Attempt to ensure that all output sent to the stream has reached its destination returning t upon success or nil if the operation would block. Implemented by the generic function ext:stream-finish-output-no-hang. When the write is not completed the function ext:stream-waiting-file-descriptor may be called to find the file descriptor and directions blocked and when ready the function ext:finish-output-no-hang may be called again to continue the operation. See also: ext:flush-output.