feat: offchain reception

[mverzilli]

Adds the ability to receive offchain messages to Aztec.nr contracts.

It works by using capsules to implement a persistent offchain message inbox, and making sure that inbox is processed whenever sync_state runs.

Apps deliver an offchain message to the inbox through a new utility function offchain_receive, generated by the Aztec macro.

An offchain::sync_inbox function, takes the responsibility to let sync_state know which messages should be processed like so:

1. Limits processed messages to those whose originating TX are available at PXE's anchor block. To this end it uses a new resolve_message_contexts oracle to determine which messages have corresponding TXs known by PXE.
2. It is resilient to reorgs. Messages are re-processed for until expiration time to make sure their effects don't get lost after re-orgs. After expiration, they are safe to be discarded.

The feature can be exercised by invoking a function that emits offchain messages on the sender side, saving the resulting offchain effects, and then calling offchain_receive on the recipient side with them.

    // Alice sends the private transfer which emits offchain messages.
    const { receipt, offchainMessages } = await contract.methods
      .transfer_offchain(paymentAmount, bob)
      .send({ from: alice });
    expect(offchainMessages.length).toBeGreaterThan(0);

    const messageForBob = offchainMessages.find(msg => msg.recipient.equals(bob));
    expect(messageForBob).toBeTruthy();

    await contract.methods
      .offchain_receive([
        {
          ciphertext: messageForBob!.payload,
          recipient: bob,
          tx_hash: receipt.txHash.hash,
          expiration_timestamp: messageForBob!.expirationTimestamp,
        },
      ])
      .simulate({ from: bob });

Known issues (can be addressed in subsequent PRs):

1. The new oracle, together with the long window offchain messages get replayed for, means we should really cache getTxEffects node calls to avoid performance regressions.
2. The current approach of PXE to caching contract sync calls, means that with the current implementation we need to wait for one block before a new message delivered to the inbox gets processed. We will solve this by adding a new oracle to invalidate the contract sync cache.
3. Scopes aren't handled properly, which might need bigger refactors that exceed offchain

Closes F-323
Closes F-327
Closes F-325

[nventuro]

Lovely. Left some comments but approving given that this is still in-progress work ultimtaely, no point in holding back the large changeset.

[nventuro]

So none would be disabling this feature, no?

[mverzilli]

yes, maybe I grew too attached to the first implementation that used extensions

[nventuro]

With such long tests, you may want to consider putting these in a separate file (which would be processing/offchain/test.nr)

[mverzilli]

will refactor later

[nventuro]

Should we leave a comment explaining the capsule issue?

[AztecBot]

❌ Failed to cherry-pick to v4-next due to conflicts. (🤖) View backport run.