Clear mempool storage if the syncer starts syncing large numbers of blocks again

[teor2345]

Motivation

If large numbers of blocks get synced, Zebra needs to clear the mempool storage and reject list, because:

• previously verified transactions might have become invalid
• previously invalid transactions might have become valid

Design

Tasks can check if they are behind the tip using SyncStatus::is_close_to_tip:

zebra/zebrad/src/components/sync/status.rs

Lines 42 to 43 in 44ac067

	/// Check if the synchronization is likely close to the chain tip.
	pub fn is_close_to_tip(&self) -> bool {

Tasks can await tip jumps using ChainTipChange::tip_change and TipAction::Reset:

zebra/zebra-state/src/service/chain_tip.rs

Line 313 in 44ac067

pub async fn tip_change(&mut self) -> Result<TipAction, watch::error::RecvError> {

[mpguerra]

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[teor2345]

@jvff I think PRs #2773 and #2764 might have already closed this ticket:

Here's the code that clears the mempool if the syncer reports large numbers of blocks:

zebra/zebrad/src/components/mempool.rs

Lines 155 to 167 in 18acec6

	if is_close_to_tip {
	let tx_downloads = Box::pin(TxDownloads::new(
	Timeout::new(self.outbound.clone(), TRANSACTION_DOWNLOAD_TIMEOUT),
	Timeout::new(self.tx_verifier.clone(), TRANSACTION_VERIFY_TIMEOUT),
	self.state.clone(),
	));
	self.active_state = ActiveState::Enabled {
	storage: Default::default(),
	tx_downloads,
	};
	} else {
	self.active_state = ActiveState::Disabled
	}

And we also clear if the state commits more than one block at the same time:

zebra/zebrad/src/components/mempool.rs

Lines 249 to 254 in 18acec6

	if let Some(tip_action) = self.chain_tip_change.last_tip_change() {
	match tip_action {
	// Clear the mempool if there has been a chain tip reset.
	TipAction::Reset { .. } => {
	storage.clear();
	}