Fixes#5287.
The PSBT spec is a bit vague when it comes to the WitnessUtxo field of
an input as it's not strictly required for witness inputs. Therefore
Electrum for example does not include the field.
We need to make the SegWit input check a bit more elaborate by looking
at the output script of the UTXO and also the redeem script in case it's
a nested SegWit spend.
Add a dust-limit to `CoinSelect` and let the fee estimation consider if a change output is needed or not, assuring that if the change is below the dust-limit it will go towards the fee instead.
Without this change the high-fee logic is never tested as it is instead caught by the dust-output logic. This change uses a higher fee rate to ensure an output value above the dust limit, while still spending 20% on fees.
We add a new funding assembler and intent type that handle channel
funding through the use of a PSBT. The PsbtIntent is in itself a
simple state machine that can be stepped through the process of
assembling the required information for the funding output, verifying
a user supplied PSBT for correctness, accepting a fully signed PSBT
and then assembling the funding wire message.
As frozen channels can only be created via the non-default channel
assembler, we extend both the ShimIntent and CannedAssembler to also
accept and expose this new channel status along with the thaw height.
In this commit, we implement the currently defined transition methods
for the new `FundingStateStep` method. At this point, we're now able to
serve the "responder" of the externally initiated channel funding flow
by being able to register and cancel a funding flow according to its
expected pending channel ID.
In this commit, we introduce a series of new abstractions for channel
funding. The end goal is to enable uses cases that construct the funding
transaction externally, eventually handing the funding outpoint to lnd.
An example of such a use case includes channel factories and external
channel funding using a hardware wallet.
We also add a new chanfunding.Assembler meant to allow external channel
funding in contexts similar to how channel factories
can be constructed. With this channel funder, we'll only obtain the
channel point and funding output from it, as this alone is enough to
carry out a funding flow as normal.
In this commit, we make an incremental change to move the existing coin
selection code into a new chanfunding package. In later commits, this
package will grow to serve all the lower level channel funding needs in
the daemon.
