SourceQueue

Offers an element to a stream and returns a CompletionStage that:

• completes with Enqueued if the element is consumed by a stream
• completes with Dropped when the stream dropped the offered element
• completes with QueueClosed when the stream is completed whilst the CompletionStage is active
• completes with Failure(f) in case of failure to enqueue element from upstream
• fails when stream is already completed

Additionally when using the backpressure overflowStrategy:

• If the buffer is full the CompletionStage won't be completed until there is space in the buffer
• Calling offer before the CompletionStage is completed, in this case it will return a failed CompletionStage

Returns a CompletionStage that will be completed if this operator completes, or will be failed when the stream is failed.