Add returnType when call other contract

[Tommo-L]

Deason

For security reason, it's not safe to call other contract without restrict on the return stack.

bool verify(int a, int b)
{
    CallSomeContract();
    return a > b;
}

//script of CallSomeContract()
PUSH 1 // this hide the parameter b of verify()
PUSH 2 // this hide the parameter a of verify()
RET

We expect CallSomeContract() to return no value. But if it maliciously returns 2 values, then these values will be automatically copied onto our stack, causing us to return wrong results. Original posted at neo-project/neo#1680 (comment)

Solution

Maybe we can use the following format:

public static T Contract.Call<T extends StackItem>(B, method, args)

---------------------------- update 11 June --------------------------------
Using the following format:

[Contract(Hash = "0x0011223344556677889900112233445566778899")]
class AContract
{
    public static extern int Funca();
}

[Contract(Hash = "0x0011223344556677889900112233445566778890")]
class BContract
{
    public static extern int Funcb();
}

class MyContract : SmartContract
{
    public static void MyMethod()
    {
        int a = A.Funca()
        int b = B.Funcb();
        a++;
    }
}

original discussed at neo-project/neo#1680 (comment)

[erikzhang]

AppcallAttribute should be replaced.

[Tommo-L]

The parameters of System.Contract.Call/CallEx have been updated.

System.Contract.Call(UInt160 contractHash, ContractParameterType returnType, string method, Array args)
System.Contract.CallEx(UInt160 contractHash, ContractParameterType returnType, string method, Array args, CallFlags callFlags)

[ShawnYun]

According to neo-project/neo#1706 . We need to modify the plan.