[core] Deflakey gcs server rpc test (#31919)

The test failed because in the current ray redis client there is a global variable using io context. and the io context is freed during destruction and when destruct the global variable it'll cause issues.

The current redis client is aweful and ugly. Since we'll move to redis plus plus, and this client doesn't cause other issues, I'll just fix the test case.

src/ray/gcs/gcs_server/test/gcs_server_rpc_test.cc

@@ -63,6 +63,7 @@ class GcsServerTest : public ::testing::Test {
     gcs_server_->Stop();
     thread_io_service_->join();
     gcs_server_.reset();
+    ray::gcs::RedisCallbackManager::instance().Clear();
   }
 
   bool AddJob(const rpc::AddJobRequest &request) {

src/ray/gcs/redis_context.cc

@@ -212,6 +212,11 @@ std::shared_ptr<RedisCallbackManager::CallbackItem> RedisCallbackManager::GetCal
   return it->second;
 }
 
+void RedisCallbackManager::Clear() {
+  std::lock_guard<std::mutex> lock(mutex_);
+  callback_items_.clear();
+}
+
 void RedisCallbackManager::RemoveCallback(int64_t callback_index) {
   std::lock_guard<std::mutex> lock(mutex_);
   callback_items_.erase(callback_index);

src/ray/gcs/redis_context.h

@@ -145,6 +145,9 @@ class RedisCallbackManager {
   /// Get a callback.
   std::shared_ptr<CallbackItem> GetCallback(int64_t callback_index) const;
 
+  /// Clear all callbacks.
+  void Clear();
+
  private:
   RedisCallbackManager() : num_callbacks_(0){};