Fix ProcessClientMessages, and add variant taking the input as a string

PiperOrigin-RevId: 853836141

Author: schoppmp

willow/src/api/server_accumulator.cc

@@ -60,10 +60,16 @@ WillowShellServerAccumulator::CreateFromSerializedState(
 
 absl::Status WillowShellServerAccumulator::ProcessClientMessages(
     willow::ClientMessageList client_messages) {
+  auto serialized_client_messages = client_messages.SerializeAsString();
   client_messages.Clear();
+  return ProcessClientMessages(std::move(serialized_client_messages));
+}
+
+absl::Status WillowShellServerAccumulator::ProcessClientMessages(
+    std::string serialized_client_messages) {
   std::unique_ptr<std::string> status_message;
   int status_code = accumulator_->ProcessClientMessages(
-      std::make_unique<std::string>(client_messages.SerializeAsString()),
+      std::make_unique<std::string>(std::move(serialized_client_messages)),
       &status_message);
   if (status_code != 0) {
     return absl::Status(absl::StatusCode(status_code), *status_message);

willow/src/api/server_accumulator.h

@@ -50,6 +50,10 @@ class WillowShellServerAccumulator {
   // an error is logged and processing continues.
   absl::Status ProcessClientMessages(willow::ClientMessageList client_messages);
 
+  // Processes a list of client messages, given as a serialized
+  // ClientMessageList proto.
+  absl::Status ProcessClientMessages(std::string serialized_client_messages);
+
   // Merges the state of `other` into the current accumulator.
   absl::Status Merge(std::unique_ptr<WillowShellServerAccumulator> other);