Send target to backend before local processing (#9323)

* Send target to backend before local processing

* Remove trailing whitespace

Author: schmidt-sebastian

Firestore/CHANGELOG.md

@@ -1,4 +1,6 @@
 # Unreleased
+- [changed] Queries are now send to the backend before the SDK starts local
+  processing, which reduces overall Query latency.
 - [changed] Add more details to the assertion failure in OrderBy::Compare() to
   help with future debugging (#9258).
 

Firestore/core/src/core/sync_engine.cc

@@ -109,16 +109,17 @@ TargetId SyncEngine::Listen(Query query) {
               "We already listen to query: %s", query.ToString());
 
   TargetData target_data = local_store_->AllocateTarget(query.ToTarget());
+  TargetId target_id = target_data.target_id();
+  remote_store_->Listen(std::move(target_data));
+
   ViewSnapshot view_snapshot =
-      InitializeViewAndComputeSnapshot(query, target_data.target_id());
+      InitializeViewAndComputeSnapshot(query, target_id);
   std::vector<ViewSnapshot> snapshots;
   // Not using the `std::initializer_list` constructor to avoid extra copies.
   snapshots.push_back(std::move(view_snapshot));
   sync_engine_callback_->OnViewSnapshots(std::move(snapshots));
 
-  // TODO(wuandy): move `target_data` into `Listen`.
-  remote_store_->Listen(target_data);
-  return target_data.target_id();
+  return target_id;
 }
 
 ViewSnapshot SyncEngine::InitializeViewAndComputeSnapshot(const Query& query,

Firestore/core/src/remote/remote_store.cc

@@ -151,20 +151,20 @@ void RemoteStore::Shutdown() {
 
 // Watch Stream
 
-void RemoteStore::Listen(const TargetData& target_data) {
+void RemoteStore::Listen(TargetData target_data) {
   TargetId target_key = target_data.target_id();
   if (listen_targets_.find(target_key) != listen_targets_.end()) {
     return;
   }
 
   // Mark this as something the client is currently listening for.
-  listen_targets_[target_key] = target_data;
+  listen_targets_[target_key] = std::move(target_data);
 
   if (ShouldStartWatchStream()) {
     // The listen will be sent in `OnWatchStreamOpen`
     StartWatchStream();
   } else if (watch_stream_->IsOpen()) {
-    SendWatchRequest(target_data);
+    SendWatchRequest(listen_targets_[target_key]);
   }
 }
 

Firestore/core/src/remote/remote_store.h

@@ -159,7 +159,7 @@ class RemoteStore : public TargetMetadataProvider,
    * It is a no-op if the target of the given target data is already being
    * listened to.
    */
-  void Listen(const local::TargetData& target_data);
+  void Listen(local::TargetData target_data);
 
   /**
    * Stops listening to the target with the given target ID.