WIP Fix to absurd fee bug on small transaction amounts

We inherited this bug from upstream, and most Zcash forks
probably have similar behavior.

Upstream issue:

https://github.com/zcash/zcash/issues/3281

src/wallet/rpcwallet.cpp

@@ -3634,8 +3634,10 @@ UniValue z_sendmany(const UniValue& params, bool fHelp)
         throw JSONRPCError(RPC_INVALID_PARAMETER, "Minimum number of confirmations cannot be less than 0");
     }
 
-    // Fee in Zatoshis, not currency format)
-    CAmount nFee = ASYNC_RPC_OPERATION_DEFAULT_MINERS_FEE;
+    // Fee in Puposhis, not currency format
+    CAmount nFee        = ASYNC_RPC_OPERATION_DEFAULT_MINERS_FEE;
+    CAmount nDefaultFee = nFee;
+
     if (params.size() > 3) {
         if (params[3].get_real() == 0.0) {
             nFee = 0;
@@ -3643,9 +3645,17 @@ UniValue z_sendmany(const UniValue& params, bool fHelp)
             nFee = AmountFromValue( params[3] );
         }
 
-        // Check that the user specified fee is sane.
-        if (nFee > nTotalOut) {
-            throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Fee %s is greater than the sum of outputs %s", FormatMoney(nFee), FormatMoney(nTotalOut)));
+        // This allows amount=0 (and all amount < nDefaultFee) transactions to use the default network fee
+        // instead of being forced to use a custom fee and leak metadata
+        if (nTotalOut < nDefaultFee) {
+            if (nFee > nDefaultFee) {
+                throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Small transaction amount %s has fee %s that is greater than the default fee %s", FormatMoney(nTotalOut), FormatMoney(nFee), FormatMoney(nDefaultFee)));
+            }
+        } else {
+            // Check that the user specified fee is sane.
+            if (nFee > nTotalOut) {
+                throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Fee %s is greater than the sum of outputs %s", FormatMoney(nFee), FormatMoney(nTotalOut)));
+            }
         }
     }