feat: use UnsafeVec to replace Vec<Mutex<>>

[jackwener]

last_locations will only be updated in record(), validate_read_locations(), and convert_writes_to_estimates(). For the same TxIdx, these functions cannot execute concurrently, so we can replace Vec<Mutex<>> with UnsafeVec.

[hai-rise]

@jackwener Good insight as always!

Actually, there can be concurrent validation tasks for the same transaction index.
There can only be one execution task when the status transitions from ReadyToExecute to Executing:

pevm/src/scheduler.rs

Lines 125 to 131 in cce4fc9

	if tx.status == IncarnationStatus::ReadyToExecute {
	tx.status = IncarnationStatus::Executing;
	return Some(Task::Execution(TxVersion {
	tx_idx,
	tx_incarnation: tx.incarnation,
	}));
	}

But a new validation task doesn't transition the status, so multiple can be scheduled for the same executed/validated transaction:

pevm/src/scheduler.rs

Lines 133 to 140 in cce4fc9

	if matches!(
	tx.status,
	IncarnationStatus::Executed | IncarnationStatus::Validated
	) {
	return Some(Task::Validation(TxVersion {
	tx_idx,
	tx_incarnation: tx.incarnation,
	}));

This happens when validation_idx is reset to a lower value. We don't want a blocking Validating status as we want the latter task to start (with the latest data) as soon as possible. A relevant optimization is to have a way to signal the on-going validation task(s) to exit when a new one (of the same transaction) starts.

Anyway, that is just extra information, as it should still be safe to have concurrent reads!
I had a local branch but with Vec<SyncUnsafeCell<LastLocations>>, which is much shorter than rolling our own UnsafeVec. Do you think it's worth re-trying this that way?

[jackwener]

Actually, there can be concurrent validation tasks for the same transaction index.

Get it! Indeed, for the same TxIdx, there may be multiple validation tasks at the same time

I had a local branch but with Vec<SyncUnsafeCell<LastLocations>>, which is much shorter than rolling our own UnsafeVec. Do you think it's worth re-trying this that way?

Yes, it's will be easier and simpler