unchanged.
> - I am not a Bitcoin dev so hopefully no embarrassing mistakes in my
>  code :-(
> 
> --joe
> 
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev