Fix unknown handling in `impl_writeable_tlv_based_enum_upgradable`

`impl_writeable_tlv_based_enum_upgradable` professed to supporting
upgrades by returning `None` from `MaybeReadable` when unknown
variants written by newer versions of LDK were read. However, it
generally didn't support this as it didn't discard bytes for
unknown types, resulting in corrupt reading.

This is fixed here for enum variants written as a TLV stream,
however we don't have a length prefix for tuple enum variants, so
the documentation on the macro is updated to mention that
downgrades are not supported for tuple variants.

diff --git a/lightning/src/util/ser_macros.rs b/lightning/src/util/ser_macros.rs
index 84d9f7a180bb301a1c18a136530a04ce75589a9e..5d6988ba1f193585c0d325c49fa679e52b4553c5 100644 (file)
--- a/lightning/src/util/ser_macros.rs
+++ b/lightning/src/util/ser_macros.rs
@@ -1065,6 +1065,10 @@ macro_rules! impl_writeable_tlv_based_enum {
 /// when [`MaybeReadable`] is practical instead of just [`Readable`] as it provides an upgrade path for
 /// new variants to be added which are simply ignored by existing clients.
 ///
+/// Note that only struct and unit variants (not tuple variants) will support downgrading, thus any
+/// new odd variants MUST be non-tuple (i.e. described using `$variant_id` and `$variant_name` not
+/// `$tuple_variant_id` and `$tuple_variant_name`).
+///
 /// [`MaybeReadable`]: crate::util::ser::MaybeReadable
 /// [`Writeable`]: crate::util::ser::Writeable
 /// [`DecodeError::UnknownRequiredFeature`]: crate::ln::msgs::DecodeError::UnknownRequiredFeature
@@ -1102,7 +1106,14 @@ macro_rules! impl_writeable_tlv_based_enum_upgradable {
                                        $($($tuple_variant_id => {
                                                Ok(Some($st::$tuple_variant_name(Readable::read(reader)?)))
                                        }),*)*
-                                       _ if id % 2 == 1 => Ok(None),
+                                       _ if id % 2 == 1 => {
+                                               // Assume that a $variant_id was written, not a $tuple_variant_id, and read
+                                               // the length prefix and discard the correct number of bytes.
+                                               let tlv_len: $crate::util::ser::BigSize = $crate::util::ser::Readable::read(reader)?;
+                                               let mut rd = $crate::util::ser::FixedLengthReader::new(reader, tlv_len.0);
+                                               rd.eat_remaining().map_err(|_| $crate::ln::msgs::DecodeError::ShortRead)?;
+                                               Ok(None)
+                                       },
                                        _ => Err($crate::ln::msgs::DecodeError::UnknownRequiredFeature),
                                }
                        }