serai/processor/scanner/src/eventuality/mod.rs

use group::GroupEncoding;

use serai_db::{Get, DbTxn, Db};

use primitives::{task::ContinuallyRan, OutputType, ReceivedOutput, Eventuality, Block};

// TODO: Localize to EventualityDb?
use crate::{
  lifetime::LifetimeStage,
  db::{OutputWithInInstruction, ReceiverScanData, ScannerDb, ScanToEventualityDb},
  BlockExt, ScannerFeed, KeyFor, SchedulerUpdate, Scheduler, sort_outputs,
};

mod db;
use db::EventualityDb;

/// The latest scannable block, which is determined by this task.
///
/// This task decides when a key retires, which impacts the scan task. Accordingly, the scanner is
/// only allowed to scan `S::WINDOW_LENGTH - 1` blocks ahead so we can safely schedule keys to
/// retire `S::WINDOW_LENGTH` blocks out.
pub(crate) fn latest_scannable_block<S: ScannerFeed>(getter: &impl Get) -> Option<u64> {
  EventualityDb::<S>::next_to_check_for_eventualities_block(getter)
    .map(|b| b + S::WINDOW_LENGTH - 1)
}

/*
  When we scan a block, we receive outputs. When this block is acknowledged, we accumulate those
  outputs into some scheduler, potentially causing certain transactions to begin their signing
  protocol.

  Despite only scanning blocks with `CONFIRMATIONS`, we cannot assume that these transactions (in
  their signed form) will only appear after `CONFIRMATIONS`. For `CONFIRMATIONS = 10`, the scanned
  block's number being `1`, the blockchain will have blocks with numbers `0 ..= 10`. While this
  implies the earliest the transaction will appear is when the block number is `11`, which is
  `1 + CONFIRMATIONS` (the number of the scanned block, plus the confirmations), this isn't
  guaranteed.

  A reorganization could occur which causes all unconfirmed blocks to be replaced, with the new
  blockchain having the signed transaction present immediately.

  This means that in order to detect Eventuality completions, we can only check block `b+1` once
  we've acknowledged block `b`, accumulated its outputs, triggered any transactions, and prepared
  for their Eventualities. This is important as both the completion of Eventualities, and the scan
  process, may cause a block to be considered notable (where notable blocks must be perfectly
  ordered).

  We do not want to fully serialize the scan flow solely because the Eventuality flow must be. If
  the time to scan, acknowledge, and intake a block ever exceeded the block time, we'd form a
  backlog.

  The solution is to form a window of blocks we can scan/acknowledge/intake, safely, such that we
  only form a backlog if the latency for a block exceeds the duration of the entire window (the
  amount of blocks in the window * the block time).

  By considering the block an Eventuality resolves not as the block it does, yet the block a window
  later, we enable the following flow:

  - The scanner scans within its window, submitting blocks for acknowledgement.
  - We have the blocks acknowledged (the consensus protocol handling this in parallel).
  - The scanner checks for Eventualities completed following acknowledged blocks.
  - If all Eventualities for a retiring multisig have been cleared, the notable block is one window
    later.
  - The start of the window shifts to the last block we've checked for Eventualities. This means
    the end of the window is the block we just set as notable, and yes, once that's scanned we can
    successfully publish a batch for it in a canonical fashion.

  This forms a backlog only if the latency of scanning, acknowledgement, and intake (including
  checking Eventualities) exceeds the window duration (the desired property).
*/
struct EventualityTask<D: Db, S: ScannerFeed, Sch: Scheduler<S>> {
  db: D,
  feed: S,
  scheduler: Sch,
}

impl<D: Db, S: ScannerFeed, Sch: Scheduler<S>> EventualityTask<D, S, Sch> {
  pub(crate) fn new(mut db: D, feed: S, scheduler: Sch, start_block: u64) -> Self {
    if EventualityDb::<S>::next_to_check_for_eventualities_block(&db).is_none() {
      // Initialize the DB
      let mut txn = db.txn();
      // We can receive outputs in `start_block`, but any descending transactions will be in the
      // next block
      EventualityDb::<S>::set_next_to_check_for_eventualities_block(&mut txn, start_block + 1);
      txn.commit();
    }

    Self { db, feed, scheduler }
  }
}

#[async_trait::async_trait]
impl<D: Db, S: ScannerFeed, Sch: Scheduler<S>> ContinuallyRan for EventualityTask<D, S, Sch> {
  async fn run_iteration(&mut self) -> Result<bool, String> {
    /*
      The set of Eventualities only increase when a block is acknowledged. Accordingly, we can only
      iterate up to (and including) the block currently pending acknowledgement. "including" is
      because even if block `b` causes new Eventualities, they'll only potentially resolve in block
      `b + 1`.

      We only know blocks will need acknowledgement *for sure* if they were scanned. The only other
      causes are key activation and retirement (both scheduled outside the scan window). This makes
      the exclusive upper bound the *next block to scan*.
    */
    let exclusive_upper_bound = {
      // Fetch the next to scan block
      let next_to_scan = ScannerDb::<S>::next_to_scan_for_outputs_block(&self.db)
        .expect("EventualityTask run before writing the start block");
      // If we haven't done any work, return
      if next_to_scan == 0 {
        return Ok(false);
      }
      next_to_scan
    };

    // Fetch the highest acknowledged block
    let highest_acknowledged = ScannerDb::<S>::highest_acknowledged_block(&self.db)
      .expect("EventualityTask run before writing the start block");

    // Fetch the next block to check
    let next_to_check = EventualityDb::<S>::next_to_check_for_eventualities_block(&self.db)
      .expect("EventualityTask run before writing the start block");

    // Check all blocks
    let mut iterated = false;
    for b in next_to_check .. exclusive_upper_bound {
      // If the prior block was notable *and* not acknowledged, break
      // This is so if it caused any Eventualities (which may resolve this block), we have them
      {
        // This `- 1` is safe as next to check is bound to be non-zero
        // This is possible since even if we receive coins in block 0, any transactions we'd make
        // would resolve in block 1 (the first block we'll check under this non-zero rule)
        let prior_block = b - 1;
        if ScannerDb::<S>::is_block_notable(&self.db, prior_block) &&
          (prior_block > highest_acknowledged)
        {
          break;
        }
      }

      iterated = true;

      let block = self.feed.block_by_number(&self.db, b).await?;

      log::debug!("checking eventuality completions in block: {} ({b})", hex::encode(block.id()));

      /*
        This is proper as the keys for the next to scan block (at most `WINDOW_LENGTH` ahead,
        which is `<= CONFIRMATIONS`) will be the keys to use here, with only minor edge cases.

        This may include a key which has yet to activate by our perception. We can simply drop
        those.

        This may not include a key which has retired by the next-to-scan block. This task is the
        one which decides when to retire a key, and when it marks a key to be retired, it is done
        with it. Accordingly, it's not an issue if such a key was dropped.
      */
      let mut keys = ScannerDb::<S>::active_keys_as_of_next_to_scan_for_outputs_block(&self.db)
        .expect("scanning for a blockchain without any keys set");
      // Since the next-to-scan block is ahead of us, drop keys which have yet to actually activate
      keys.retain(|key| b <= key.activation_block_number);

      let mut txn = self.db.txn();

      // Fetch the data from the scanner
      let scan_data = ScanToEventualityDb::recv_scan_data(&mut txn, b);
      assert_eq!(scan_data.block_number, b);
      let ReceiverScanData { block_number: _, received_external_outputs, forwards, returns } =
        scan_data;
      let mut outputs = received_external_outputs;

      for key in &keys {
        let completed_eventualities = {
          let mut eventualities = EventualityDb::<S>::eventualities(&txn, key.key);
          let completed_eventualities = block.check_for_eventuality_resolutions(&mut eventualities);
          EventualityDb::<S>::set_eventualities(&mut txn, key.key, &eventualities);
          completed_eventualities
        };

        for tx in completed_eventualities.keys() {
          log::info!("eventuality resolved by {}", hex::encode(tx.as_ref()));
        }

        // Fetch all non-External outputs
        let mut non_external_outputs = block.scan_for_outputs(key.key);
        non_external_outputs.retain(|output| output.kind() != OutputType::External);
        // Drop any outputs less than the dust limit
        non_external_outputs.retain(|output| {
          let balance = output.balance();
          balance.amount.0 >= self.feed.dust(balance.coin).0
        });

        /*
          Now that we have all non-External outputs, we filter them to be only the outputs which
          are from transactions which resolve our own Eventualities *if* the multisig is retiring.
          This implements step 6 of `spec/processor/Multisig Rotation.md`.

          We may receive a Change output. The only issue with accumulating this would be if it
          extends the multisig's lifetime (by increasing the amount of outputs yet to be
          forwarded). By checking it's one we made, either:
          1) It's a legitimate Change output to be forwarded
          2) It's a Change output created by a user burning coins (specifying the Change address),
             which can only be created while the multisig is actively handling `Burn`s (therefore
            ensuring this multisig cannot be kept alive ad-infinitum)

          The commentary on Change outputs also applies to Branch/Forwarded. They'll presumably get
          ignored if not usable however.
        */
        if key.stage == LifetimeStage::Finishing {
          non_external_outputs
            .retain(|output| completed_eventualities.contains_key(&output.transaction_id()));
        }

        // Now, we iterate over all Forwarded outputs and queue their InInstructions
        for output in
          non_external_outputs.iter().filter(|output| output.kind() == OutputType::Forwarded)
        {
          let Some(eventuality) = completed_eventualities.get(&output.transaction_id()) else {
            // Output sent to the forwarding address yet not actually forwarded
            continue;
          };
          let Some(forwarded) = eventuality.forwarded_output() else {
            // This was a TX made by us, yet someone burned to the forwarding address
            continue;
          };

          let (return_address, in_instruction) =
            ScannerDb::<S>::return_address_and_in_instruction_for_forwarded_output(
              &txn, &forwarded,
            )
            .expect("forwarded an output yet didn't save its InInstruction to the DB");
          ScannerDb::<S>::queue_output_until_block(
            &mut txn,
            b + S::WINDOW_LENGTH,
            &OutputWithInInstruction { output: output.clone(), return_address, in_instruction },
          );
        }

        // Accumulate all of these outputs
        outputs.extend(non_external_outputs);
      }

      // Update the scheduler
      let mut scheduler_update = SchedulerUpdate { outputs, forwards, returns };
      scheduler_update.outputs.sort_by(sort_outputs);
      scheduler_update.forwards.sort_by(sort_outputs);
      scheduler_update.returns.sort_by(|a, b| sort_outputs(&a.output, &b.output));
      // Intake the new Eventualities
      let new_eventualities = self.scheduler.update(&mut txn, scheduler_update);
      for (key, new_eventualities) in new_eventualities {
        let key = {
          let mut key_repr = <KeyFor<S> as GroupEncoding>::Repr::default();
          assert_eq!(key.len(), key_repr.as_ref().len());
          key_repr.as_mut().copy_from_slice(&key);
          KeyFor::<S>::from_bytes(&key_repr).unwrap()
        };

        keys
          .iter()
          .find(|serai_key| serai_key.key == key)
          .expect("queueing eventuality for key which isn't active");

        let mut eventualities = EventualityDb::<S>::eventualities(&txn, key);
        for new_eventuality in new_eventualities {
          eventualities.active_eventualities.insert(new_eventuality.lookup(), new_eventuality);
        }
        EventualityDb::<S>::set_eventualities(&mut txn, key, &eventualities);
      }

      // Now that we've intaked any Eventualities caused, check if we're retiring any keys
      for key in &keys {
        if key.stage == LifetimeStage::Finishing {
          let eventualities = EventualityDb::<S>::eventualities(&txn, key.key);
          // TODO: This assumes the Scheduler is empty
          if eventualities.active_eventualities.is_empty() {
            log::info!(
              "key {} has finished and is being retired",
              hex::encode(key.key.to_bytes().as_ref())
            );

            // Retire this key `WINDOW_LENGTH` blocks in the future to ensure the scan task never
            // has a malleable view of the keys.
            ScannerDb::<S>::retire_key(&mut txn, b + S::WINDOW_LENGTH, key.key);
          }
        }
      }

      // Update the next-to-check block
      EventualityDb::<S>::set_next_to_check_for_eventualities_block(&mut txn, next_to_check);
      txn.commit();
    }

    // Run dependents if we successfully checked any blocks
    Ok(iterated)
  }
}