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Make light clients gossip block announces to each other #2226

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Apr 8, 2022
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Make light clients gossip block announces to each other #2226

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db60c21
Make light clients gossip block announces to each other
tomaka Apr 8, 2022
5effbe9
CHANGELOG PR
tomaka Apr 8, 2022
ea51d5f
Oops, useful -> useless
tomaka Apr 8, 2022
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15 changes: 15 additions & 0 deletions bin/light-base/src/network_service.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -49,6 +49,7 @@ use smoldot::{
connection::{self, handshake},
multiaddr::Multiaddr,
peer_id::PeerId,
peers,
read_write::ReadWrite,
},
network::{protocol, service},
Expand Down Expand Up @@ -927,6 +928,20 @@ impl<TPlat: Platform> NetworkService<TPlat> {
sent_peers
}

/// See [`service::ChainNetwork::send_block_announce`].
pub async fn send_block_announce(
self: Arc<Self>,
target: &PeerId,
chain_index: usize,
scale_encoded_header: &[u8],
is_best: bool,
) -> Result<(), peers::QueueNotificationError> {
self.inner
.network
.send_block_announce(&target, chain_index, scale_encoded_header, is_best)
.await
}

/// See [`service::ChainNetwork::discover`].
///
/// The `important_nodes` parameter indicates whether these nodes are considered note-worthy
Expand Down
99 changes: 92 additions & 7 deletions bin/light-base/src/sync_service/standalone.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -28,7 +28,7 @@ use smoldot::{
trie::proof_verify,
};
use std::{
collections::HashMap,
collections::{HashMap, HashSet},
marker::PhantomData,
num::{NonZeroU32, NonZeroU64},
sync::Arc,
Expand Down Expand Up @@ -120,7 +120,7 @@ pub(super) async fn start_standalone_chain<TPlat: Platform>(
// verifying storage proof.
// If the state is one of the "verifying" states, perform the actual verification
// and set ̀`queue_empty` to `false`.
let (task_update, has_done_verif) = task.process_one_verification_queue();
let (task_update, has_done_verif) = task.process_one_verification_queue().await;
task = task_update;

if has_done_verif {
Expand Down Expand Up @@ -579,7 +579,7 @@ impl<TPlat: Platform> Task<TPlat> {
/// verification.
///
/// Returns ̀`self` and a boolean indicating whether something has been processed.
fn process_one_verification_queue(mut self) -> (Self, bool) {
async fn process_one_verification_queue(mut self) -> (Self, bool) {
// Note that `process_one` moves out of `sync` and provides the value back in its
// return value.
match self.sync.process_one() {
Expand Down Expand Up @@ -609,6 +609,7 @@ impl<TPlat: Platform> Task<TPlat> {
all::ProcessOne::VerifyHeader(verify) => {
// Header to verify.
let verified_hash = verify.hash();
let verified_height = verify.height();
match verify.perform(TPlat::now_from_unix_epoch(), ()) {
all::HeaderVerifyOutcome::Success {
sync, is_new_best, ..
Expand All @@ -626,18 +627,102 @@ impl<TPlat: Platform> Task<TPlat> {
self.network_up_to_date_best = false;
}

// Notify of the new block.
self.dispatch_all_subscribers({
let (parent_hash, scale_encoded_header) = {
// TODO: the code below is `O(n)` complexity
let header = self
.sync
.non_finalized_blocks_unordered()
.find(|h| h.hash() == verified_hash)
.unwrap();
(*header.parent_hash, header.scale_encoding_vec())
};

// Announce the newly-verified block to all the light client sources that
// might not be aware of it. We can never be guaranteed that a certain
// source does *not* know about a block, however it is not a big problem
// to send a block announce to a source that already knows about that
// block. For this reason, the list of sources we send the block announce
// to is `all_sources - sources_that_know_it`.
//
// Note that not sending block announces to sources that already know that
// block means that these sources might also miss the fact that our local
// best block has been updated. This is in practice not a problem either.
//
// Block announces are intentionally sent only to light clients, and not
// to full nodes. Block announces coming from light clients are useful to
// full nodes, as they can't download the block body (which they need)
// from that light client.
//
// Announcing blocks to other light clients increases the likelihood that
// equivocations are detected by light clients. This is especially
// important for light clients, as they try to connect to as few full
// nodes as possible.
let sources_to_announce_to = {
let mut all_sources = self
.sync
.sources()
.filter(|s| matches!(self.sync[*s].1, protocol::Role::Light))
.collect::<HashSet<_, fnv::FnvBuildHasher>>();
for knows in self
.sync
.knows_non_finalized_block(verified_height, &verified_hash)
{
all_sources.remove(&knows);
}
all_sources
};

for source_id in sources_to_announce_to {
// The `PeerId` needs to be cloned, otherwise `self` would have to
// stay borrowed accross an `await`, which isn't possible because it
// doesn't implement `Sync`.
let (source_peer_id, _source_role) = &self.sync[source_id].clone();
debug_assert!(matches!(_source_role, protocol::Role::Light));

if self
.network_service
.clone()
.send_block_announce(
&source_peer_id,
self.network_chain_index,
&scale_encoded_header,
is_new_best,
)
.await
.is_ok()
{
log::debug!(
target: &self.log_target,
"Network <= BlockAnnounce(peer_id={}, hash={})",
source_peer_id,
HashDisplay(&verified_hash)
);

// Update the sync state machine with the fact that the target of
// the block announce now knows this block.
//
// This code is never called for full nodes. When it comes to full
// nodes, we want track knowledge about block bodies and storage
// rather than just headers.
//
// Note that `try_add_known_block_to_source` might have
// no effect, which is not a problem considering that this
// block tracking is mostly about optimizations and
// politeness.
self.sync.try_add_known_block_to_source(
source_id,
verified_height,
verified_hash,
);
}
}

// Notify of the new block.
self.dispatch_all_subscribers({
Notification::Block(BlockNotification {
is_new_best,
scale_encoded_header: header.scale_encoding_vec(),
parent_hash: *header.parent_hash,
scale_encoded_header,
parent_hash,
})
});
}
Expand Down
4 changes: 4 additions & 0 deletions bin/wasm-node/CHANGELOG.md
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,10 @@

## Unreleased

### Changed

- Block announces are now propagated to other peers that are also light clients. Light clients should try to connect to as few full nodes as possible (to save resources), but doing so can leave them vulnerable to eclipse attacks. By having light clients connect to other light clients and making them gossip block announces to each other, we increase the likelihood that they detect situations where a given validator generates two blocks during the same slot and is trying to show one of the block only to some peers and the other block to the rest. ([#2226](https://github.com/paritytech/smoldot/pull/2226))

## 0.6.15 - 2022-04-07

### Fixed
Expand Down