Notify superseded plugin processes and support SDK draining

cli/src/upgrades.rs

@@ -12,6 +12,7 @@ pub(crate) struct OldProcess {
 pub(crate) struct Upgrade {
     plugin: String,
     previous: String,
+    current: String,
     grace: Duration,
     processes: Vec<OldProcess>,
     services: Vec<(Service, Option<watch::Receiver<bool>>)>,
@@ -67,6 +68,7 @@ impl Host {
         Upgrade {
             plugin: plugin.into(),
             previous,
+            current: current.into(),
             grace: Duration::from_secs_f64(grace),
             processes,
             services,
@@ -74,6 +76,13 @@ impl Host {
     }
     /// Lets old calls finish, stops them after grace, and starts each lifecycle hook once.
     pub(crate) fn begin_upgrade(&self, upgrade: Upgrade) {
+        // Queue the notice before returning from activation, without waiting on guest code.
+        for process in &upgrade.processes {
+            self.send(
+                &process.pid,
+                json!({"jsonrpc":"2.0","method":"rpc.superseded","params":{"current":upgrade.current}}),
+            );
+        }
         let host = self.clone();
         tokio::spawn(async move {
             if host.0.shutdown.load(Ordering::SeqCst) {