fuzz: Add cross-version ChannelMonitor roundtrip coverage

Author: Atishyy27

fuzz/Cargo.toml

@@ -11,6 +11,7 @@ lightning-invoice = { path = "../lightning-invoice" }
 lightning-liquidity = { path = "../lightning-liquidity" }
 lightning-rapid-gossip-sync = { path = "../lightning-rapid-gossip-sync" }
 lightning-persister = { path = "../lightning-persister", features = ["tokio"]}
+lightning_0_2 = { package = "lightning", version = "0.2.0", features = ["_test_utils"] }
 bech32 = "0.11.0"
 bitcoin = { version = "0.32.4", features = ["secp-lowmemory"] }
 tokio = { version = "~1.35", default-features = false, features = ["rt-multi-thread"] }

fuzz/src/chanmon_consistency.rs

@@ -971,9 +971,25 @@ impl<'a> HarnessNode<'a> {
 				};
 				// Use a different value of `use_old_mons` if we have another monitor
 				// (only for node B) by shifting `use_old_mons` one in base-3.
-				use_old_mons /= 3;
+				let mut monitor_bytes = serialized_mon.clone();
+				if use_old_mons % 3 == 2 {
+                    let keys_manager_0_2 = lightning_0_2::sign::KeysManager::new(&[0u8; 32], 0, 0, false);
+                    let read_res: Result<(
+                        bitcoin::BlockHash, 
+                        lightning_0_2::chain::channelmonitor::ChannelMonitor<lightning_0_2::sign::InMemorySigner>
+                    ), _> = lightning_0_2::util::ser::ReadableArgs::read(
+                        &mut &monitor_bytes[..],
+                        (&keys_manager_0_2, &keys_manager_0_2)
+                    );
+
+                    if let Ok((block_hash_0_2, old_monitor)) = read_res {
+                        let mut reserialized = Vec::new();
+                        lightning_0_2::util::ser::Writeable::write(&(block_hash_0_2, &old_monitor), &mut reserialized).unwrap();
+                        monitor_bytes = reserialized;
+                    }
+                }
 				let mon = <(BlockLocator, ChannelMonitor<TestChannelSigner>)>::read(
-					&mut &serialized_mon[..],
+                    &mut &monitor_bytes[..],
 					(&*self.keys_manager, &*self.keys_manager),
 				)
 				.expect("Failed to read monitor");