referrerpolicy=no-referrer-when-downgrade
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
// Copyright (C) Parity Technologies (UK) Ltd.
// This file is part of Polkadot.

// Polkadot is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// Polkadot is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.

//! A [`SelectChain`] implementation designed for relay chains.
//!
//! This uses information about parachains to inform GRANDPA and BABE
//! about blocks which are safe to build on and blocks which are safe to
//! finalize.
//!
//! To learn more about chain-selection rules for Relay Chains, please see the
//! documentation on [chain-selection][chain-selection-guide]
//! in the implementers' guide.
//!
//! This is mostly a wrapper around a subsystem which implements the
//! chain-selection rule, which leaves the code to be very simple.
//!
//! However, this does apply the further finality constraints to the best
//! leaf returned from the chain selection subsystem by calling into other
//! subsystems which yield information about approvals and disputes.
//!
//! [chain-selection-guide]: https://paritytech.github.io/polkadot-sdk/book/protocol-chain-selection.html

#![cfg(feature = "full-node")]

use super::{HeaderProvider, HeaderProviderProvider};
use futures::channel::oneshot;
use polkadot_node_primitives::MAX_FINALITY_LAG as PRIMITIVES_MAX_FINALITY_LAG;
use polkadot_node_subsystem::messages::{
	ApprovalDistributionMessage, ApprovalVotingMessage, ApprovalVotingParallelMessage,
	ChainSelectionMessage, DisputeCoordinatorMessage, HighestApprovedAncestorBlock,
};
use polkadot_node_subsystem_util::metrics::{self, prometheus};
use polkadot_overseer::{AllMessages, Handle};
use polkadot_primitives::{Block as PolkadotBlock, BlockNumber, Hash, Header as PolkadotHeader};
use sp_consensus::{Error as ConsensusError, SelectChain};
use std::sync::Arc;

pub use sc_service::SpawnTaskHandle;

/// The maximum amount of unfinalized blocks we are willing to allow due to approval checking
/// or disputes.
///
/// This is a safety net that should be removed at some point in the future.
// In sync with `MAX_HEADS_LOOK_BACK` in `approval-voting`
// and `MAX_BATCH_SCRAPE_ANCESTORS` in `dispute-coordinator`.
const MAX_FINALITY_LAG: polkadot_primitives::BlockNumber = PRIMITIVES_MAX_FINALITY_LAG;

const LOG_TARGET: &str = "parachain::chain-selection";

/// Prometheus metrics for chain-selection.
#[derive(Debug, Default, Clone)]
pub struct Metrics(Option<MetricsInner>);

#[derive(Debug, Clone)]
struct MetricsInner {
	approval_checking_finality_lag: prometheus::Gauge<prometheus::U64>,
	disputes_finality_lag: prometheus::Gauge<prometheus::U64>,
}

impl metrics::Metrics for Metrics {
	fn try_register(registry: &prometheus::Registry) -> Result<Self, prometheus::PrometheusError> {
		let metrics = MetricsInner {
			approval_checking_finality_lag: prometheus::register(
				prometheus::Gauge::with_opts(
					prometheus::Opts::new(
						"polkadot_parachain_approval_checking_finality_lag",
						"How far behind the head of the chain the Approval Checking protocol wants to vote",
					)
				)?,
				registry,
			)?,
			disputes_finality_lag: prometheus::register(
				prometheus::Gauge::with_opts(
					prometheus::Opts::new(
						"polkadot_parachain_disputes_finality_lag",
						"How far behind the head of the chain the Disputes protocol wants to vote",
					)
				)?,
				registry,
			)?,
		};

		Ok(Metrics(Some(metrics)))
	}
}

impl Metrics {
	fn note_approval_checking_finality_lag(&self, lag: BlockNumber) {
		if let Some(ref metrics) = self.0 {
			metrics.approval_checking_finality_lag.set(lag as _);
		}
	}

	fn note_disputes_finality_lag(&self, lag: BlockNumber) {
		if let Some(ref metrics) = self.0 {
			metrics.disputes_finality_lag.set(lag as _);
		}
	}
}

/// Determines whether the chain is a relay chain
/// and hence has to take approval votes and disputes
/// into account.
enum IsDisputesAwareWithOverseer<B: sc_client_api::Backend<PolkadotBlock>> {
	Yes(SelectRelayChainInner<B, Handle>),
	No,
}

impl<B> Clone for IsDisputesAwareWithOverseer<B>
where
	B: sc_client_api::Backend<PolkadotBlock>,
	SelectRelayChainInner<B, Handle>: Clone,
{
	fn clone(&self) -> Self {
		match self {
			Self::Yes(ref inner) => Self::Yes(inner.clone()),
			Self::No => Self::No,
		}
	}
}

/// A chain-selection implementation which provides safety for relay chains.
pub struct SelectRelayChain<B: sc_client_api::Backend<PolkadotBlock>> {
	longest_chain: sc_consensus::LongestChain<B, PolkadotBlock>,
	selection: IsDisputesAwareWithOverseer<B>,
}

impl<B> Clone for SelectRelayChain<B>
where
	B: sc_client_api::Backend<PolkadotBlock>,
	SelectRelayChainInner<B, Handle>: Clone,
{
	fn clone(&self) -> Self {
		Self { longest_chain: self.longest_chain.clone(), selection: self.selection.clone() }
	}
}

impl<B> SelectRelayChain<B>
where
	B: sc_client_api::Backend<PolkadotBlock> + 'static,
{
	/// Use the plain longest chain algorithm exclusively.
	pub fn new_longest_chain(backend: Arc<B>) -> Self {
		gum::debug!(target: LOG_TARGET, "Using {} chain selection algorithm", "longest");

		Self {
			longest_chain: sc_consensus::LongestChain::new(backend.clone()),
			selection: IsDisputesAwareWithOverseer::No,
		}
	}

	/// Create a new [`SelectRelayChain`] wrapping the given chain backend
	/// and a handle to the overseer.
	pub fn new_with_overseer(
		backend: Arc<B>,
		overseer: Handle,
		metrics: Metrics,
		spawn_handle: Option<SpawnTaskHandle>,
		approval_voting_parallel_enabled: bool,
	) -> Self {
		gum::debug!(target: LOG_TARGET, "Using dispute aware relay-chain selection algorithm",);

		SelectRelayChain {
			longest_chain: sc_consensus::LongestChain::new(backend.clone()),
			selection: IsDisputesAwareWithOverseer::Yes(SelectRelayChainInner::new(
				backend,
				overseer,
				metrics,
				spawn_handle,
				approval_voting_parallel_enabled,
			)),
		}
	}

	/// Allow access to the inner chain, for usage during the node setup.
	pub fn as_longest_chain(&self) -> &sc_consensus::LongestChain<B, PolkadotBlock> {
		&self.longest_chain
	}
}

#[async_trait::async_trait]
impl<B> SelectChain<PolkadotBlock> for SelectRelayChain<B>
where
	B: sc_client_api::Backend<PolkadotBlock> + 'static,
{
	async fn leaves(&self) -> Result<Vec<Hash>, ConsensusError> {
		match self.selection {
			IsDisputesAwareWithOverseer::Yes(ref selection) => selection.leaves().await,
			IsDisputesAwareWithOverseer::No => self.longest_chain.leaves().await,
		}
	}

	async fn best_chain(&self) -> Result<PolkadotHeader, ConsensusError> {
		match self.selection {
			IsDisputesAwareWithOverseer::Yes(ref selection) => selection.best_chain().await,
			IsDisputesAwareWithOverseer::No => self.longest_chain.best_chain().await,
		}
	}

	async fn finality_target(
		&self,
		target_hash: Hash,
		maybe_max_number: Option<BlockNumber>,
	) -> Result<Hash, ConsensusError> {
		if let IsDisputesAwareWithOverseer::Yes(ref selection) = self.selection {
			selection
				.finality_target_with_longest_chain(target_hash, maybe_max_number)
				.await
		} else {
			self.longest_chain.finality_target(target_hash, maybe_max_number).await
		}
	}
}

/// A chain-selection implementation which provides safety for relay chains
/// but does not handle situations where the overseer is not yet connected.
pub struct SelectRelayChainInner<B, OH> {
	backend: Arc<B>,
	overseer: OH,
	metrics: Metrics,
	spawn_handle: Option<SpawnTaskHandle>,
	approval_voting_parallel_enabled: bool,
}

impl<B, OH> SelectRelayChainInner<B, OH>
where
	B: HeaderProviderProvider<PolkadotBlock>,
	OH: OverseerHandleT,
{
	/// Create a new [`SelectRelayChainInner`] wrapping the given chain backend
	/// and a handle to the overseer.
	pub fn new(
		backend: Arc<B>,
		overseer: OH,
		metrics: Metrics,
		spawn_handle: Option<SpawnTaskHandle>,
		approval_voting_parallel_enabled: bool,
	) -> Self {
		SelectRelayChainInner {
			backend,
			overseer,
			metrics,
			spawn_handle,
			approval_voting_parallel_enabled,
		}
	}

	fn block_header(&self, hash: Hash) -> Result<PolkadotHeader, ConsensusError> {
		match HeaderProvider::header(self.backend.header_provider(), hash) {
			Ok(Some(header)) => Ok(header),
			Ok(None) =>
				Err(ConsensusError::ChainLookup(format!("Missing header with hash {:?}", hash,))),
			Err(e) => Err(ConsensusError::ChainLookup(format!(
				"Lookup failed for header with hash {:?}: {:?}",
				hash, e,
			))),
		}
	}

	fn block_number(&self, hash: Hash) -> Result<BlockNumber, ConsensusError> {
		match HeaderProvider::number(self.backend.header_provider(), hash) {
			Ok(Some(number)) => Ok(number),
			Ok(None) =>
				Err(ConsensusError::ChainLookup(format!("Missing number with hash {:?}", hash,))),
			Err(e) => Err(ConsensusError::ChainLookup(format!(
				"Lookup failed for number with hash {:?}: {:?}",
				hash, e,
			))),
		}
	}
}

impl<B, OH> Clone for SelectRelayChainInner<B, OH>
where
	B: HeaderProviderProvider<PolkadotBlock> + Send + Sync,
	OH: OverseerHandleT,
{
	fn clone(&self) -> Self {
		SelectRelayChainInner {
			backend: self.backend.clone(),
			overseer: self.overseer.clone(),
			metrics: self.metrics.clone(),
			spawn_handle: self.spawn_handle.clone(),
			approval_voting_parallel_enabled: self.approval_voting_parallel_enabled,
		}
	}
}

#[derive(thiserror::Error, Debug)]
enum Error {
	// Oneshot for requesting leaves from chain selection got canceled - check errors in that
	// subsystem.
	#[error("Request for leaves from chain selection got canceled")]
	LeavesCanceled(oneshot::Canceled),
	#[error("Request for leaves from chain selection got canceled")]
	BestLeafContainingCanceled(oneshot::Canceled),
	// Requesting recent disputes oneshot got canceled.
	#[error("Request for determining the undisputed chain from DisputeCoordinator got canceled")]
	DetermineUndisputedChainCanceled(oneshot::Canceled),
	#[error("Request approved ancestor from approval voting got canceled")]
	ApprovedAncestorCanceled(oneshot::Canceled),
	/// Chain selection returned empty leaves.
	#[error("ChainSelection returned no leaves")]
	EmptyLeaves,
}

/// Decoupling trait for the overseer handle.
///
/// Required for testing purposes.
#[async_trait::async_trait]
pub trait OverseerHandleT: Clone + Send + Sync {
	async fn send_msg<M: Send + Into<AllMessages>>(&mut self, msg: M, origin: &'static str);
}

#[async_trait::async_trait]
impl OverseerHandleT for Handle {
	async fn send_msg<M: Send + Into<AllMessages>>(&mut self, msg: M, origin: &'static str) {
		Handle::send_msg(self, msg, origin).await
	}
}

impl<B, OH> SelectRelayChainInner<B, OH>
where
	B: HeaderProviderProvider<PolkadotBlock>,
	OH: OverseerHandleT + 'static,
{
	/// Get all leaves of the chain, i.e. block hashes that are suitable to
	/// build upon and have no suitable children.
	async fn leaves(&self) -> Result<Vec<Hash>, ConsensusError> {
		let (tx, rx) = oneshot::channel();

		self.overseer
			.clone()
			.send_msg(ChainSelectionMessage::Leaves(tx), std::any::type_name::<Self>())
			.await;

		let leaves = rx
			.await
			.map_err(Error::LeavesCanceled)
			.map_err(|e| ConsensusError::Other(Box::new(e)))?;

		gum::trace!(target: LOG_TARGET, ?leaves, "Chain selection leaves");

		Ok(leaves)
	}

	/// Among all leaves, pick the one which is the best chain to build upon.
	async fn best_chain(&self) -> Result<PolkadotHeader, ConsensusError> {
		// The Chain Selection subsystem is supposed to treat the finalized
		// block as the best leaf in the case that there are no viable
		// leaves, so this should not happen in practice.
		let best_leaf = *self
			.leaves()
			.await?
			.first()
			.ok_or_else(|| ConsensusError::Other(Box::new(Error::EmptyLeaves)))?;

		gum::trace!(target: LOG_TARGET, ?best_leaf, "Best chain");

		self.block_header(best_leaf)
	}

	/// Get the best descendant of `target_hash` that we should attempt to
	/// finalize next, if any. It is valid to return the `target_hash` if
	/// no better block exists.
	///
	/// This will search all leaves to find the best one containing the
	/// given target hash, and then constrain to the given block number.
	///
	/// It will also constrain the chain to only chains which are fully
	/// approved, and chains which contain no disputes.
	pub(crate) async fn finality_target_with_longest_chain(
		&self,
		target_hash: Hash,
		maybe_max_number: Option<BlockNumber>,
	) -> Result<Hash, ConsensusError> {
		let mut overseer = self.overseer.clone();

		let subchain_head = {
			let (tx, rx) = oneshot::channel();
			overseer
				.send_msg(
					ChainSelectionMessage::BestLeafContaining(target_hash, tx),
					std::any::type_name::<Self>(),
				)
				.await;

			let best = rx
				.await
				.map_err(Error::BestLeafContainingCanceled)
				.map_err(|e| ConsensusError::Other(Box::new(e)))?;

			gum::trace!(target: LOG_TARGET, ?best, "Best leaf containing");

			match best {
				// No viable leaves containing the block.
				None => return Ok(target_hash),
				Some(best) => best,
			}
		};

		let target_number = self.block_number(target_hash)?;

		// 1. Constrain the leaf according to `maybe_max_number`.
		let subchain_head = match maybe_max_number {
			None => subchain_head,
			Some(max) => {
				if max <= target_number {
					if max < target_number {
						gum::warn!(
							LOG_TARGET,
							max_number = max,
							target_number,
							"`finality_target` max number is less than target number",
						);
					}
					return Ok(target_hash)
				}
				// find the current number.
				let subchain_header = self.block_header(subchain_head)?;

				if subchain_header.number <= max {
					gum::trace!(target: LOG_TARGET, ?subchain_head, "Constrained sub-chain head",);
					subchain_head
				} else {
					let (ancestor_hash, _) =
						crate::grandpa_support::walk_backwards_to_target_block(
							self.backend.header_provider(),
							max,
							&subchain_header,
						)
						.map_err(|e| ConsensusError::ChainLookup(format!("{:?}", e)))?;
					gum::trace!(
						target: LOG_TARGET,
						?ancestor_hash,
						"Grandpa walk backwards sub-chain head"
					);
					ancestor_hash
				}
			},
		};

		let initial_leaf = subchain_head;
		let initial_leaf_number = self.block_number(initial_leaf)?;

		// 2. Constrain according to `ApprovedAncestor`.
		let (subchain_head, subchain_number, subchain_block_descriptions) = {
			let (tx, rx) = oneshot::channel();
			if self.approval_voting_parallel_enabled {
				overseer
					.send_msg(
						ApprovalVotingParallelMessage::ApprovedAncestor(
							subchain_head,
							target_number,
							tx,
						),
						std::any::type_name::<Self>(),
					)
					.await;
			} else {
				overseer
					.send_msg(
						ApprovalVotingMessage::ApprovedAncestor(subchain_head, target_number, tx),
						std::any::type_name::<Self>(),
					)
					.await;
			}
			match rx
				.await
				.map_err(Error::ApprovedAncestorCanceled)
				.map_err(|e| ConsensusError::Other(Box::new(e)))?
			{
				// No approved ancestors means target hash is maximal vote.
				None => (target_hash, target_number, Vec::new()),
				Some(HighestApprovedAncestorBlock { number, hash, descriptions }) =>
					(hash, number, descriptions),
			}
		};

		gum::trace!(target: LOG_TARGET, ?subchain_head, "Ancestor approval restriction applied",);

		let lag = initial_leaf_number.saturating_sub(subchain_number);
		self.metrics.note_approval_checking_finality_lag(lag);

		// Messages sent to `approval-distribution` are known to have high `ToF`, we need to spawn a
		// task for sending the message to not block here and delay finality.
		if let Some(spawn_handle) = &self.spawn_handle {
			let mut overseer_handle = self.overseer.clone();
			let approval_voting_parallel_enabled = self.approval_voting_parallel_enabled;
			let lag_update_task = async move {
				if approval_voting_parallel_enabled {
					overseer_handle
						.send_msg(
							ApprovalVotingParallelMessage::ApprovalCheckingLagUpdate(lag),
							std::any::type_name::<Self>(),
						)
						.await;
				} else {
					overseer_handle
						.send_msg(
							ApprovalDistributionMessage::ApprovalCheckingLagUpdate(lag),
							std::any::type_name::<Self>(),
						)
						.await;
				}
			};

			spawn_handle.spawn(
				"approval-checking-lag-update",
				Some("relay-chain-selection"),
				Box::pin(lag_update_task),
			);
		}

		let (lag, subchain_head) = {
			// Prevent sending flawed data to the dispute-coordinator.
			if Some(subchain_block_descriptions.len() as _) !=
				subchain_number.checked_sub(target_number)
			{
				gum::error!(
					LOG_TARGET,
					present_block_descriptions = subchain_block_descriptions.len(),
					target_number,
					subchain_number,
					"Mismatch of anticipated block descriptions and block number difference.",
				);
				return Ok(target_hash)
			}
			// 3. Constrain according to disputes:
			let (tx, rx) = oneshot::channel();
			overseer
				.send_msg(
					DisputeCoordinatorMessage::DetermineUndisputedChain {
						base: (target_number, target_hash),
						block_descriptions: subchain_block_descriptions,
						tx,
					},
					std::any::type_name::<Self>(),
				)
				.await;

			// Try to fetch response from `dispute-coordinator`. If an error occurs we just log it
			// and return `target_hash` as maximal vote. It is safer to contain this error here
			// and not push it up the stack to cause additional issues in GRANDPA/BABE.
			let (lag, subchain_head) =
				match rx.await.map_err(Error::DetermineUndisputedChainCanceled) {
					// If request succeeded we will receive (block number, block hash).
					Ok((subchain_number, subchain_head)) => {
						// The total lag accounting for disputes.
						let lag_disputes = initial_leaf_number.saturating_sub(subchain_number);
						self.metrics.note_disputes_finality_lag(lag_disputes);
						(lag_disputes, subchain_head)
					},
					Err(e) => {
						gum::error!(
							target: LOG_TARGET,
							error = ?e,
							"Call to `DetermineUndisputedChain` failed",
						);
						// We need to return a sane finality target. But, we are unable to ensure we
						// are not finalizing something that is being disputed or has been concluded
						// as invalid. We will be conservative here and not vote for finality above
						// the ancestor passed in.
						return Ok(target_hash)
					},
				};
			(lag, subchain_head)
		};

		gum::trace!(
			target: LOG_TARGET,
			?subchain_head,
			"Disputed blocks in ancestry restriction applied",
		);

		// 4. Apply the maximum safeguard to the finality lag.
		if lag > MAX_FINALITY_LAG {
			// We need to constrain our vote as a safety net to
			// ensure the network continues to finalize.
			let safe_target = initial_leaf_number - MAX_FINALITY_LAG;

			if safe_target <= target_number {
				gum::warn!(target: LOG_TARGET, ?target_hash, "Safeguard enforced finalization");
				// Minimal vote needs to be on the target number.
				Ok(target_hash)
			} else {
				// Otherwise we're looking for a descendant.
				let initial_leaf_header = self.block_header(initial_leaf)?;
				let (forced_target, _) = crate::grandpa_support::walk_backwards_to_target_block(
					self.backend.header_provider(),
					safe_target,
					&initial_leaf_header,
				)
				.map_err(|e| ConsensusError::ChainLookup(format!("{:?}", e)))?;

				gum::warn!(
					target: LOG_TARGET,
					?forced_target,
					"Safeguard enforced finalization of child"
				);

				Ok(forced_target)
			}
		} else {
			Ok(subchain_head)
		}
	}
}