Emitted when the session requires authentication.
If credentials are available call authenticate on auth. If
these credentials fail, the signal will be emitted again, with retrying set to true, which
will continue until you return without calling authenticate on
auth.
Note that this may be emitted before msg's body has been fully read.
If you call pause_message on msg before returning, then you can
authenticate auth asynchronously (as long as you @ref it
to make sure it doesn't get destroyed), and then unpause msg when you are ready for it to continue.
| msg |
the Message being sent |
| auth |
the Auth to authenticate |
| retrying |
true if this is the second (or later) attempt |