network: Investigate importing block with unknown parent

[lexnv]

Long running litep2p and libp2p nodes produced a high number of warnings wrt:

litep2p
| 516        | warn       | 💔 Error importing block .*: .* ( block has an unknown parent    ) |

libp2p
| 506        | warn       | 💔 Error importing block .*: .* ( block has an unknown parent    ) |

The warning is coming from:

polkadot-sdk/substrate/client/network/sync/src/strategy/chain_sync.rs

Lines 1857 to 1861 in 01e0fc2

	e @ Err(BlockImportError::UnknownParent) | e @ Err(BlockImportError::Other(_)) => {
	warn!(target: LOG_TARGET, "💔 Error importing block {hash:?}: {}", e.unwrap_err());
	self.state_sync = None;
	self.restart();
	},

And this is causing the sync process to restart, which may affect other issues like:

• Banned node for "Same state requested multiple times" #4924

polkadot-sdk/substrate/client/network/sync/src/strategy/chain_sync.rs

Lines 1303 to 1306 in 01e0fc2

	/// Restart the sync process. This will reset all pending block requests and return an iterator
	/// of new block requests to make to peers. Peers that were downloading finality data (i.e.
	/// their state was `DownloadingJustification`) are unaffected and will stay in the same state.
	fn restart(&mut self) {

[lexnv]

Might be coming from:

• loosing queue_blocks status information

polkadot-sdk/substrate/client/network/sync/src/strategy/chain_sync.rs

Lines 1401 to 1407 in 01e0fc2

	/// What is the status of the block corresponding to the given hash?
	fn block_status(&self, hash: &B::Hash) -> Result<BlockStatus, ClientError> {
	if self.queue_blocks.contains(hash) {
	return Ok(BlockStatus::Queued)
	}
	self.client.block_status(*hash)
	}

• Note: we might not track the queued_blocks properly (ie if self.queue_blocks.len() > MAX_IMPORTING_BLOCKS > 2k blocks)

• lower level

polkadot-sdk/substrate/client/state-db/src/noncanonical.rs

Lines 276 to 286 in 01e0fc2

	if number == front_block_number {
	if !self
	.last_canonicalized
	.as_ref()
	.map_or(false, |&(ref h, n)| h == parent_hash && n == number - 1)
	{
	return Err(StateDbError::InvalidParent)
	}
	} else if !self.parents.contains_key(parent_hash) {
	return Err(StateDbError::InvalidParent)
	}