Stop writing signer data as a part of channels

This breaks backwards compatibility with versions of LDK prior to
0.0.113 as they expect to always read signer data.

This also substantially reduces allocations during `ChannelManager`
serialization, as we currently don't pre-allocate the `Vec` that
the signer gets written in to. We could alternatively pre-allocate
that `Vec`, but we've been set up to skip the write entirely for a
while, and 0.0.113 was released nearly a year ago. Users
downgrading to LDK 0.0.112 and before at this point should not be
expected.

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -6892,7 +6892,7 @@ impl<SP: Deref> InboundV1Channel<SP> where SP::Target: SignerProvider {
 }
 
 const SERIALIZATION_VERSION: u8 = 3;
-const MIN_SERIALIZATION_VERSION: u8 = 2;
+const MIN_SERIALIZATION_VERSION: u8 = 3;
 
 impl_writeable_tlv_based_enum!(InboundHTLCRemovalReason,;
 	(0, FailRelay),
@@ -6972,14 +6972,6 @@ impl<SP: Deref> Writeable for Channel<SP> where SP::Target: SignerProvider {
 
 		self.context.latest_monitor_update_id.write(writer)?;
 
-		let mut key_data = VecWriter(Vec::new());
-		// TODO (taproot|arik): Introduce serialization distinction for non-ECDSA signers.
-		self.context.holder_signer.as_ecdsa().expect("Only ECDSA signers may be serialized").write(&mut key_data)?;
-		assert!(key_data.0.len() < core::usize::MAX);
-		assert!(key_data.0.len() < core::u32::MAX as usize);
-		(key_data.0.len() as u32).write(writer)?;
-		writer.write_all(&key_data.0[..])?;
-
 		// Write out the old serialization for shutdown_pubkey for backwards compatibility, if
 		// deserialized from that format.
 		match self.context.shutdown_scriptpubkey.as_ref().and_then(|script| script.as_legacy_pubkey()) {

pending_changelog/113-channel-ser-compat.txt

@@ -0,0 +1,4 @@
+ * `ChannelManager`s written with LDK 0.0.119 are no longer readable by versions
+   of LDK prior to 0.0.113. Users wishing to downgrade to LDK 0.0.112 or before
+   can read an 0.0.119-serialized `ChannelManager` with a version of LDK from
+   0.0.113 to 0.0.118, re-serialize it, and then downgrade.