Backend/Bitcoin: enabled RBF in all outgoing transactions

It's arguable if this will be enabled by default on
upstream NBitcoin (my PR is being debated[1]) so I'll
enable it from GWallet in the meantime.

[1] MetacoSA/NBitcoin#355

Author: knocte

ReadMe.md

@@ -7,7 +7,7 @@ GWallet is a minimalistic and pragmatist lightweight wallet for people that want
 | Branch   | Description                                                              | CI status                                                                                                                               |
 | -------- | ------------------------------------------------------------------------ | --------------------------------------------------------------------------------------------------------------------------------------  |
 | stable   | ETC & ETH support, console-based frontend, cold-storage support          | [![Build status badge](http://gitlab.com/knocte/gwallet/badges/stable/build.svg)](https://gitlab.com/knocte/gwallet/commits/stable)     |
-| master   | +BTC support (SegWit by default); soon: LTC                              | [![Build status badge](http://gitlab.com/knocte/gwallet/badges/master/build.svg)](https://gitlab.com/knocte/gwallet/commits/master)     |
+| master   | +BTC support (SegWit & RBF support); soon: LTC                           | [![Build status badge](http://gitlab.com/knocte/gwallet/badges/master/build.svg)](https://gitlab.com/knocte/gwallet/commits/master)     |
 | frontend | +Xamarin.Forms frontends in progress (now: Android, iOS; soon: gtk, UWP) | [![Build status badge](http://gitlab.com/knocte/gwallet/badges/frontend/build.svg)](https://gitlab.com/knocte/gwallet/commits/frontend) |
 
 
@@ -25,7 +25,6 @@ GWallet will always be standing in the shoulders of giants, which means we have
 This list is the (intended) order of preference for new features:
 
 - Xamarin.Forms frontends (in progress, see the 'frontend' branch)...
-- RBF support.
 - LTC support.
 - Fee selection for custom priority.
 - Multi-sig support.

src/GWallet.Backend/Bitcoin/BitcoinAccount.fs

@@ -94,7 +94,13 @@ module internal Account =
             seq {
                 for input in transactionDraft.Inputs do
                     let inputTx = Transaction(input.RawTransaction)
-                    transaction.AddInput(inputTx, input.OutputIndex) |> ignore
+                    let inputAdded = transaction.AddInput(inputTx, input.OutputIndex)
+
+                    // mark RBF=enabled by default
+                    inputAdded.Sequence <- Sequence(0)
+                    if not inputAdded.Sequence.IsRBF then
+                        failwith "input should have been marked as RBF by default"
+
                     yield Coin(inputTx, uint32 input.OutputIndex)
             } |> List.ofSeq
 
@@ -103,6 +109,12 @@ module internal Account =
             let txOut = TxOut(Money(output.ValueInSatoshis), destAddress)
             transaction.Outputs.Add(txOut)
 
+        if not transaction.RBF then
+            failwith "transaction should have been marked as RBF by default"
+        if transaction.LockTime.IsTimeLock then
+            failwith "transaction shouldn't be marked as time lock"
+        if transaction.LockTime.Height <> 0 then
+            failwith "transaction height shouldn't be different than 0"
         transaction,coins
 
     type internal UnspentTransactionOutputInfo =