Only encode non nil block numbers

[ferglor]

AUTO-11111

The node upgrade test for automation 2.0 was panicking due to a nil block number; this PR modifies buildCallOpts to allow for block 0 to be specified, in order to prevent a nil pointer.

Node upgrade test passing here: https://github.com/smartcontractkit/chainlink/actions/runs/9746647010

[infiloop2]

should this change be done within buildCallOpts so that if other services are using the function they don't get a nil pointer?

maybe just the if condition in line 469 can be removed

[ferglor]

Maybe! I wanted to see what happens with just this change so far; buildCallOpts has been updated to initialise with whatever block we pass rather than directly with nil, and I wanted to limit the change for now so that we pass a non-nil value here.

[ferglor]

I'm working off the assumption that specifying block 0 actually means the latest block is used - is that correct?

[ferglor]

2.0 upgrade passes: https://github.com/smartcontractkit/chainlink/actions/runs/9744742196/job/26891191178

[ferglor]

Updated PR still passing with 2.0 upgrade: https://github.com/smartcontractkit/chainlink/actions/runs/9746647010

[infiloop2]

did we check what eth calls do when 0 is passed as block number?

[infiloop2]

actually i see this function can also return an error. Maybe it makes more sense to return an error instead of setting it to 0?

[ferglor]

In the CallOpts struct, BlockNumber is documented as optional - should we be erroring when trying to set an optional field? Maybe as an alternative, we shouldn't change this function, and only set the optional BlockNumber arg on eth_call when opts.BlockNumber is non-nil?

[infiloop2]

oh interesting. BlockNumber is documented as optional . Does nil not qualify as optional? or is it different than not setting this field at all, something like

	opts := bind.CallOpts{
		Context:     ctx,
	}

[ferglor]

So the problem is that we're passing a nil value to hexutil.EncodeBig, i.e. hexutil.EncodeBig(opts.BlockNumber). So I was thinking of changing this:

checkReqs[i] = rpc.BatchElem{
	Method: "eth_call",
	Args: []interface{}{
		map[string]interface{}{
			"to":   r.addr.Hex(),
			"data": hexutil.Bytes(payload),
		},
		hexutil.EncodeBig(opts.BlockNumber),
	},
	Result: &result,
}

To something like this:

args := []interface{}{
  map[string]interface{}{
	  "to":   r.addr.Hex(),
	  "data": hexutil.Bytes(payload),
  }}

if opts.BlockNumber != nil {
    args = append(args, hexutil.EncodeBig(opts.BlockNumber)
}

checkReqs[i] = rpc.BatchElem{
	Method: "eth_call",
	Args: args,
	Result: &result,
}

[infiloop2]

yeah i think that makes sense. We'll need to do it in all places where args are made?