No - multiline auto-flush does not work with the UDP input yet. AFAIK the file input and the beats input are the only input plugins that have been modified to support auto-flush.
It is not possible make auto-flushing work in the codec alone without modifying the input code as well.
The reason why the multiline (or any) codec can't support auto-flush without the input being modified is because the codecs are in-band event generators. This means that the call to decode happens in the input thread's execution context. The input thread calls the codec's decode method with a closure to call back on. Auto-flush is something that occurs out-of-band i.e. in another thread at some time later when the input thread is doing some other work - the auto-flush code/thread does not have a reference to the input to call back on when it has an auto-flush generated event give to Logstash for further processing.
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