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Fix wrong ref counting #1358

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merged 3 commits into from
Sep 1, 2023
Merged

Fix wrong ref counting #1358

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052c14b
Fix wrong ref counting
Sep 1, 2023
0ecc208
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Sep 1, 2023
2706a61
Merge branch 'master' into rk-fix-wrong-refcounting
eskimor Sep 1, 2023
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4 changes: 2 additions & 2 deletions polkadot/node/core/dispute-coordinator/src/initialized.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1271,7 +1271,7 @@ impl Initialized {
if import_result.has_fresh_byzantine_threshold_against() {
let blocks_including = self.scraper.get_blocks_including_candidate(&candidate_hash);
for (parent_block_number, parent_block_hash) in &blocks_including {
gum::trace!(
gum::warn!(
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target: LOG_TARGET,
?candidate_hash,
?parent_block_number,
Expand All @@ -1282,7 +1282,7 @@ impl Initialized {
if blocks_including.len() > 0 {
ctx.send_message(ChainSelectionMessage::RevertBlocks(blocks_including)).await;
} else {
gum::debug!(
gum::warn!(
target: LOG_TARGET,
?candidate_hash,
?session,
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11 changes: 8 additions & 3 deletions polkadot/node/core/dispute-coordinator/src/scraping/candidates.rs
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Expand Up @@ -135,11 +135,14 @@ impl ScrapedCandidates {
}

pub fn insert(&mut self, block_number: BlockNumber, candidate_hash: CandidateHash) {
self.candidates.insert(candidate_hash);
self.candidates_by_block_number
if self
.candidates_by_block_number
.entry(block_number)
.or_default()
.insert(candidate_hash);
.insert(candidate_hash)
{
self.candidates.insert(candidate_hash);
}
Comment on lines +138 to +145
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How is this different from previous logic? I don't really understand, it should have worked for multiple blocks per height previously. This if will fail only in case of insertion of the same CandidateHash, which shouldn't happen, but even then it would be the same as previously.

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Oh, it's because of ref counting. nvm

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No, because the inserts in candidates is ref counted. Therefore if we insert at the same height multiple times, we would increase the refcount each time. But in cleanup we would only decrement it once, hence we would never clean it.

Demonstration: The modified test fails without that change (pruning does not happen).

In case of forks there can easily be the same candidate included twice at the same height.

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But this will only happen if we insert the same candidate (hash), not different candidates at the same height, no? How is this possible?

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I guess if there are multiple forks at the same height including the same candidate.

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Yes same hash, same height.

     - B
A<
     - B'

A candidate gets backed in A, afterwards we get a fork. Bitfields will be imported in both forks. Result: same candidate included at the same height.

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This is really nasty. Good catch!

}

// Used only for tests to verify the pruning doesn't leak data.
Expand All @@ -159,6 +162,8 @@ mod scraped_candidates_tests {
let mut candidates = ScrapedCandidates::new();
let target = CandidateHash(BlakeTwo256::hash(&vec![1, 2, 3]));
candidates.insert(1, target);
// Repeated inserts at same height should be fine:
candidates.insert(1, target);

assert!(candidates.contains(&target));

Expand Down