wallet rbf: add new in/out if necessary

lib/primitives/mtx.js

@@ -1227,11 +1227,25 @@ class MTX extends TX {
   async fund(coins, options) {
     assert(options, 'Options are required.');
     assert(options.changeAddress, 'Change address is required.');
-    assert(this.inputs.length === 0, 'TX is already funded.');
+    if (!options.bumpFee)
+      assert(this.inputs.length === 0, 'TX is already funded.');
 
     // Select necessary coins.
     const select = await this.selectCoins(coins, options);
 
+    // Bump Fee mode:
+    // We want the coin selector to ignore the values of
+    // all existing inputs and outputs, but still consider their size.
+    // Inside the coin selector this is done by making a copy of the TX
+    // and then setting the total output value to 0
+    // (input value is already ignored).
+    // The coin selector will add coins to cover the fee on the entire TX
+    // including paying for any inputs it adds.
+    // Now that coin selection is done, we add back in the fee paid by
+    // the original existing inputs and outputs so we can set the change value.
+    if (options.bumpFee)
+      select.fee += this.getFee();
+
     // Add coins to transaction.
     for (const coin of select.chosen)
       this.addCoin(coin);

lib/wallet/coinselector.js

@@ -71,6 +71,7 @@ class CoinSelector {
     this.changeAddress = null;
     this.inputs = new BufferMap();
     this.useSelectEstimate = false;
+    this.bumpFee = false;
 
     // Needed for size estimation.
     this.getAccount = null;
@@ -158,6 +159,11 @@ class CoinSelector {
       this.useSelectEstimate = options.useSelectEstimate;
     }
 
+    if (options.bumpFee != null) {
+      assert(typeof options.bumpFee === 'boolean');
+      this.bumpFee = options.bumpFee;
+    }
+
     if (options.changeAddress) {
       const addr = options.changeAddress;
       if (typeof addr === 'string') {
@@ -209,7 +215,7 @@ class CoinSelector {
 
   async init(coins) {
     this.coins = coins.slice();
-    this.outputValue = this.tx.getOutputValue();
+    this.outputValue = this.bumpFee ? 0 : this.tx.getOutputValue();
     this.chosen = [];
     this.change = 0;
     this.fee = CoinSelector.MIN_FEE;

lib/wallet/wallet.js

@@ -1143,7 +1143,8 @@ class Wallet extends EventEmitter {
       rate: rate,
       maxFee: options.maxFee,
       useSelectEstimate: options.useSelectEstimate,
-      getAccount: this.getAccountByAddress.bind(this)
+      getAccount: this.getAccountByAddress.bind(this),
+      bumpFee: options.bumpFee
     });
 
     assert(mtx.getFee() <= CoinSelector.MAX_FEE, 'TX exceeds MAX_FEE.');
@@ -1376,27 +1377,42 @@ class Wallet extends EventEmitter {
       }
     }
 
-    if (!change)
-      throw new Error('Transaction has no change output.');
-
-    // Start by reducing absolute fee to 0
-    change.value += oldFee;
-    if (mtx.getFee() !== 0)
-      throw new Error('Arithmetic error for change.');
-
-    // Pay for replacement fees: BIP 125 rule #3
-    change.value -= oldFee;
-
-    // Pay for our own current fee: BIP 125 rule #4
-    change.value -= currentFee;
-
-    if (change.value < 0)
-      throw new Error('Change output insufficient for fee.');
-
-    // If change output is below dust, give it all to fee
-    if (change.isDust()) {
-      mtx.outputs.splice(mtx.changeIndex, 1);
-      mtx.changeIndex = -1;
+    if (change) {
+      // Start by reducing absolute fee to 0
+      change.value += oldFee;
+      if (mtx.getFee() !== 0)
+        throw new Error('Arithmetic error for change.');
+
+      // Pay for replacement fees: BIP 125 rule #3
+      change.value -= oldFee;
+
+      // Pay for our own current fee: BIP 125 rule #4
+      change.value -= currentFee;
+
+      // We need to add more inputs.
+      // This will obviously also fail the dust test next
+      // and conveniently remove the change output for us.
+      if (change.value < 0)
+        throw new Error('Change value insufficient to bump fee.');
+
+      // If change output is below dust,
+      // give it all to fee (remove change output)
+      if (change.isDust()) {
+        mtx.outputs.splice(mtx.changeIndex, 1);
+        mtx.changeIndex = -1;
+      }
+    } else {
+      // We need to add more inputs (and maybe a change output) to increase the
+      // fee. Since the original inputs and outputs already paid for
+      // their own fee (rule #3) all we have to do is pay for this
+      // new TX's fee (rule #4).
+      await this.fund(
+        mtx,
+        {
+          bumpFee: true,  // set bump fee mode to ignore existing output values
+          rate,           // rate in s/kvB for the rule 4 fee
+          depth: 1        // replacements can not add new unconfirmed coins
+        });
     }
 
     if (!sign)
@@ -1420,7 +1436,7 @@ class Wallet extends EventEmitter {
       await this.wdb.send(ntx);
     }
 
-    return ntx;
+    return mtx;
   }
 
   /**

test/wallet-rbf-test.js

@@ -186,4 +186,35 @@ describe('Wallet RBF', function () {
     });
     await node.rpc.generateToAddress([1, aliceReceive]);
   });
+
+  it('should bump a tx with no change by adding new in/out pair', async () => {
+    const coins = await alice.getCoins();
+    let coin;
+    for (coin of coins) {
+      if (!coin.coinbase)
+        break;
+    }
+    const mtx = new MTX();
+    mtx.addCoin(coin);
+    mtx.addOutput(bobReceive, coin.value - 200);
+    mtx.inputs[0].sequence = 0xfffffffd;
+    await alice.sign(mtx);
+    const tx = mtx.toTX();
+    assert.strictEqual(tx.inputs.length, 1);
+    assert.strictEqual(tx.outputs.length, 1);
+    await alice.wdb.addTX(tx);
+    await alice.wdb.send(tx);
+    await forEvent(node.mempool, 'tx');
+
+    const rtx = await alice.bumpTXFee(tx.hash(), 2000 /* satoshis per kvB */, true, null);
+    assert.strictEqual(rtx.inputs.length, 2);
+    assert.strictEqual(rtx.outputs.length, 2);
+    assert(rtx.getRate() >= 2000 && rtx.getRate() < 3000);
+
+    await forEvent(node.mempool, 'tx');
+    assert(!node.mempool.hasEntry(tx.hash()));
+    assert(node.mempool.hasEntry(rtx.hash()));
+
+    await node.rpc.generateToAddress([1, aliceReceive]);
+  });
 });