Auto merge of #3592 - Eirik0:z-sendmany-better-error, r=bitcartel

Better error message when sending to both sprout and sapling

When trying to send to both Sprout and Sapling (not currently supported with z_sendmany) we were getting the following error in our operation result: `general exception: boost::bad_get: failed value get using boost::get`.

This PR changes this to fail with a better error message before the async operation begins:
```
error code: -8
error message:
Cannot send to both Sprout and Sapling addresses using z_sendmany
```

qa/rpc-tests/wallet_sapling.py

@@ -4,6 +4,7 @@
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 from test_framework.test_framework import BitcoinTestFramework
+from test_framework.authproxy import JSONRPCException
 from test_framework.util import (
     assert_equal,
     start_nodes,
@@ -144,5 +145,18 @@ class WalletSaplingTest(BitcoinTestFramework):
         self.nodes[2].z_importkey(sk1, "yes")
         assert_equal(self.nodes[2].z_getbalance(saplingAddr1), Decimal('5'))
 
+        # Make sure we get a useful error when trying to send to both sprout and sapling
+        node4_sproutaddr = self.nodes[3].z_getnewaddress('sprout')
+        node4_saplingaddr = self.nodes[3].z_getnewaddress('sapling')
+        try:
+            self.nodes[1].z_sendmany(
+                taddr1,
+                [{'address': node4_sproutaddr, 'amount': 2.5}, {'address': node4_saplingaddr, 'amount': 2.4999}],
+                1, 0.0001
+            )
+            raise AssertionError("Should have thrown an exception")
+        except JSONRPCException as e:
+            assert_equal("Cannot send to both Sprout and Sapling addresses using z_sendmany", e.error['message'])
+
 if __name__ == '__main__':
     WalletSaplingTest().main()

src/wallet/rpcwallet.cpp

@@ -3687,6 +3687,9 @@ UniValue z_sendmany(const UniValue& params, bool fHelp)
     std::vector<SendManyRecipient> zaddrRecipients;
     CAmount nTotalOut = 0;
 
+    bool containsSproutOutput = false;
+    bool containsSaplingOutput = false;
+
     for (const UniValue& o : outputs.getValues()) {
         if (!o.isObject())
             throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, expected object");
@@ -3710,6 +3713,16 @@ UniValue z_sendmany(const UniValue& params, bool fHelp)
                 bool toSprout = !toSapling;
                 noSproutAddrs = noSproutAddrs && toSapling;
 
+                containsSproutOutput |= toSprout;
+                containsSaplingOutput |= toSapling;
+
+                // Sending to both Sprout and Sapling is currently unsupported using z_sendmany
+                if (containsSproutOutput && containsSaplingOutput) {
+                    throw JSONRPCError(
+                        RPC_INVALID_PARAMETER,
+                        "Cannot send to both Sprout and Sapling addresses using z_sendmany");
+                }
+
                 // If we are sending from a shielded address, all recipient
                 // shielded addresses must be of the same type.
                 if (fromSprout && toSapling) {