orderwatch,rpc: Supply parsed contract events in emitted OrderEvents

[fabioberger]

Currently, OrderEvents emitted by Mesh represent the order's net fillability changes introduced by all the transactions within a block. Some use-cases require knowing about all smart contract events that led to this net fillability change.

Example: If an order is filled and cancelled within a block, we only emit a CANCELLED order event (since this is the final state of the order). We do include the transaction hashes of both the fill & cancel transactions, but accessing the underlying smart contract events requires clients to do further Ethereum JSON-RPC calls. Since we already have the parsed smart contract events in Mesh, we could simply include them in the response.

Proposed new OrderEvent type:

// OrderEvent is the order event emitted by Mesh nodes on the "orders" topic
// when calling JSON-RPC method `mesh_subscribe`
type OrderEvent struct {
	OrderHash                common.Hash    `json:"orderHash"`
	SignedOrder              *SignedOrder   `json:"signedOrder"`
	Kind                     OrderEventKind `json:"kind"`
	FillableTakerAssetAmount *big.Int       `json:"fillableTakerAssetAmount"`
	Transactions []Transaction `json:"transactions"`
}

// Transaction is a succinct representation of an Ethereum transaction
type Transaction struct {
	TxHash common.Hash `json:"orderHash"`
	Logs   []ParsedLog `json:"logs"`
}

// ParsedLog represents a parsed smart contract event/log
type ParsedLog struct {
	Name       string            `json:"name"`
	Parameters map[string]string `json:"parameters"`
}

Example OrderEvent:

{
    "jsonrpc": "2.0",
    "method": "mesh_subscription",
    "params": {
        "subscription": "0xcd0c3e8af590364c09d0fa6a1210faf5",
        "result": [
            {
                "orderHash": "0x96e6eb6174dbf0458686bdae44c9a330d9a9eb563962512a7be545c4ecc13fd4",
                "signedOrder": {
                    "makerAddress": "0x50f84bbee6fb250d6f49e854fa280445369d64d9",
                    "makerAssetData": "0xf47261b00000000000000000000000000f5d2fb29fb7d3cfee444a200298f468908cc942",
                    "makerAssetAmount": "4424020538752105500000",
                    "makerFee": "0",
                    "takerAddress": "0x0000000000000000000000000000000000000000",
                    "takerAssetData": "0xf47261b0000000000000000000000000c02aaa39b223fe8d0a0e5c4f27ead9083c756cc2",
                    "takerAssetAmount": "1000000000000000061",
                    "takerFee": "0",
                    "senderAddress": "0x0000000000000000000000000000000000000000",
                    "exchangeAddress": "0x4f833a24e1f95d70f028921e27040ca56e09ab0b",
                    "feeRecipientAddress": "0xa258b39954cef5cb142fd567a46cddb31a670124",
                    "expirationTimeSeconds": "1559422407",
                    "salt": "1559422141994",
                    "signature": "0x1cf16c2f3a210965b5e17f51b57b869ba4ddda33df92b0017b4d8da9dacd3152b122a73844eaf50ccde29a42950239ba36a525ed7f1698a8a5e1896cf7d651aed203"
                },
                "kind": "CANCELLED",
                "fillableTakerAssetAmount": 0,
                "transactions": [
                    {
                        "txHash": "0x84aaae84147fc42fc77b33e2d3e05d86272663792d9cacaa8dc89f207b4d0642",
                        "logs": [
                            {
                                "name": "Fill",
                                "parameters": {
                                        "makerAddress":        "0x50f84bbee6fb250d6f49e854fa280445369d64d9",
                                        "takerAddress":        "0xab1a3e8af590364c09d0fa6a12103ada1bccda31",
                                        "senderAddress":       "0x0000000000000000000000000000000000000000",
                                        "feeRecipientAddress": "0xa258b39954cef5cb142fd567a46cddb31a670124",
                                        "makerAssetFilledAmount": "2212010269376052700000",
                                        "takerAssetFilledAmount": "500000000000000000",
                                        "makerFeePaid":           "0",
                                        "takerFeePaid":           "0",
                                        "orderHash":              "0x96e6eb6174dbf0458686bdae44c9a330d9a9eb563962512a7be545c4ecc13fd4",
                                        "makerAssetData":      "0xf47261b00000000000000000000000000f5d2fb29fb7d3cfee444a200298f468908cc942",
                                        "takerAssetData":      "0xf47261b0000000000000000000000000c02aaa39b223fe8d0a0e5c4f27ead9083c756cc2",
                                },
                            },
                        ],
                    },
                    {
                        "txHash": "0x9e6830a7044b39e107f410e4f765995fd0d3d69d5c3b3582a1701b9d68167560",
                        "logs": [
                            {
                                "name": "Cancel",
                                "parameters": {
                                    "makerAddress":        "0x50f84bbee6fb250d6f49e854fa280445369d64d9",
                                    "feeRecipientAddress": "0xa258b39954cef5cb142fd567a46cddb31a670124",
                                    "senderAddress":       "0x0000000000000000000000000000000000000000",
                                    "orderHash":           "0x96e6eb6174dbf0458686bdae44c9a330d9a9eb563962512a7be545c4ecc13fd4",
                                    "makerAssetData":      "0xf47261b00000000000000000000000000f5d2fb29fb7d3cfee444a200298f468908cc942",
                                    "takerAssetData":      "0xf47261b0000000000000000000000000c02aaa39b223fe8d0a0e5c4f27ead9083c756cc2",
                                },
                            },
                        ],
                    },
                ]
            }
        ]
    }
}

The benefit of this approach is a much better developer experience for use-cases that require the smart contract events (e.g., market-makers who need all contract events for trade execution, operations and bookkeeping purposes). The downside is that each event is not larger in size.

[fabioberger]

Open question: Are only the Exchange contract's Fill, Cancel and CancelUpTo events important, or also token Transfer, Approval events that impact order fillability?