Implement EstimateRouteFee

[huumn]

It doesn't appear to be implemented. I'm pretty sure we can do this by probing using the library, but this would be nice to have.

The request/response are pretty simple so I imagine wrapping it is less of a lift than other things.

lightning/grpc/protos/router.proto

Lines 56 to 60 in c754858

	/*
	EstimateRouteFee allows callers to obtain a lower bound w.r.t how much it
	may cost to send an HTLC to the target end destination.
	*/
	rpc EstimateRouteFee (RouteFeeRequest) returns (RouteFeeResponse);

[alexbosworth]

Yes there is probeForRoute

[huumn]

If anyone wants a shim for this, something like this should work (untested):

async function estimateRouteFee ({ lnd, dest, amt_sat, request, timeout }) {
  return await new Promise((resolve, reject) => {
    lnd.router.estimateRouteFee({ dest: Buffer.from(dest, 'hex'), amt_sat, payment_request: request, timeout }, (err, res) => {
      if (err) {
        reject(err)
      } else {
        if (res.failure_reason) {
          reject(new Error(`Unable to estimate route: ${res.failure_reason}`))
        }

        resolve({
          routingFeeMsat: res.routing_fee_msat,
          timeLockDelay: res.time_lock_delay
        })
      }
    })
  })
}

[bnonni]

Would love to help out on this issue if it hasn't already been completed. Any guidance in addition to the above commentary is welcomed.

[alexbosworth]

Sounds good, I would avoid the destination based arguments and stick to only the payment request argument

For the response i would interpret the error instead of returning the error in the success. Potentially just return the callback with an error if the PaymentFailureReason is not reason none

[bnonni]

Thank you 🙏🏼 will follow up with any further questions etc. as I move along