Document exactly our CLTV sanitization policy for final incoming HTLCs

We want to avoid a third-party channel closure, where a random node
by sending us a payment expiring at current height, would trigger our
onchain logic to close the channel due to a near-expiration.

PaymentReceived and unknown HTLC cancellation must happen before
counterparty close channel for timing out onchain.

Author: Antoine Riard

lightning/src/ln/channel.rs

@@ -1727,8 +1727,6 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
 			return Err(ChannelError::Close("Remote provided CLTV expiry in seconds instead of block height"));
 		}
 
-		//TODO: Check msg.cltv_expiry further? Do this in channel manager?
-
 		if self.channel_state & ChannelState::LocalShutdownSent as u32 != 0 {
 			if let PendingHTLCStatus::Forward(_) = pending_forward_state {
 				panic!("ChannelManager shouldn't be trying to add a forwardable HTLC after we've started closing");

lightning/src/ln/channelmanager.rs

@@ -1039,7 +1039,11 @@ impl<ChanSigner: ChannelKeys, M: Deref, T: Deref, K: Deref, F: Deref> ChannelMan
 
 				// OUR PAYMENT!
 				// final_expiry_too_soon
-				if (msg.cltv_expiry as u64) < self.latest_block_height.load(Ordering::Acquire) as u64 + (CLTV_CLAIM_BUFFER + LATENCY_GRACE_PERIOD_BLOCKS) as u64 {
+				// We have to have some headroom to broadcast on chain if we have the preimage, so make sure we have at least
+				// HTLC_FAIL_BACK_BUFFER blocks to go.
+				// Also, ensure that, in the case of an unknown payment hash, our payment logic has enough time to fail the HTLC backward
+				// before our onchain logic triggers a channel closure (see HTLC_FAIL_BACK_BUFFER rational).
+				if (msg.cltv_expiry as u64) <= self.latest_block_height.load(Ordering::Acquire) as u64 + HTLC_FAIL_BACK_BUFFER as u64 + 1 {
 					return_err!("The final CLTV expiry is too soon to handle", 17, &[0;0]);
 				}
 				// final_incorrect_htlc_amount

lightning/src/util/events.rs

@@ -52,7 +52,10 @@ pub enum Event {
 	/// Indicates we've received money! Just gotta dig out that payment preimage and feed it to
 	/// ChannelManager::claim_funds to get it....
 	/// Note that if the preimage is not known or the amount paid is incorrect, you must call
-	/// ChannelManager::fail_htlc_backwards to free up resources for this HTLC.
+	/// ChannelManager::fail_htlc_backwards to free up resources for this HTLC before
+	/// counterparty close channel for timing out HTLC onchain. You should assume counterparty
+	/// goes onchain few blocks before HTLC cltv_expiry to get its commitment already confirmed
+	/// at timelock expiration.
 	/// The amount paid should be considered 'incorrect' when it is less than or more than twice
 	/// the amount expected.
 	/// If you fail to call either ChannelManager::claim_funds or