RPC gracefulReload fails for mongo_replset plugin

[jeevamanivel]

We are using mongo_replset plugin to forward logs to Mongo DB. When we reload fluentd config using fluentd HTTP RPC api, instead of restarting fluentd, it fails with below error

Reload URL : http://127.0.0.1:24444/api/config.gracefulReload

Config:

<system>
rpc_endpoint 127.0.0.1:24444
</system>
       <source>
	  @type forward
	  @id mongo_forward
	  port 24224 
	  bind 0.0.0.0
	  @label @LOGS
	</source>
	<label @LOGS>
		<match mongo_forward>
			@type mongo_replset
			nodes HOST_PORT
			database DB_NAME
			collection COLLECTION_NAME
			replica_set REPLICA_NAME
			include_time_key false
			date_keys datekey
			<buffer tag>
				@type file
				path /fluentd/work/buf
				chunk_limit_size 256m
				total_limit_size 500g
				compress gzip
				queued_chunks_limit_size 10
				flush_thread_count 10
				flush_mode interval
				flush_interval 2s
				flush_thread_burst_interval 1.0
				flush_at_shutdown true
				overflow_action throw_exception
				retry_max_times 3
				retry_exponential_backoff_base 2
				retry_type exponential_backoff
				retry_timeout 24h
				retry_wait 2s
				retry_forever false
			</buffer>
		</match>
	</label>

Error

2023-03-23 09:48:34 +0530 [debug]: fluent/log.rb:309:debug: preparing shutdown output plugin type=:mongo_replset plugin_id="object:a50"
2023-03-23 09:48:34 +0530 [info]: fluent/log.rb:330:info: shutting down output plugin type=:mongo_replset plugin_id="object:a50"
2023-03-23 09:48:34 +0530 [warn]: fluent/log.rb:351:warn: unexpected error while shutting down on output plugin plugin=Fluent::Plugin::MongoOutputReplset plugin_id="object:a50" error_class=NoMethodError error="undefined method `close' for nil:NilClass\n\n      @client.close\n             ^^^^^^"
#<Thread:0x0000029a74c9d590 C:/Ruby31-x64/lib/ruby/gems/3.1.0/gems/fluentd-1.15.3-x64-mingw-ucrt/lib/fluent/root_agent.rb:264 run> terminated with exception (report_on_exception is true):
C:/Ruby31-x64/lib/ruby/gems/3.1.0/gems/fluentd-1.15.3-x64-mingw-ucrt/lib/fluent/log.rb:566:in `caller_line': undefined method `[]' for nil:NilClass (NoMethodError)

        line = caller(depth+1)[0]
                              ^^^
        from C:/Ruby31-x64/lib/ruby/gems/3.1.0/gems/fluentd-1.15.3-x64-mingw-ucrt/lib/fluent/log.rb:467:in `dump_stacktrace'
        from C:/Ruby31-x64/lib/ruby/gems/3.1.0/gems/fluentd-1.15.3-x64-mingw-ucrt/lib/fluent/log.rb:357:in `warn_backtrace'
        from C:/Ruby31-x64/lib/ruby/gems/3.1.0/gems/fluentd-1.15.3-x64-mingw-ucrt/lib/fluent/root_agent.rb:287:in `rescue in block (3 levels) in shutdown'
        from C:/Ruby31-x64/lib/ruby/gems/3.1.0/gems/fluentd-1.15.3-x64-mingw-ucrt/lib/fluent/root_agent.rb:266:in `block (3 levels) in shutdown'
C:/Ruby31-x64/lib/ruby/gems/3.1.0/gems/fluent-plugin-mongo-1.6.0/lib/fluent/plugin/out_mongo.rb:189:in `shutdown': undefined method `close' for nil:NilClass (NoMethodError)

      @client.close
             ^^^^^^
        from C:/Ruby31-x64/lib/ruby/gems/3.1.0/gems/fluentd-1.15.3-x64-mingw-ucrt/lib/fluent/root_agent.rb:280:in `block (3 levels) in shutdown'
2023-03-23 09:48:34 +0530 [debug]: fluent/log.rb:309:debug: Got Win32 event "fluentd_21228_STOP_EVENT_THREAD"
2023-03-23 09:48:34 +0530 [error]: fluent/log.rb:372:error: Failed to reload config file: undefined method `[]' for nil:NilClass

        line = caller(depth+1)[0]
                              ^^^
Unexpected error undefined method `[]' for nil:NilClass

        line = caller(depth+1)[0]
                              ^^^
  C:/Ruby31-x64/lib/ruby/gems/3.1.0/gems/fluentd-1.15.3-x64-mingw-ucrt/lib/fluent/log.rb:566:in `caller_line'
  C:/Ruby31-x64/lib/ruby/gems/3.1.0/gems/fluentd-1.15.3-x64-mingw-ucrt/lib/fluent/log.rb:467:in `dump_stacktrace'
  C:/Ruby31-x64/lib/ruby/gems/3.1.0/gems/fluentd-1.15.3-x64-mingw-ucrt/lib/fluent/log.rb:357:in `warn_backtrace'
  C:/Ruby31-x64/lib/ruby/gems/3.1.0/gems/fluentd-1.15.3-x64-mingw-ucrt/lib/fluent/root_agent.rb:287:in `rescue in block (3 levels) in shutdown'
  C:/Ruby31-x64/lib/ruby/gems/3.1.0/gems/fluentd-1.15.3-x64-mingw-ucrt/lib/fluent/root_agent.rb:266:in `block (3 levels) in shutdown'