Assert that all onion error messages are correct len in tests

When we're constructing an HTLCFailReason, we should check that we
set the data to at least the correct length for the given failure
code, which we do here.

Fixes #1879

lightning/src/ln/onion_utils.rs

@@ -642,11 +642,48 @@ impl_writeable_tlv_based_enum!(HTLCFailReasonRepr,
 
 impl HTLCFailReason {
 	pub(super) fn reason(failure_code: u16, data: Vec<u8>) -> Self {
+		const BADONION: u16 = 0x8000;
+		const PERM: u16 = 0x4000;
+		const NODE: u16 = 0x2000;
+		const UPDATE: u16 = 0x1000;
+
+		     if failure_code == 1  | PERM { debug_assert!(data.is_empty()) }
+		else if failure_code == 2  | NODE { debug_assert!(data.is_empty()) }
+		else if failure_code == 3  | PERM | NODE { debug_assert!(data.is_empty()) }
+		else if failure_code == 4  | BADONION | PERM { debug_assert_eq!(data.len(), 32) }
+		else if failure_code == 5  | BADONION | PERM { debug_assert_eq!(data.len(), 32) }
+		else if failure_code == 6  | BADONION | PERM { debug_assert_eq!(data.len(), 32) }
+		else if failure_code == 7  | UPDATE {
+			debug_assert_eq!(data.len() - 2, u16::from_be_bytes(data[0..2].try_into().unwrap()) as usize) }
+		else if failure_code == 8  | PERM { debug_assert!(data.is_empty()) }
+		else if failure_code == 9  | PERM { debug_assert!(data.is_empty()) }
+		else if failure_code == 10 | PERM { debug_assert!(data.is_empty()) }
+		else if failure_code == 11 | UPDATE {
+			debug_assert_eq!(data.len() - 2, u16::from_be_bytes(data[8..10].try_into().unwrap()) as usize) }
+		else if failure_code == 12 | UPDATE {
+			debug_assert_eq!(data.len() - 2, u16::from_be_bytes(data[8..10].try_into().unwrap()) as usize) }
+		else if failure_code == 13 | UPDATE {
+			debug_assert_eq!(data.len() - 2, u16::from_be_bytes(data[4..6].try_into().unwrap()) as usize) }
+		else if failure_code == 14 | UPDATE {
+			debug_assert_eq!(data.len() - 2, u16::from_be_bytes(data[0..2].try_into().unwrap()) as usize) }
+		else if failure_code == 15 | PERM { debug_assert_eq!(data.len(), 12) }
+		else if failure_code == 18 { debug_assert_eq!(data.len(), 4) }
+		else if failure_code == 19 { debug_assert_eq!(data.len(), 8) }
+		else if failure_code == 20 | UPDATE {
+			debug_assert_eq!(data.len() - 2, u16::from_be_bytes(data[2..4].try_into().unwrap()) as usize) }
+		else if failure_code == 21 { debug_assert!(data.is_empty()) }
+		else if failure_code == 22 | PERM { debug_assert!(data.len() <= 11) }
+		else if failure_code == 23 { debug_assert!(data.is_empty()) }
+		else if failure_code & BADONION != 0 {
+			// We set some bogus BADONION failure codes in test, so ignore unknown ones.
+		}
+		else { debug_assert!(false, "Unknown failure code: {}", failure_code) }
+
 		Self(HTLCFailReasonRepr::Reason { failure_code, data })
 	}
 
 	pub(super) fn from_failure_code(failure_code: u16) -> Self {
-		Self(HTLCFailReasonRepr::Reason { failure_code, data: Vec::new() })
+		Self::reason(failure_code, Vec::new())
 	}
 
 	pub(super) fn from_msg(msg: &msgs::UpdateFailHTLC) -> Self {