Emitted when the server has successfully read a request.
message will have all of its request-side information
filled in, and if the message was authenticated,
will have information about that. This signal is emitted
before any (non-early) handlers are called for the message,
and if it sets the message’s #status_code, then normal
handler processing will be skipped.
The default handler is called before the handlers added via
The data is owned by the caller of the function.