WIP prototype migrations

document/migrations/core/src/payload.rs

@@ -0,0 +1,47 @@
+use crate::MigrationError;
+use serde::Serialize;
+use serde::de::DeserializeOwned;
+
+/// Codec of a serialized payload. Mirrors the `document-format` codec table entries relevant to
+/// migrations so migration crates don't depend on `document-format`; the runner maps between the two.
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum PayloadCodec {
+	Json,
+	MessagePack,
+}
+
+/// One serialized document payload (the registry, a single history record, the session) plus its
+/// codec. History framing is the runner's concern; migrations always see one record per payload.
+#[derive(Clone, Debug)]
+pub struct Payload {
+	pub bytes: Vec<u8>,
+	pub codec: PayloadCodec,
+}
+
+impl Payload {
+	pub fn new(bytes: Vec<u8>, codec: PayloadCodec) -> Self {
+		Self { bytes, codec }
+	}
+
+	/// Decode into a typed shape, usually a frozen mirror struct owned by the migration crate.
+	pub fn decode<T: DeserializeOwned>(&self) -> Result<T, MigrationError> {
+		match self.codec {
+			PayloadCodec::Json => serde_json::from_slice(&self.bytes).map_err(|e| MigrationError::Decode(e.to_string())),
+			PayloadCodec::MessagePack => rmp_serde::from_slice(&self.bytes).map_err(|e| MigrationError::Decode(e.to_string())),
+		}
+	}
+
+	/// Encode a typed shape with the given codec.
+	pub fn encode<T: Serialize>(value: &T, codec: PayloadCodec) -> Result<Self, MigrationError> {
+		let bytes = match codec {
+			PayloadCodec::Json => serde_json::to_vec(value).map_err(|e| MigrationError::Encode(e.to_string()))?,
+			PayloadCodec::MessagePack => rmp_serde::to_vec(value).map_err(|e| MigrationError::Encode(e.to_string()))?,
+		};
+		Ok(Self { bytes, codec })
+	}
+
+	/// Encode a typed shape, keeping this payload's codec.
+	pub fn encode_as<T: Serialize>(&self, value: &T) -> Result<Self, MigrationError> {
+		Self::encode(value, self.codec)
+	}
+}