wallet: allow mintxfee=0

[delta1]

Allow -mintxfee=0, so the elements wallet can create transactions with 0 value fee outputs.

Elements can already accept 0 value fees with -minrelaytxfee=0 and -blockmintxfee=0, but before this change the wallet would not create a transaction with 0 feerate.

Note: this check for 0 was inherited from upstream and subsequently removed in bitcoin/bitcoin@f11eb1f

[tomt1664]

HasValidFee is a consensus check (in CheckTxInputs) which was introduced with CA.

I'm not sure why the fee == 0 was originally added, as a transaction without any fee output (i.e. also zero fee) is consensus valid. (the fee == 0 check in only applied to individual fee outputs, not the total fees for the tx).

I've verified that removing this fee == 0 check will cause a hard-fork if a zero fee output tx is confirmed in a block.

Suggestion: we add a -con_ config parameter (e.g. -con_allowanyfee) that removes this optionally.

[delta1]

why the fee == 0 was originally added

I think that part was inherited from upstream initially

add a -con_ config parameter (e.g. -con_allowanyfee) that removes this optionally

Good idea, thanks

[tomt1664]

Modified so that the wallet now creates a transaction without a fee output if the mintxfee=0 and fee rate set to zero.

This is consensus valid and so the fee == 0 check in HasValidFee does not prevent this.

Functional test updated to verify number of transaction outputs and no fee output in zero fee case.

[delta1]

Hey @tomt1664 the macos failure is because the wallet in that job is sqlite (descriptors) only - I'm able to repro on macos by running test/functional/wallet_send_zero_fee.py --descriptors

[delta1]

ACK ab5777e

Ran tests locally