Add migrate_kv_store_data util method

.. allowing to migrate data from one store to another.

lightning/src/util/persist.rs

@@ -176,6 +176,53 @@ pub trait MigratableKVStore: KVStore {
 	fn list_all_keys(&self) -> Result<Vec<(String, String, String)>, io::Error>;
 }
 
+/// Migrates all data from one store to another.
+///
+/// This operation assumes that `target_store` is empty, i.e., any data present under copied keys
+/// might get overriden. User must ensure `source_store` is not modified during operation,
+/// otherwise no consistency guarantees can be given.
+///
+/// Will abort and return an error if any IO operation fails. Note that in this case the
+/// `target_store` might get left in an intermediate state.
+///
+/// **Caution**: Will delete all data from `source_store` if `move_data` is set.
+pub fn migrate_kv_store_data<S: MigratableKVStore, T: MigratableKVStore>(
+	source_store: &mut S, target_store: &mut T, move_data: bool,
+) -> Result<(), io::Error> {
+	let keys_to_migrate = source_store.list_all_keys()?;
+
+	// First copy over all data.
+	for (primary_namespace, secondary_namespace, key) in &keys_to_migrate {
+		let data = source_store.read(primary_namespace, secondary_namespace, key)?;
+		target_store.write(primary_namespace, secondary_namespace, key, &data)?;
+
+		// Now validate what we've written.
+		let read_data = target_store.read(primary_namespace, secondary_namespace, &key)?;
+		if data != read_data {
+			let err =
+				io::Error::new(io::ErrorKind::InvalidData, "Failed to validate migrated data");
+			return Err(err);
+		}
+	}
+
+	// If we succeeded and `move_data` is set, remove all migrated keys.
+	if move_data {
+		for (primary_namespace, secondary_namespace, key) in &keys_to_migrate {
+			source_store.remove(primary_namespace, secondary_namespace, &key, false)?;
+		}
+
+		if !source_store.list_all_keys()?.is_empty() {
+			let err = io::Error::new(
+				io::ErrorKind::InvalidData,
+				"Source store is not empty. Was it modified during migration? This should never happen!"
+			);
+			return Err(err);
+		}
+	}
+
+	Ok(())
+}
+
 /// Trait that handles persisting a [`ChannelManager`], [`NetworkGraph`], and [`WriteableScore`] to disk.
 ///
 /// [`ChannelManager`]: crate::ln::channelmanager::ChannelManager