Fix absurd fee bug reported in #3281, with tests

qa/rpc-tests/wallet.py

@@ -463,6 +463,62 @@ class WalletTest (BitcoinTestFramework):
 
         assert_equal("not an integer" in errorString, True);
 
+        myzaddr     = self.nodes[0].z_getnewaddress()
+        recipients  = [ {"address": myzaddr, "amount": Decimal('0.0') } ]
+        errorString = ''
+
+        # Make sure that amount=0 transactions can use the default fee
+        # without triggering "absurd fee" errors
+        try:
+            myopid = self.nodes[0].z_sendmany(myzaddr, recipients)
+            assert(myopid)
+        except JSONRPCException,e:
+            errorString = e.error['message']
+            print errorString
+            assert(False)
+
+        # This fee is larger than the default fee and since amount=0
+        # it should trigger error
+        fee         = Decimal('0.1')
+        recipients  = [ {"address": myzaddr, "amount": Decimal('0.0') } ]
+        minconf     = 1
+        errorString = ''
+
+        try:
+            myopid = self.nodes[0].z_sendmany(myzaddr, recipients, minconf, fee)
+        except JSONRPCException,e:
+            errorString = e.error['message']
+            assert('Small transaction amount' in errorString)
+
+        #### This fee is less than default and greater than amount, but still valid
+        fee         = Decimal('0.0000001')
+        recipients  = [ {"address": myzaddr, "amount": Decimal('0.00000001') } ]
+        minconf     = 1
+        errorString = ''
+
+        try:
+            myopid = self.nodes[0].z_sendmany(myzaddr, recipients, minconf, fee)
+            assert(myopid)
+        except JSONRPCException,e:
+            errorString = e.error['message']
+            print errorString
+            assert(False)
+
+        ### Make sure amount=0, fee=0 transaction are valid to add to mempool
+        # though miners decide whether to add to a block
+        fee         = Decimal('0.0')
+        minconf     = 1
+        recipients  = [ {"address": myzaddr, "amount": Decimal('0.0') } ]
+        errorString = ''
+
+        try:
+            myopid = self.nodes[0].z_sendmany(myzaddr, recipients, minconf, fee)
+            assert(myopid)
+        except JSONRPCException,e:
+            errorString = e.error['message']
+            print errorString
+            assert(False)
+
 
 if __name__ == '__main__':
     WalletTest ().main ()

src/wallet/rpcwallet.cpp

@@ -3698,7 +3698,9 @@ UniValue z_sendmany(const UniValue& params, bool fHelp)
     }
 
     // Fee in Zatoshis, not currency format)
-    CAmount nFee = ASYNC_RPC_OPERATION_DEFAULT_MINERS_FEE;
+    CAmount nFee        = ASYNC_RPC_OPERATION_DEFAULT_MINERS_FEE;
+    CAmount nDefaultFee = nFee;
+
     if (params.size() > 3) {
         if (params[3].get_real() == 0.0) {
             nFee = 0;
@@ -3707,9 +3709,18 @@ UniValue z_sendmany(const UniValue& params, bool fHelp)
         }
 
         // 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
+        // or anything less than nDefaultFee 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)));
+            }
+	}
     }
 
     // Use input parameters as the optional context info to be returned by z_getoperationstatus and z_getoperationresult.