Merge bitcoin/bitcoin#27195: bumpfee: allow send coins back to yourself

be72663a1521bc6cdf16d43a4feae7c5b57735c0 test: bumpfee, add coverage for "send coins back to yourself" (furszy)
7bffec6715a75bb2c8631177d39f984aabc656ba bumpfee: enable send coins back to yourself (furszy)

Pull request description:

  Simple example:

  1) User_1 sends 0.1 btc to user_2 on a low fee transaction.
  2) After few hours, the tx is still in the mempool, user_2
     is not interested anymore, so user_1 decides to cancel
     it by sending coins back to himself.
  3) User_1 has the bright idea of opening the explorer and
     copy the change output address of the transaction. Then
     call bumpfee providing such output (in the "outputs" arg).

  Currently, this is not possible. The wallet fails with
  "Unable to create transaction. Transaction must have at least
  one recipient" error.
  The error reason is because we discard the provided output
  from the recipients list and set it inside the coin control
  so the process adds it later (when the change is calculated).
  But.. there is no later if the tx has no outputs.

ACKs for top commit:
  ishaanam:
    reACK be72663a1521bc6cdf16d43a4feae7c5b57735c0
  achow101:
    ACK be72663a1521bc6cdf16d43a4feae7c5b57735c0

Tree-SHA512: c2c38290a998f9b426a830d9624c7feb730158980ac186f8fb0138d5e200935d6538307bc60a2c3d0b7b6ee2b4ffb77a1e98baf8feb1d20a7d825f6055ac377f

1 files changed, 16 insertions, 0 deletions

diff --git a/src/wallet/feebumper.cpp b/src/wallet/feebumper.cpp
index d127c41c43..b6b1fa1d3e 100644
--- a/src/wallet/feebumper.cpp
+++ b/src/wallet/feebumper.cpp
@@ -231,6 +231,7 @@ Result CreateRateBumpTransaction(CWallet& wallet, const uint256& txid, const CCo
     // is one). If outputs vector is non-empty, replace original
     // outputs with its contents, otherwise use original outputs.
     std::vector<CRecipient> recipients;
+    CAmount new_outputs_value = 0;
     const auto& txouts = outputs.empty() ? wtx.tx->vout : outputs;
     for (const auto& output : txouts) {
         if (!OutputIsChange(wallet, output)) {
@@ -241,6 +242,21 @@ Result CreateRateBumpTransaction(CWallet& wallet, const uint256& txid, const CCo
             ExtractDestination(output.scriptPubKey, change_dest);
             new_coin_control.destChange = change_dest;
         }
+        new_outputs_value += output.nValue;
+    }
+
+    // If no recipients, means that we are sending coins to a change address
+    if (recipients.empty()) {
+        // Just as a sanity check, ensure that the change address exist
+        if (std::get_if<CNoDestination>(&new_coin_control.destChange)) {
+            errors.emplace_back(Untranslated("Unable to create transaction. Transaction must have at least one recipient"));
+            return Result::INVALID_PARAMETER;
+        }
+
+        // Add change as recipient with SFFO flag enabled, so fees are deduced from it.
+        // If the output differs from the original tx output (because the user customized it) a new change output will be created.
+        recipients.emplace_back(CRecipient{GetScriptForDestination(new_coin_control.destChange), new_outputs_value, /*fSubtractFeeFromAmount=*/true});
+        new_coin_control.destChange = CNoDestination();
     }
 
     if (coin_control.m_feerate) {