Closes #2639. Adds optional limit parameter with a default value of 50.

The new parameter is to satisfy the principle of least astonishment
by providing a sensible default for the maximum number of transparent
inputs to shield.  If users do not configure -mempooltxinputlimit
it is possible for them to create transactions with hundreds of
inputs which suffer from mining delay, due to the current state of
the network where some miners have configured -mempooltxinputlimit
as a way to deal with the problem of quadratic hashing.

qa/rpc-tests/wallet_shieldcoinbase.py

@@ -114,6 +114,20 @@ class WalletShieldCoinbaseTest (BitcoinTestFramework):
             errorString = e.error['message']
         assert_equal("Insufficient coinbase funds" in errorString, True)
 
+        # Shielding will fail because limit parameter must be at least 0
+        try:
+            self.nodes[0].z_shieldcoinbase("*", myzaddr, Decimal('0.001'), -1)
+        except JSONRPCException,e:
+            errorString = e.error['message']
+        assert_equal("Limit on maximum number of utxos cannot be negative" in errorString, True)
+
+        # Shielding will fail because limit parameter is absurdly large
+        try:
+            self.nodes[0].z_shieldcoinbase("*", myzaddr, Decimal('0.001'), 99999999999999)
+        except JSONRPCException,e:
+            errorString = e.error['message']
+        assert_equal("JSON integer out of range" in errorString, True)
+
         # Shield coinbase utxos from node 0 of value 40, standard fee of 0.00010000
         result = self.nodes[0].z_shieldcoinbase(mytaddr, myzaddr)
         self.wait_and_assert_operationid_status(0, result['opid'])
@@ -151,14 +165,15 @@ class WalletShieldCoinbaseTest (BitcoinTestFramework):
 
         # Shielding the 800 utxos will occur over two transactions, since max tx size is 100,000 bytes.
         # We don't verify shieldingValue as utxos are not selected in any specific order, so value can change on each test run.
-        result = self.nodes[0].z_shieldcoinbase(mytaddr, myzaddr, 0)
+        # We set an unrealistically high limit parameter of 99999, to verify that max tx size will constrain the number of utxos.
+        result = self.nodes[0].z_shieldcoinbase(mytaddr, myzaddr, 0, 99999)
         assert_equal(result["shieldingUTXOs"], Decimal('662'))
         assert_equal(result["remainingUTXOs"], Decimal('138'))
         remainingValue = result["remainingValue"]
         opid1 = result['opid']
 
         # Verify that utxos are locked (not available for selection) by queuing up another shielding operation
-        result = self.nodes[0].z_shieldcoinbase(mytaddr, myzaddr)
+        result = self.nodes[0].z_shieldcoinbase(mytaddr, myzaddr, 0, 0)
         assert_equal(result["shieldingValue"], Decimal(remainingValue))
         assert_equal(result["shieldingUTXOs"], Decimal('138'))
         assert_equal(result["remainingValue"], Decimal('0'))
@@ -176,8 +191,9 @@ class WalletShieldCoinbaseTest (BitcoinTestFramework):
         self.sync_all()
 
         # Verify maximum number of utxos which node 2 can shield is limited by option -mempooltxinputlimit
+        # This option is used when the limit parameter is set to 0.
         mytaddr = self.nodes[2].getnewaddress()
-        result = self.nodes[2].z_shieldcoinbase(mytaddr, myzaddr, 0)
+        result = self.nodes[2].z_shieldcoinbase(mytaddr, myzaddr, Decimal('0.0001'), 0)
         assert_equal(result["shieldingUTXOs"], Decimal('7'))
         assert_equal(result["remainingUTXOs"], Decimal('13'))
         self.wait_and_assert_operationid_status(2, result['opid'])
@@ -185,5 +201,23 @@ class WalletShieldCoinbaseTest (BitcoinTestFramework):
         self.nodes[1].generate(1)
         self.sync_all()
 
+        # Verify maximum number of utxos which node 0 can shield is set by default limit parameter of 50
+        self.nodes[0].generate(200)
+        self.sync_all()
+        mytaddr = self.nodes[0].getnewaddress()
+        result = self.nodes[0].z_shieldcoinbase(mytaddr, myzaddr, Decimal('0.0001'))
+        assert_equal(result["shieldingUTXOs"], Decimal('50'))
+        assert_equal(result["remainingUTXOs"], Decimal('50'))
+        self.wait_and_assert_operationid_status(0, result['opid'])
+
+        # Verify maximum number of utxos which node 0 can shield can be set by the limit parameter
+        result = self.nodes[0].z_shieldcoinbase(mytaddr, myzaddr, Decimal('0.0001'), 33)
+        assert_equal(result["shieldingUTXOs"], Decimal('33'))
+        assert_equal(result["remainingUTXOs"], Decimal('17'))
+        self.wait_and_assert_operationid_status(0, result['opid'])
+        sync_blocks(self.nodes)  # don't sync on mempool as node 2 will reject thix tx due to its mempooltxinputlimit
+        self.nodes[1].generate(1)
+        self.sync_all()
+
 if __name__ == '__main__':
     WalletShieldCoinbaseTest().main()