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.
| auth | the Auth to authenticate |
| msg | the Message being sent |
| retrying |
true if this is the second (or later) attempt |