Support retrieving newer txs only in light wallet REST API

[ndorf]

Currently, the light wallet REST API only supports retrieving the entire transaction history. This proposed extension to the protocol allows clients to request only those transactions newer than the ones they already know.

Two optional fields are added to the get_address_txs request object: since_tx_id, the highest transaction id known to the client, and since_tx_block_hash, the block_hash of the same transaction. If these fields are absent, the entire transaction history is returned, as before.

A since_tx_id field is also added to the response object; its value may differ from the value in the request, and clients must be sure to honor the former, by forgetting any txs with a higher id before processing the response.

This logic, along with the new block hash fields, allow detection and correct handling of blockchain reorgs. Upon receiving the request, the server compares the provided tx id and block_hash with the current blockchain state. If they match, then the server can simply return the newer txs, exactly as requested, and the since_tx_id of the response should be the same as the request. If they don't match, then the specified transaction must have been part of a blockchain reorg, in which case some or all of the previous history must be returned, and the since_tx_id of the response will differ from the request.

If the server keeps track of reorged blocks, and where they previously were in the blockchain, then it can return only those txs that are newer than the split. (In this case the returned since_tx_id should be the highest id in the last block common to both chains.) However, it is not obligated to do so: it can simply return the entire transaction history anytime a reorg is detected (in which case the returned since_tx_id should be zero.)

Requesting feedback from @vtnerd and @moneroexamples

[paulshapiro]

I support deprecating the full list and calling this standard in clients.

[moneroexamples]

Yes, I was proposing something similar like that before as well. There is no reason to scan entire blockchain if you know that your wallet is 3 days old. You could also perhaps just return always at minim last 10 translations or so, to protect against reorgs?

[ndorf]

@moneroexamples this is orthogonal to scanning, it only relates to the client's retrieval of tx history.

[AAH20]

check this https://github.com/monero-project/monero/issues/7896 to enforce your contributions with latest api security standards , also feel free to share your opinion on the overall security and transparency situation of the project.

[j-berman]

Adding: I think this is a good idea! Spec looks good to me.

EDIT: nevermind below, sorry. It seems it would come at the cost of an additional DB read, so not worth it.

Small thing, unless I'm misunderstanding, it seems a little confusing that in the reorg case, the updated since_tx_id could be the highest tx in a block. Might it be a tiny bit more intuitive to return the user's highest tx_id from before the split (edit: and 0 if none)? I feel like a consumer of the API would find it marginally easier to grok, but doesn't really make a big difference either way.

[ndorf]

it can simply return the entire transaction history anytime a reorg is detected (in which case the returned since_tx_id should be zero.)

Changed the wording to explicitly allow simply omitting this field altogether; the client needs to handle that case anyway, for backward compatibility.