f Further document edge case where HTLC claiming can fail

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -1973,9 +1973,13 @@ impl<ChanSigner: ChannelKeys, M: Deref, T: Deref, K: Deref, F: Deref> ChannelMan
 					return Ok(())
 				},
 				Err(e) => {
-					// TODO: There is probably a channel manager somewhere that needs to
-					// learn the preimage as the channel may be about to hit the chain.
 					// TODO: Do something with e?
+					// This should only occur if we are claiming an HTLC at the same time as the
+					// HTLC is being failed (eg because a block is being connected and this caused
+					// an HTLC to time out). This should, of course, only occur if the user is the
+					// one doing the claiming (as it being a part of a peer claim would imply we're
+					// about to lose funds) and only if the lock in claim_funds was dropped as a
+					// previous HTLC was failed (thus not for an MPP payment).
 					debug_assert!(false, "This shouldn't be reachable except in absurdly rare cases between monitor updates and HTLC timeouts: {:?}", e);
 					return Err(None)
 				},