Fix a bug when a migration is followed by a rebalance

Nodes would stabilize on different encoding formats for the values,
some having the pre-migration format and some having the post-migration
format. This would be reflected in the Merkle trees never converging
and thus having an infinite resync loop.

1 files changed, 18 insertions, 9 deletions

diff --git a/src/table/data.rs b/src/table/data.rs
index 7af5f552..d7787b6b 100644
--- a/src/table/data.rs
+++ b/src/table/data.rs
@@ -130,22 +130,31 @@ where
 		let tree_key = self.tree_key(update.partition_key(), update.sort_key());
 
 		let changed = (&self.store, &self.merkle_todo).transaction(|(store, mkl_todo)| {
-			let (old_entry, new_entry) = match store.get(&tree_key)? {
-				Some(prev_bytes) => {
+			let (old_entry, old_bytes, new_entry) = match store.get(&tree_key)? {
+				Some(old_bytes) => {
 					let old_entry = self
-						.decode_entry(&prev_bytes)
+						.decode_entry(&old_bytes)
 						.map_err(sled::transaction::ConflictableTransactionError::Abort)?;
 					let mut new_entry = old_entry.clone();
 					new_entry.merge(&update);
-					(Some(old_entry), new_entry)
+					(Some(old_entry), Some(old_bytes), new_entry)
 				}
-				None => (None, update.clone()),
+				None => (None, None, update.clone()),
 			};
 
-			if Some(&new_entry) != old_entry.as_ref() {
-				let new_bytes = rmp_to_vec_all_named(&new_entry)
-					.map_err(Error::RmpEncode)
-					.map_err(sled::transaction::ConflictableTransactionError::Abort)?;
+			// Scenario 1: the value changed, so of course there is a change
+			let value_changed = Some(&new_entry) != old_entry.as_ref();
+
+			// Scenario 2: the value didn't change but due to a migration in the
+			// data format, the messagepack encoding changed. In this case
+			// we have to write the migrated value in the table and update
+			// the associated Merkle tree entry.
+			let new_bytes = rmp_to_vec_all_named(&new_entry)
+				.map_err(Error::RmpEncode)
+				.map_err(sled::transaction::ConflictableTransactionError::Abort)?;
+			let encoding_changed = Some(&new_bytes[..]) != old_bytes.as_ref().map(|x| &x[..]);
+
+			if value_changed || encoding_changed {
 				let new_bytes_hash = blake2sum(&new_bytes[..]);
 				mkl_todo.insert(tree_key.clone(), new_bytes_hash.as_slice())?;
 				store.insert(tree_key.clone(), new_bytes)?;