One contract accessing data from another fails

Question

I am trying to get two contracts to interact:

pragma solidity ^0.5.10;


contract C2 {

    address public addr;

    constructor(address per) public {
        addr = per;
    }

    function remote_get_number() external view returns(uint) {
        return C1(addr).get_a_number();
    }

    function remote_set_number(uint num) external {
        C1(addr).set_a_number(num);
    }
}

contract C1 {

    uint public a_number;

    function get_a_number() external view returns(uint) {
        return(a_number);
    }

    function set_a_number(uint num) external  {
        a_number = num;
    }
}

I create an instance of C1, then create a C2 instance using the address of the C1 instance as the argument of the C2 constructor.

If I call, say remote_set_number(42), it works correctly; calling get_a_number() returns 42. Likewise for any uint value. However, when I call remote_get_number(), event though all that does is return the result of a C1(addr).remote_get_number() call, the value returned is always 0.

How do I get a call of a function in one contract to correctly return a value when it is called by a function in another? It seems to me that this follows the patterns given in the Solidity documentation, so I have no idea why it's not working. Can anyone shed a little light?

(This was done using Injected Web3 on a private blockchain in Remix)

UPDATE:

I tried updating to the latest stable geth (1.9.3), and now I get the following error for any interaction between contracts: Gas estimation errored with the following message (see below). The transaction execution will likely fail. Do you want to force sending? Internal JSON-RPC error.

I have tried it with two different blockchains, with different genesis blocks. #1:

{
    "config": {
        "chainId":123 ,
        "homesteadBlock": 0,
        "eip155Block": 0,
        "eip158Block": 0
    },

    "nonce": "0x0000000000000042",     "timestamp": "0x0",
    "parentHash": "0x0000000000000000000000000000000000000000000000000000000000000000",
    "extraData": "0x00",     "gasLimit": "0x8000000",     "difficulty": "0x400",
    "mixhash": "0x0000000000000000000000000000000000000000000000000000000000000000",
    "coinbase": "0xDB725f67f09228Ea1075f36a0fDb568B9aeBd54C",
    "alloc":{
      "DB725f67f09228Ea1075f36a0fDb568B9aeBd54C":{
        "balance":"6660000080800000000777"
      }
  }
}

and #2:

{
    "config": {
        "chainId":123 ,
        "homesteadBlock": 0,
        "byzantiumBlock": 0,
        "constantinopleBlock": 0,
        "petersburgBlock": 0,
        "eip155Block": 0,
        "eip158Block": 0
    },

    "nonce": "0x0000000000000042",     "timestamp": "0x0",
    "parentHash": "0x0000000000000000000000000000000000000000000000000000000000000000",
    "extraData": "0x00",     "gasLimit": "0x8000000",     "difficulty": "0x80",
    "mixhash": "0x0000000000000000000000000000000000000000000000000000000000000000",
    "coinbase": "0xB10Ec08f54343133875f0244e2A435C19789ec28",
    "alloc":{
      "B10Ec08f54343133875f0244e2A435C19789ec28":{
        "balance":"6660010005550000100777"
      }
  }
}

I have tried using a different versions of solidity - but I've had the same results with Solidity 0.4.1, 0.4.4, 0.5.1, and 0.5.11.

Answer 1

The contract looks fine and I confirmed with a little superficial fiddling in Remix using the JavaScript VM.

A common stumbling block when transitioning to "real" chains is the need to wait for transactions to confirm. If you are setting and checking without someone first mining a block that contains the set transaction, then you will read the old value (because it is still current).

When you send a transaction, the receipt you get is not confirmation that the transaction has executed successfully - only confirmation that your node knows you want it sent. Big difference.

Try slowing everything down and make sure something is mining.

Hope it helps.