Fix error in getLogs method

[danielattilasimon]

When listening on an event filter with a null parameter followed by
a non-null parameter, it would keep throwing the error:

{ code: -32600, message: "data types must start with 0x" }

Turns out params.topics were being encoded as e.g.:

  "topics": [
    ["0x000000000000000000000000a94f5374fce5edbc8e2a8697c15331677e6ebf0b"],
    [null],
    [
      "0x000000000000000000000000a94f5374fce5edbc8e2a8697c15331677e6ebf0b",
      "0x0000000000000000000000000aff3454fce5edbc8cca8697c15331677e6ebccc"
    ]
  ]

instead of:

  "topics": [
    "0x000000000000000000000000a94f5374fce5edbc8e2a8697c15331677e6ebf0b",
    null,
    [
      "0x000000000000000000000000a94f5374fce5edbc8e2a8697c15331677e6ebf0b",
      "0x0000000000000000000000000aff3454fce5edbc8cca8697c15331677e6ebccc"
    ]
  ]

Fix this by flattening the array if there is only one topic in a
position.

[ricmoo]

Thanks. Looking into this now. I'd have thought null and [ null ] would both match the same filter, but I'll fix this right away. :)

[ricmoo]

Can you provide an example of how you generated that first filter?

[ricmoo]

When I try, I get:

const abi = [ "event Transfer(address indexed from, address indexed to, uint value)" ];
const provider = ethers.getDefaultProvider();

const contract = new ethers.Contract("dai.tokens.ethers.eth", abi, provider);

 const filter = contract.filters.Transfer(null, to);
console.log(filter);
// { address: 'dai.tokens.ethers.eth',
//  topics:
//   [ '0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef',
//     null,
//     '0x000000000000000000000000a5407eae9ba41422680e2e00537571bcc53efbfd' ] }

[ricmoo]

(oh wiat... You are finding it happens during serialization... one sec)

[danielattilasimon]

Exactly, if you call deserializeTopics("0x000000000000000000000000a94f5374fce5edbc8e2a8697c15331677e6ebf0b&null&0x000000000000000000000000a94f5374fce5edbc8e2a8697c15331677e6ebf0b|0x0000000000000000000000000aff3454fce5edbc8cca8697c15331677e6ebccc") you get my example.

[ricmoo]

Perfect. Yes, I've reproduced it and verified it isn't a problem in a. few other places you had me worried about. ;)

[ricmoo]

I have merged the fix locally. I'm also pinging a few other people so I can convince myself why [ [ null ], A ] isn't the same as [ null, A ]. I need to make sure I haven't made other weird assumptions. :)

[danielattilasimon]

Thanks so much for looking at this so quickly!

In my opinion the spec is too vague on the definition of topics. The best I can find is the note in https://github.com/ethereum/EIPs/blob/master/EIPS/eip-1474.md#parameters-34

Otherwise, topics is only defined as a Data[], when it should probably be something like (null | Data | Data[])[].

[ricmoo]

Yeah... I have the same page open now too. "Specification" might be too string a word. :)

I'm going to assume null can't be inside a data for now, until I hear back. A few outstanding questions I have are:

1. Can [ [ A, B, null ], C ] be simplified to [ null, C ] (or is it invalid)?
2. Can [ A, [ ], B ] be simplified to never-gonna-happen?

But I'll commit these changes for now until I hear back. :)

[ricmoo]

(also these are things I plan to address in the ethers docs, so after this research there is some place with the results ;))

[danielattilasimon]

Thanks again for such a quick turnaround! I see you've already published a new package with the fix included.

[ricmoo]

Yes, thanks! Let me know if it works for you. :)

Perfect PR, btw. You even named variables the exact same way I do. :)

[danielattilasimon]

Already integrated the new release, works perfectly!

[danielattilasimon]

After looking into the geth sources, I think the API weirdness comes down to how the data is represented by the backend, and the mechanics of JSON encoding in Go. I know very little about Go though, so I could be wrong.

Topics are typed as a [][]common.Hash, a slice of slices of hashes.
https://github.com/ethereum/go-ethereum/blob/5d7e5b00be3eb04a334366d78b6ab742772c8e0a/eth/filters/filter.go#L51-L62

In Go, nil functions as an empty slice:
https://tour.golang.org/moretypes/12

A nil slice is encoded as null when marshalling to JSON, and vice-versa:
https://golang.org/pkg/encoding/json/#Marshal
https://golang.org/pkg/encoding/json/#Unmarshal

The type [][]common.Hash represents an array of Hash-arrays. In this JSON representation:

  "topics": [
    ["0x000000000000000000000000a94f5374fce5edbc8e2a8697c15331677e6ebf0b"],
    [null],
    [
      "0x000000000000000000000000a94f5374fce5edbc8e2a8697c15331677e6ebf0b",
      "0x0000000000000000000000000aff3454fce5edbc8cca8697c15331677e6ebccc"
    ]
  ]

null (which will be translated to nil) stands in a position where a hash is expected. I haven't looked into the parsing of hashes, but common.Hash is a fixed-size array of bytes, so I'm assuming that a null/nil will be rejected.

On the other hand, this should work:

  "topics": [
    ["0x000000000000000000000000a94f5374fce5edbc8e2a8697c15331677e6ebf0b"],
    [],
    [
      "0x000000000000000000000000a94f5374fce5edbc8e2a8697c15331677e6ebf0b",
      "0x0000000000000000000000000aff3454fce5edbc8cca8697c15331677e6ebccc"
    ]
  ]

[danielattilasimon]

Shall we close this now that it's fixed?

[ricmoo]

Oh yes! Thanks. :)