Closes zcash#1854. z_sendmany selects more utxos to avoid dust change…

… output.

qa/rpc-tests/wallet_protectcoinbase.py

@@ -127,6 +127,15 @@ def run_test (self):
         assert_equal(Decimal(resp["private"]), Decimal('9.9998'))
         assert_equal(Decimal(resp["total"]), Decimal('39.9998'))
 
+        # z_sendmany will return an error if there is transparent change output considered dust.
+        # UTXO selection in z_sendmany sorts in ascending order, so smallest utxos are consumed first.
+        # At this point in time, unspent notes all have a value of 10.0 and standard z_sendmany fee is 0.0001.
+        recipients = []
+        amount = Decimal('10.0') - Decimal('0.00010000') - Decimal('0.00000001')    # this leaves change at 1 zatoshi less than dust threshold
+        recipients.append({"address":self.nodes[0].getnewaddress(), "amount":amount })
+        myopid = self.nodes[0].z_sendmany(mytaddr, recipients)
+        self.wait_and_assert_operationid_status(myopid, "failed", "Insufficient transparent funds, have 10.00, need 0.00000545 more to avoid creating invalid change output 0.00000001 (dust threshold is 0.00000546)")
+
         # Send will fail because send amount is too big, even when including coinbase utxos
         errorString = ""
         try:

src/wallet/asyncrpcoperation_sendmany.cpp

@@ -215,6 +215,14 @@ bool AsyncRPCOperation_sendmany::main_impl() {
     CAmount selectedUTXOAmount = 0;
     bool selectedUTXOCoinbase = false;
     if (isfromtaddr_) {
+        // Get dust threshold
+        CKey secret;
+        secret.MakeNewKey(true);
+        CScript scriptPubKey = GetScriptForDestination(secret.GetPubKey().GetID());
+        CTxOut out(CAmount(1), scriptPubKey);
+        CAmount dustThreshold = out.GetDustThreshold(minRelayTxFee);
+        CAmount dustChange = -1;
+
         std::vector<SendManyInputUTXO> selectedTInputs;
         for (SendManyInputUTXO & t : t_inputs_) {
             bool b = std::get<3>(t);
@@ -224,9 +232,21 @@ bool AsyncRPCOperation_sendmany::main_impl() {
             selectedUTXOAmount += std::get<2>(t);
             selectedTInputs.push_back(t);
             if (selectedUTXOAmount >= targetAmount) {
-                break;
+                // Select another utxo if there is change less than the dust threshold.
+                dustChange = selectedUTXOAmount - targetAmount;
+                if (dustChange == 0 || dustChange >= dustThreshold) {
+                    break;
+                }
             }
         }
+
+        // If there is transparent change, is it valid or is it dust?
+        if (dustChange < dustThreshold && dustChange != 0) {
+            throw JSONRPCError(RPC_WALLET_INSUFFICIENT_FUNDS,
+                strprintf("Insufficient transparent funds, have %s, need %s more to avoid creating invalid change output %s (dust threshold is %s)",
+                FormatMoney(t_inputs_total), FormatMoney(dustThreshold - dustChange), FormatMoney(dustChange), FormatMoney(dustThreshold)));
+        }
+
         t_inputs_ = selectedTInputs;
         t_inputs_total = selectedUTXOAmount;
 
@@ -772,6 +792,11 @@ bool AsyncRPCOperation_sendmany::find_utxos(bool fAcceptCoinbase=false) {
         t_inputs_.push_back(utxo);
     }
 
+    // sort in ascending order, so smaller utxos appear first
+    std::sort(t_inputs_.begin(), t_inputs_.end(), [](SendManyInputUTXO i, SendManyInputUTXO j) -> bool {
+        return ( std::get<2>(i) < std::get<2>(j));
+    });
+
     return t_inputs_.size() > 0;
 }