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Allow download of old blocks from peers with equal or lower difficulty #9226

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Jul 27, 2018
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Allow download of old blocks from peers with equal or lower difficulty #9226

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16 changes: 13 additions & 3 deletions ethcore/sync/src/chain/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -761,14 +761,24 @@ impl ChainSync {
}
}

// Only ask for old blocks if the peer has a higher difficulty
if force || higher_difficulty {
// Only ask for old blocks if the peer has a higher difficulty than the last imported old block
let last_imported_old_block_difficulty = self.old_blocks.as_mut().and_then(|d| {
io.chain().block_total_difficulty(BlockId::Number(d.last_imported_block_number()))
});

if force || last_imported_old_block_difficulty.map_or(true, |ld| peer_difficulty.map_or(true, |pd| pd > ld)) {
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I'm not sure we should request old blocks if the peer's difficulty is unknown, or our last imported old block diff. is unknown.

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In the case of the peer's difficulty being unknown, I was copying the existing behaviour: https://github.com/paritytech/parity-ethereum/blob/82a7ed2f912f22df78ee2417964f82d5a2ecfc43/ethcore/sync/src/chain/mod.rs#L731. So I guess we're being optimistic and sending the request anyway, we're assuming that most of the time we know the peer difficulty.

Not so sure about the case where the imported block diff is not known. That probably would mean (assuming old_blocks is Some) that last_imported_block_number has not been imported for whatever reason. So what to do in that case? Better to send the request anyway or not? Might be okay since it should almost always be Some, and in case it is None it might be okay to ask for it again.

if let Some(request) = self.old_blocks.as_mut().and_then(|d| d.request_blocks(io, num_active_peers)) {
SyncRequester::request_blocks(self, io, peer_id, request, BlockSet::OldBlocks);
return;
}
} else {
trace!(target: "sync", "peer {} is not suitable for asking old blocks", peer_id);
trace!(
target: "sync",
"peer {:?} is not suitable for requesting old blocks, last_imported_old_block_difficulty={:?}, peer_difficulty={:?}",
peer_id,
last_imported_old_block_difficulty,
peer_difficulty
);
self.deactivate_peer(io, peer_id);
}
},
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