[2.6][T4] Transaction API returns empty response when canEnterTransactionPool is false and no error is pushed by the transaction's handler

Expected Behavior When canEnterTransactionPool fails after a transaction is posted to the api, there should be an error message in the payload.

Current Behavior When a transaction is posted to the API, it goes through core-transaction-pool to validate it and check if it can enter the pool. However, when handler.canEnterTransactionPool(transaction, this.pool, this); (https://github.com/ArkEcosystem/core/blob/develop/packages/core-transaction-pool/src/processor.ts#L220) is false, and the custom transaction doesn’t push any error (just true or false), the validation response becomes {"data":{"accept":[],"broadcast":[],"excess":[],"invalid":[]}}. No error description is in the payload.

Possible Solution If no errors have been pushed but canEnterTransactionPool returns false, core-transaction-pool should push a generic error that the handler’s canEnterTransactionPool validation has failed.

Steps to Reproduce (for bugs)

1. Create a custom transaction without pushError calls in canEnterTransactionPool and make it return false.
2. Post a transaction to the transactions api endpoint.
3. Observe empty payload.

[vasild]

Yes, the interface is not very robust - canEnterTransactionPool() may forget to push the error and return false.

I do not like chasing a sloppy canEnterTransactionPool() after it has been called. That would be slippery - for example the errors array may have some entries from beforehand and be non-empty after canEnterTransactionPool() has completed and still that function may have forgotten to add the error. And even if chased properly - we still have the issue with a nearly useless generic error message, without the actual reason for the rejection.

A more robust interface would be:

• canEnterTransactionPool() does not call pushError(). If it needs the processor argument just for that, then drop that argument.
• Instead of true/false it would return an array of errors with an empty array designating success / no errors.
• Then the caller of canEnterTransactionPool() does pushError() for all elements of the returned array.

The point is that then there is no way for canEnterTransactionPool() to signal an error and still forget to give an explanation about it.

[vasild]

Closing now that https://github.com/ArkEcosystem/core/pull/3342 is merged.