Correctly update the last block hash on disconnect

When a block is disconnected, the hash of the disconnected block was
used to update the last connected block. However, this amounts to a
no-op because these hashes should be equal. Successive disconnections
would update the hash but leave it one block off.

Normally, this not a problem because the last block_disconnected should
be followed by block_connected since the former is triggered by a chain
re-org. However, this assumes the user calls the API correctly and that
no failure occurs that would prevent block_connected from being called
(e.g., if fetching the connected block fails).

Instead, update the last block hash with the disconnected block's
previous block hash.

Author: jkczyz

lightning/src/chain/channelmonitor.rs

@@ -2090,8 +2090,7 @@ impl<Signer: Sign> ChannelMonitorImpl<Signer> {
 		      F::Target: FeeEstimator,
 		      L::Target: Logger,
 	{
-		let block_hash = header.block_hash();
-		log_trace!(logger, "Block {} at height {} disconnected", block_hash, height);
+		log_trace!(logger, "Block {} at height {} disconnected", header.block_hash(), height);
 
 		if let Some(_) = self.onchain_events_waiting_threshold_conf.remove(&(height + ANTI_REORG_DELAY - 1)) {
 			//We may discard:
@@ -2101,7 +2100,7 @@ impl<Signer: Sign> ChannelMonitorImpl<Signer> {
 
 		self.onchain_tx_handler.block_disconnected(height, broadcaster, fee_estimator, logger);
 
-		self.last_block_hash = block_hash;
+		self.last_block_hash = header.prev_blockhash;
 	}
 
 	/// Filters a block's `txdata` for transactions spending watched outputs or for any child

lightning/src/ln/channelmanager.rs

@@ -3327,7 +3327,7 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 				}
 			});
 
-			*self.last_block_hash.lock().unwrap() = header.block_hash();
+			*self.last_block_hash.lock().unwrap() = header.prev_blockhash;
 			self.latest_block_height.fetch_sub(1, Ordering::AcqRel);
 		}