_Motivation_
Currently, the wallet uses a fSubtractFeeAmount (SFFO) flag on the
recipients objects for all forms of sending calls. According to the
commit discussion, this flag was chiefly introduced to permit sweeping
without manually calculating the fees of transactions. However, the flag
leads to unintuitive behavior and makes it more complicated to test
many wallet RPCs exhaustively. We proposed to introduce a dedicated
`sendall` RPC with the intention to cover this functionality.
Since the proposal, it was discovered in further discussion that our
proposed `sendall` rpc and SFFO have subtly different scopes of
operation.
• sendall:
Use _specific UTXOs_ to pay a destination the remainder after fees.
• SFFO:
Use a _specific budget_ to pay an address the remainder after fees.
While `sendall` will simplify cases of spending from specific UTXOs,
emptying a wallet, or burning dust, we realized that there are some
cases in which SFFO is used to pay other parties from a limited budget,
which can often lead to the creation of change outputs. This cannot be
easily replicated using `sendall` as it would require manual computation
of the appropriate change amount.
As such, sendall cannot replace all uses of SFFO, but it still has a
different use case and will aid in simplifying some wallet calls and
numerous wallet tests.
_Sendall call details_
The proposed sendall call builds a transaction from a specific subset of
the wallet's UTXO pool (by default all of them) and assigns the funds to
one or more receivers. Receivers can either be specified with a specific
amount or receive an equal share of the remaining unassigned funds. At
least one recipient must be provided without assigned amount to collect
the remainder. The `sendall` call will never create change. The call has
a `send_max` option that changes the default behavior of spending all
UTXOs ("no UTXO left behind"), to maximizing the output amount of the
transaction by skipping uneconomic UTXOs. The `send_max` option is
incompatible with providing a specific set of inputs.
The final step of send either produces a PSBT or the final transaction.
We extract these steps to a new helper function `FinishTransaction()` to
reuse them in `sendall`.
Coin selection requires knowing the weight of a transaction so that fees
can be estimated. However for external inputs, the weight may not be
avialble, and solving data may not be enough as the input could be one
that we do not support. By allowing users to specify input weights,
those external inputs can be included in the transaction.
Additionally, if the weight for an input is specified, that value will
always be used, regardless of whether the input is in the wallet or
solving data is available. This allows us to account for scenarios where
the wallet may be more conservative and estimate a larger input than may
actually be created.
For example, we assume the maximum DER signature size, but an external
input may be signed by a wallet which does nonce grinding in order to get
a smaller signature. In that case, the user can specify the smaller
input weight to avoid overpaying transaction fees.
c0405ee27f rpc: Document that DEFAULT is for Taproot, ALL for everything else (Andrew Chow)
d3992669df psbt: Actually use SIGHASH_DEFAULT (Andrew Chow)
eb9a1a2c59 psbt: Make sighash_type std::optional<int> (Andrew Chow)
Pull request description:
Make the behavior align with the help text by actually using SIGHASH_DEFAULT as the default sighash for signing PSBTs.
ACKs for top commit:
Sjors:
re-utACK c0405ee27f
Tree-SHA512: 5199fb41de416b2f10ac451f824e7c94b428ba11fdb9e50f0027c692e959ce5813a340c34a4e52d7aa128e12008303d80939a693eff36a869720e45442119828
