[Access] Pending response from send and subscribe should return immediately

[peterargue]

Problem Definition

The SendAndSubscribe streaming grpc endpoint allows a user to send a transaction and immediately subscribe to status updates. Currently, it takes one or more seconds before receiving the first response indicating the transaction is pending. Since the transaction was submitted to that streaming node, it should know that the tx is pending immediately.

I suspect the issue is that it waits until the next block is finalized before triggering, then searches several blocks to catch up with the block the tx was included in.

Proposed Solution

Instead of waiting, add the pending status message immediately after creating the subscription:

flow-go/engine/access/rpc/backend/backend_stream_transactions.go

Line 67 in f6814f8

return b.subscriptionHandler.Subscribe(ctx, nextHeight, b.getTransactionStatusResponse(&txInfo))

Definition of Done

When calling SendAndSubscribe, the pending response should be received immediately with no delay.