output windows: check shorter service timeout on shutdown

lib/fluent/plugin/output.rb

@@ -372,6 +372,13 @@ def configure(conf)
             buf_type = Plugin.lookup_type_from_class(@buffer.class)
             log.warn "'flush_at_shutdown' is false, and buffer plugin '#{buf_type}' is not persistent buffer."
             log.warn "your configuration will lose buffered data at shutdown. please confirm your configuration again."
+          else
+            if Fluent.windows? && @buffer.persistent?
+              service_timeout = read_wait_to_kill_service_timeout
+              if service_timeout && service_timeout <= 5000 # default value might varies on windows client/server
+                log.warn "your WaitToKillServiceTimeout=#{service_timeout} registry configuration seems too short. Recommend to extend that value not to corrupt flushing buffer with forcibly shutdown"
+              end
+            end
           end
 
           if (@flush_mode != :interval) && buffer_conf.has_key?('flush_interval')
@@ -418,6 +425,22 @@ def configure(conf)
         self
       end
 
+      def read_wait_to_kill_service_timeout
+        if Fluent.windows?
+          begin
+            require "win32/registry"
+            Win32::Registry::HKEY_LOCAL_MACHINE.open("SYSTEM\\CurrentControlSet\\Control",
+                                                     Win32::Registry::KEY_READ) do |reg|
+              reg["WaitToKillServiceTimeout"].to_i
+            end
+          rescue => e
+            log.warn "'flush_at_shutdown' is true, but can't check WaitToKillServiceTimeout registry configuration", error: e
+          end
+        else
+          nil # not supported
+        end
+      end
+
       def keep_buffer_config_compat
         # Need this to call `@buffer_config.disable_chunk_backup` just as before,
         # since some plugins may use this option in this way.

test/command/test_fluentd.rb

@@ -1760,4 +1760,22 @@ def create_config_include_dir_configuration(config_path, config_dir, yaml_format
       end
     end
   end
+
+  sub_test_case "test suspicious shorter service timeout on windows" do
+    test 'warn the default value of timekey (1d) is used as-is' do
+      omit "skip service timeout on windows test case" unless Fluent.windows?
+      conf_path = create_conf_file("warning.conf", <<~EOF)
+                    <system>
+                      config_include_dir ""
+                    </system>
+                    <match>
+                      @type file
+                      path #{@tmp_dir}/test.log
+                      flush_at_shutdown
+                    </match>
+                  EOF
+      message="your WaitToKillServiceTimeout=5000 registry configuration seems too short. Recommend to extend that value not to corrupt flushing buffer with forcibly shutdown"
+      assert_log_matches(create_cmdline(conf_path, '--dry-run'), message)
+    end
+  end
 end