Add support for model migrations

src/table/schema.rs

@@ -0,0 +1,77 @@
+use async_trait::async_trait;
+use serde::{Deserialize, Serialize};
+
+use garage_util::data::*;
+use garage_util::error::Error;
+
+
+pub trait PartitionKey {
+	fn hash(&self) -> Hash;
+}
+
+pub trait SortKey {
+	fn sort_key(&self) -> &[u8];
+}
+
+pub trait Entry<P: PartitionKey, S: SortKey>:
+	PartialEq + Clone + Serialize + for<'de> Deserialize<'de> + Send + Sync
+{
+	fn partition_key(&self) -> &P;
+	fn sort_key(&self) -> &S;
+
+	fn merge(&mut self, other: &Self);
+}
+
+#[derive(Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct EmptyKey;
+impl SortKey for EmptyKey {
+	fn sort_key(&self) -> &[u8] {
+		&[]
+	}
+}
+impl PartitionKey for EmptyKey {
+	fn hash(&self) -> Hash {
+		[0u8; 32].into()
+	}
+}
+
+impl PartitionKey for String {
+	fn hash(&self) -> Hash {
+		hash(self.as_bytes())
+	}
+}
+impl SortKey for String {
+	fn sort_key(&self) -> &[u8] {
+		self.as_bytes()
+	}
+}
+
+impl PartitionKey for Hash {
+	fn hash(&self) -> Hash {
+		self.clone()
+	}
+}
+impl SortKey for Hash {
+	fn sort_key(&self) -> &[u8] {
+		self.as_slice()
+	}
+}
+
+#[async_trait]
+pub trait TableSchema: Send + Sync {
+	type P: PartitionKey + Clone + PartialEq + Serialize + for<'de> Deserialize<'de> + Send + Sync;
+	type S: SortKey + Clone + Serialize + for<'de> Deserialize<'de> + Send + Sync;
+	type E: Entry<Self::P, Self::S>;
+	type Filter: Clone + Serialize + for<'de> Deserialize<'de> + Send + Sync;
+
+    // Action to take if not able to decode current version:
+    // try loading from an older version
+    fn try_migrate(_bytes: &[u8]) -> Option<Self::E> {
+        None
+    }
+
+	async fn updated(&self, old: Option<Self::E>, new: Option<Self::E>) -> Result<(), Error>;
+	fn matches_filter(_entry: &Self::E, _filter: &Self::Filter) -> bool {
+		true
+	}
+}