fix(pageserver): is_archived should be optional (#8902)

Set the field to optional, otherwise there will be decode errors when
newer version of the storage controller receives the JSON from older
version of the pageservers.

Signed-off-by: Alex Chi Z <chi@neon.tech>

libs/pageserver_api/src/models.rs

@@ -716,12 +716,17 @@ pub struct TimelineInfo {
     pub pg_version: u32,
 
     pub state: TimelineState,
-    pub is_archived: bool,
 
     pub walreceiver_status: String,
 
+    // ALWAYS add new fields at the end of the struct with `Option` to ensure forward/backward compatibility.
+    // Backward compatibility: you will get a JSON not containing the newly-added field.
+    // Forward compatibility: a previous version of the pageserver will receive a JSON. serde::Deserialize does
+    // not deny unknown fields by default so it's safe to set the field to some value, though it won't be
+    // read.
     /// The last aux file policy being used on this timeline
     pub last_aux_file_policy: Option<AuxFilePolicy>,
+    pub is_archived: Option<bool>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]

pageserver/src/http/routes.rs

@@ -468,7 +468,7 @@ async fn build_timeline_info_common(
         pg_version: timeline.pg_version,
 
         state,
-        is_archived,
+        is_archived: Some(is_archived),
 
         walreceiver_status,