Subject: Re: [bitcoindev] Re: Great Consensus Cleanup Revival

From: Eric Voskuil <[email-masked]>
Date: Sun, 23 Jun 2024 17:35:30 -0700 (PDT)

Thanks for the responses Antoine.

>  As discussed here it would let node implementations cache block failures 
at an earlier stage of validation. Not a large gain, but still nice to have.

It is not clear to me how determining the coinbase size can be done at an 
earlier stage of validation than detection of the non-null coinbase. The 
former requires parsing the coinbase to determine its size, the latter 
requires parsing it to know if the point is null. Both of these can be 
performed as early as immediately following the socket read.

size check

(1) requires new consensus rule: 64 byte transactions (or coinbases?) are 
invalid.
(2) creates a consensus "seam"  (complexity) in txs, where < 64 bytes and > 
64 bytes are potentially valid.
(3) can be limited to reading/skipping header (80 bytes) plus parsing 0 - 
65 coinbase bytes.

point check

(1) requires no change.
(2) creates no consensus seam.
(3) can be limited to reading/skipping header (80 bytes) plus parsing 6 - 
43 coinbase bytes.

Not only is this not a large (performance) gain, it's not one at all.

> It would also avoid a large footgun for anyone implementing a software 
verifying an SPV proof verifier and not knowing the intricacies of the 
protocol...

It seems to me that introducing an arbitrary tx size validity may create 
more potential implementation bugs than it resolves. And certainly anyone 
implementing such a verifier must know many intricacies of the protocol. 
This does not remove one, it introduces another - as there is not only a 
bifurcation around tx size but one around the question of whether this rule 
is active.
 
> Finally, it would get rid of a large footgun in general. 

I do not see this. I see a very ugly perpetual seam which will likely 
result in unexpected complexities over time.

> Certainly, unique block hashes would be a useful property for Bitcoin to 
have. It's not far-fetched to expect current or future Bitcoin-related 
software to rely on this.

This does not produce unmalleable block hashes. Duplicate tx hash 
malleation remains in either case, to the same effect. Without a resolution 
to both issues this is an empty promise.

The only possible benefit that I can see here is the possible very small 
bandwidth savings pertaining to SPV proofs. I would have a very hard time 
justifying adding any consensus rule to achieve only that result.

Best,
Eric

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