Add #[must_use] to Subscription (#35616)

GitOrigin-RevId: 771a32ff04284bbb0857a82749886ae7f9d0d54e

Author: goffrie

crates/application/src/snapshot_import/mod.rs

@@ -564,7 +564,11 @@ async fn wait_for_import_worker<RT: Runtime>(
         match &snapshot_import.state {
             ImportState::Uploaded | ImportState::InProgress { .. } => {
                 let token = tx.into_token()?;
-                application.subscribe(token).await?;
+                application
+                    .subscribe(token)
+                    .await?
+                    .wait_for_invalidation()
+                    .await;
             },
             ImportState::WaitingForConfirmation { .. }
             | ImportState::Completed { .. }

crates/database/benches/subscriptions.rs

@@ -213,7 +213,9 @@ fn create_subscriptions(
     let mut subscription_manager = SubscriptionManager::new_for_testing();
     let tokens = create_tokens(tablet_id, terms_by_frequency, prefix, max_distance, count);
     for token in tokens {
-        subscription_manager.subscribe_for_testing(token).unwrap();
+        // this drops the Subscription but in these tests we don't run the
+        // worker that removes dropped subscriptions
+        _ = subscription_manager.subscribe_for_testing(token).unwrap();
     }
     subscription_manager
 }

crates/database/src/subscription.rs

@@ -394,6 +394,7 @@ enum SubscriptionState {
 }
 
 /// A subscription on a set of read keys from a prior read-only transaction.
+#[must_use]
 pub struct Subscription {
     valid_ts: Arc<AtomicI64>, // -1 means invalid
     valid: watch::Receiver<SubscriptionState>,
@@ -870,7 +871,7 @@ mod tests {
             let test = async move {
                 let mut id_generator = TestIdGenerator::new();
                 let mut subscription_manager = SubscriptionManager::new_for_testing();
-                subscription_manager.subscribe_for_testing(token.clone()).unwrap();
+                _ = subscription_manager.subscribe_for_testing(token.clone()).unwrap();
                 let notifications =
                     notify_subscribed_tokens(
                         &mut id_generator, &mut subscription_manager, vec![mismatch]