1 files changed, 159 insertions, 0 deletions
diff --git a/src/util/migrate.rs b/src/util/migrate.rs
new file mode 100644
index 00000000..1229fd9c
--- /dev/null
+++ b/src/util/migrate.rs
@@ -0,0 +1,159 @@
+use serde::{Deserialize, Serialize};
+
+/// Indicates that this type has an encoding that can be migrated from
+/// a previous version upon upgrades of Garage.
+pub trait Migrate: Serialize + for<'de> Deserialize<'de> + 'static {
+	/// A sequence of bytes to add at the beginning of the serialized
+	/// string, to identify that the data is of this version.
+	const VERSION_MARKER: &'static [u8] = b"";
+
+	/// The previous version of this data type, from which items of this version
+	/// can be migrated.
+	type Previous: Migrate;
+
+	/// The migration function that transforms a value decoded in the old format
+	/// to an up-to-date value.
+	fn migrate(previous: Self::Previous) -> Self;
+
+	/// Decode an encoded version of this type, going through a migration if necessary.
+	fn decode(bytes: &[u8]) -> Option<Self> {
+		let marker_len = Self::VERSION_MARKER.len();
+		if bytes.get(..marker_len) == Some(Self::VERSION_MARKER) {
+			if let Ok(value) = rmp_serde::decode::from_read_ref::<_, Self>(&bytes[marker_len..]) {
+				return Some(value);
+			}
+		}
+
+		Self::Previous::decode(bytes).map(Self::migrate)
+	}
+
+	/// Encode this type with optionnal version marker
+	fn encode(&self) -> Result<Vec<u8>, rmp_serde::encode::Error> {
+		let mut wr = Vec::with_capacity(128);
+		wr.extend_from_slice(Self::VERSION_MARKER);
+		let mut se = rmp_serde::Serializer::new(&mut wr)
+			.with_struct_map()
+			.with_string_variants();
+		self.serialize(&mut se)?;
+		Ok(wr)
+	}
+}
+
+/// Indicates that this type has no previous encoding version to be migrated from.
+pub trait InitialFormat: Serialize + for<'de> Deserialize<'de> + 'static {
+	/// A sequence of bytes to add at the beginning of the serialized
+	/// string, to identify that the data is of this version.
+	const VERSION_MARKER: &'static [u8] = b"";
+}
+
+impl<T: InitialFormat> Migrate for T {
+	const VERSION_MARKER: &'static [u8] = <T as InitialFormat>::VERSION_MARKER;
+
+	type Previous = NoPrevious;
+
+	fn migrate(_previous: Self::Previous) -> Self {
+		unreachable!();
+	}
+}
+
+/// Internal type used by InitialFormat, not meant for general use.
+#[derive(Serialize, Deserialize)]
+pub enum NoPrevious {}
+
+impl Migrate for NoPrevious {
+	type Previous = NoPrevious;
+
+	fn migrate(_previous: Self::Previous) -> Self {
+		unreachable!();
+	}
+
+	fn decode(_bytes: &[u8]) -> Option<Self> {
+		None
+	}
+
+	fn encode(&self) -> Result<Vec<u8>, rmp_serde::encode::Error> {
+		unreachable!()
+	}
+}
+
+#[cfg(test)]
+mod test {
+	use super::*;
+
+	#[derive(Serialize, Deserialize, PartialEq, Eq, Debug)]
+	struct V1 {
+		a: usize,
+		b: String,
+	}
+	impl InitialFormat for V1 {}
+
+	#[derive(Serialize, Deserialize, PartialEq, Eq, Debug)]
+	struct V2 {
+		a: usize,
+		b: Vec<String>,
+		c: String,
+	}
+	impl Migrate for V2 {
+		const VERSION_MARKER: &'static [u8] = b"GtestV2";
+		type Previous = V1;
+		fn migrate(prev: V1) -> V2 {
+			V2 {
+				a: prev.a,
+				b: vec![prev.b],
+				c: String::new(),
+			}
+		}
+	}
+
+	#[test]
+	fn test_v1() {
+		let x = V1 {
+			a: 12,
+			b: "hello".into(),
+		};
+		let x_enc = x.encode().unwrap();
+		let y = V1::decode(&x_enc).unwrap();
+		assert_eq!(x, y);
+	}
+
+	#[test]
+	fn test_v2() {
+		let x = V2 {
+			a: 12,
+			b: vec!["hello".into(), "world".into()],
+			c: "plop".into(),
+		};
+		let x_enc = x.encode().unwrap();
+		assert_eq!(&x_enc[..V2::VERSION_MARKER.len()], V2::VERSION_MARKER);
+		let y = V2::decode(&x_enc).unwrap();
+		assert_eq!(x, y);
+	}
+
+	#[test]
+	fn test_migrate() {
+		let x = V1 {
+			a: 12,
+			b: "hello".into(),
+		};
+		let x_enc = x.encode().unwrap();
+
+		let xx = V1::decode(&x_enc).unwrap();
+		assert_eq!(x, xx);
+
+		let y = V2::decode(&x_enc).unwrap();
+		assert_eq!(
+			y,
+			V2 {
+				a: 12,
+				b: vec!["hello".into()],
+				c: "".into(),
+			}
+		);
+
+		let y_enc = y.encode().unwrap();
+		assert_eq!(&y_enc[..V2::VERSION_MARKER.len()], V2::VERSION_MARKER);
+
+		let z = V2::decode(&y_enc).unwrap();
+		assert_eq!(y, z);
+	}
+}