Re: [bitcoin-dev] BIP30 and BIP34 interaction (was Re: [BIP Proposal] Buried Deployments)

[Eric Voskuil <eric@voskuil.org>]

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Also, it's important to take note of the motivation behind not banning
duplicate tx hashes outright. Doing so would require that spent tx
hashes are retained forever. A pruning node will have no way of knowing
whether a new tx duplicates the hash of a preceding tx. Any
implementation that does retain such hashes and dismisses new txs on
that basis would fork against pruning nodes.

e

On 11/16/2016 04:43 PM, Eric Voskuil wrote:
>> This means that all future transactions will have different txids...
> rules do guarantee it.
> 
> No, it means that the chance is small, there is a difference.
> 
> If there is an address collision, someone may lose some money. If there
> is a tx hash collision, and implementations handle this differently, it
> will produce a chain split. As such this is not something that a node
> can just dismiss. If they do they are implementing a hard fork.
> 
> e
> 
> On 11/16/2016 04:31 PM, Tier Nolan via bitcoin-dev wrote:
>>
>>
>> On Thu, Nov 17, 2016 at 12:10 AM, Eric Voskuil via bitcoin-dev
>> <bitcoin-dev@lists.linuxfoundation.org
>> <mailto:bitcoin-dev@lists.linuxfoundation.org>> wrote:
>>
>>     Both of these cases resulted from exact duplicate txs, which BIP34 now
>>     precludes. However nothing precludes different txs from having the same
>>     hash.
>>
>>
>> The only way to have two transactions have the same txid is if their
>> parents are identical, since the txids of the parents are included in a
>> transaction.
>>
>> Coinbases have no parents, so it used to be possible for two of them to
>> be identical.
>>
>> Duplicate outputs weren't possible in the database, so the later
>> coinbase transaction effectively overwrote the earlier one.
>>
>> The happened for two coinbases.  That is what the exceptions are for.
>>
>> Neither of the those coinbases were spent before the overwrite
>> happened.  I don't even think those coinbases were spent at all.
>>
>> This means that every activate coinbase transaction has a unique hash
>> and all new coinbases will be unique.
>>
>> This means that all future transactions will have different txids.
>>
>> There might not be an explicit rule that says that txids have to be
>> unique, but barring a break of the hash function, they rules do
>> guarantee it.
> 


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