Use special strings for methodSig to handle contract call by pre-compiled ABI data

[songge-cb]

Motivation

This PR is to add some logic to the preprocess API to handle fallback pattern contract call. This type of contract call does not have a method signature.

Solution

We will use either empty string or a unique string "NO-METHOD-SIG" to indicate it. The pre-compiled ABI data will be passed in as methodArgs, either in the string format or the first element of an array.

Testing

• Tested Preprocess API locally by passing empty string and "NO-METHOD-SIG" as methodSig.

Open questions

[xiaying-peng]

do we need to handle this case?

[songge-cb]

For this specific project, no. Doing it for generalization.

[xiaying-peng]

Will calling fallback ever have more than 1 args?

[songge-cb]

As far as I know, no. The arg is the pre-compiled data, which is one piece.

[xiaying-peng]

Let's simplify your impl like this:

func preprocessArgs(methodSig string, methodArgs interface{}) (interface{}, error) {
	if methodSig == "" || methodSig == NoMethodSig {
		switch args := methodArgs.(type) {
		case []interface{}:
			if len(args) == 1 {
				if argStr, ok := args[0].(string); ok {
					return argStr, nil
				}
				return nil, fmt.Errorf("failed to convert method arg \"%T\" to string", args[0])
			}
		case []string:
			if len(args) == 1 {
				return args[0], nil
			}
		}
	}

	return methodArgs, nil
}

[songge-cb]

Updated the PR.

[cb-heimdall]

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