rpc: Audit contract reader returns error from List when contract has no items

[cthulhu-rider]

im exploring Audit contract in public NeoFS networks. I called list method of the Audit contract to analyze all stored results. As it turned out, there are no results in the contract, but I found out about this in an unexpected way

Current Behavior

Error:      	Received unexpected error:
        	            	not an array

Expected Behavior

no error and number of results: 0

Possible Solution

i dug down to the result stack: there is 1 element of type Null. The error is originated by https://pkg.go.dev/github.com/nspcc-dev/[email protected]/pkg/rpcclient/unwrap#Array

i see next alternatives:

1. make unwrap.Array to return nil, nil
2. make unwrap.Array to return ErrNull
3. add new function ArrayOrNull
4. do manual stack surgery

being a user, i like 1 the most, but this is a change that could break some code which also treats null as an error. 3 will prevent this, but there is a whole family of ArrayOf* functions, and generated code . 4 is the worst to me - unwrap package and generated RPC callers were conceived to avoid this

another challenge is to teach codegen what to do - fail or not on null

Steps to Reproduce

func TestAuditResults(t *testing.T) {
	h, err := util.Uint160DecodeStringLE("85fe181f4aa3cbdc94023d97c69001ece0730398")
	require.NoError(t, err)
	c, err := rpcclient.New(context.Background(), "https://rpc1.morph.fs.neo.org:40341", rpcclient.Options{})
	require.NoError(t, err)
	require.NoError(t, c.Init())

	list, err := auditcontract.NewReader(invoker.New(c, nil), h).List()
	require.NoError(t, err)
	fmt.Println("number of results:", len(list))
}

Context

there are some other contracts and/or methods returning arrays, so same issue is likely related to them

Regression

no

Your Environment

• github.com/nspcc-dev/[email protected]
• github.com/nspcc-dev/[email protected] (latest)

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P.S.

i'd add context to the error, who is not an array, of course you can guess, but you can also doubt it

[roman-khimov]

See nspcc-dev/neo-go#2795.