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

qa/rpc-tests/wallet_protectcoinbase.py

@@ -127,6 +127,15 @@ class WalletProtectCoinbaseTest (BitcoinTestFramework):
         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: