Re: [bitcoin-dev] New PSBT version proposal

[Rusty Russell <rusty@rustcorp.com.au>]

Hi Andrew et al,

        Very excited to see this progress; thanks for doing all the
work!  Sorry for the delayed feedback, I didn't get to this before the
break.

> Additionally, I would like to add a new global field:
> * PSBT_GLOBAL_UNDER_CONSTRUCTION = 0x05
>    * Key: empty
>    * Value: A single byte as a boolean. 0 for False, 1 for True. All 
> other values ore prohibited. Must be omitted for PSBTv0, may be omitted 
> in PSBTv2.
>
> PSBT_GLOBAL_UNDER_CONSTRUCTION is used to signal whether inputs and 
> outputs can be added to the PSBT. This flag may be set to True when 
> inputs and outputs are being updated, signed, and finalized. However 
> care must be taken when there are existing signatures. If this field is 
> omitted or set to False, no further inputs and outputs may be added to 
> the PSBT.

I wonder if this can be flagged simply by omitting the (AFAICT
redundant) PSBT_GLOBAL_INPUT_COUNT and PSBT_GLOBAL_OUTPUT_COUNT?  What
are the purposes of those fields?

For our uses, there would be no signatures at this stage; it's simply a
subdivision of the Creator role.  This role would be terminated by
removing the under-construction marker.  For this, it could be clear
that such an under-construction PSBT SHOULD NOT be signed.

Otherwise, if an explicit marker is required, I would omit the value and
simply use its existence to as a flag.  Having two "false" values is
simply asking for trouble.

Thanks!
Rusty.
PS.  Perhaps we should change the name to PBT (Partial Bitcoin
     Transaction) now, since it's more than just signing...