wallet: feature: transfer amount with fee included

To transfer ~5 XMR to an address such that your balance drops by exactly 5 XMR, provide a `subtractfeefrom` flag to the `transfer` command. For example:

    transfer 76bDHojqFYiFCCYYtzTveJ8oFtmpNp3X1TgV2oKP7rHmZyFK1RvyE4r8vsJzf7SyNohMnbKT9wbcD3XUTgsZLX8LU5JBCfm 5 subtractfeefrom=all

If my walet balance was exactly 30 XMR before this transaction, it will be exactly 25 XMR afterwards and the destination address will receive slightly
less than 5 XMR. You can manually select which destinations fund the transaction fee and which ones do not by providing the destination index.
For example:

    transfer 75sr8AAr... 3 74M7W4eg... 4 7AbWqDZ6... 5 subtractfeefrom=0,2

This will drop your balance by exactly 12 XMR including fees and will spread the fee cost proportionally (3:5 ratio) over destinations with addresses
`75sr8AAr...` and `7AbWqDZ6...`, respectively.

Disclaimer: This feature was paid for by @LocalMonero.

1 files changed, 10 insertions, 0 deletions

diff --git a/src/wallet/wallet_errors.h b/src/wallet/wallet_errors.h
index fcf2ddd93..1f7e1c75d 100644
--- a/src/wallet/wallet_errors.h
+++ b/src/wallet/wallet_errors.h
@@ -85,6 +85,7 @@ namespace tools
     //         tx_too_big
     //         zero_amount
     //         zero_destination
+    //         subtract_fee_from_bad_index
     //       wallet_rpc_error *
     //         daemon_busy
     //         no_connection_to_daemon
@@ -779,6 +780,15 @@ namespace tools
       }
     };
     //----------------------------------------------------------------------------------------------------
+    struct subtract_fee_from_bad_index : public transfer_error
+    {
+      explicit subtract_fee_from_bad_index(std::string&& loc, long bad_index)
+        : transfer_error(std::move(loc),
+          "subtractfeefrom: bad index: " + std::to_string(bad_index) + " (indexes are 0-based)")
+      {
+      }
+    };
+    //----------------------------------------------------------------------------------------------------
     struct wallet_rpc_error : public wallet_logic_error
     {
       const std::string& request() const { return m_request; }