Attempts a TLS handshake on this.
On the client side, it is never necessary to call this method; although the connection needs to perform a handshake after connecting (or after sending a "STARTTLS"-type command), TlsConnection will handle this for you automatically when you try to send or receive data on the connection. You can call handshake manually if you want to know whether the initial handshake succeeded or failed (as opposed to just immediately trying to use this to read or write, in which case, if it fails, it may not be possible to tell if it failed before or after completing the handshake), but beware that servers may reject client authentication after the handshake has completed, so a successful handshake does not indicate the connection will be usable.
Likewise, on the server side, although a handshake is necessary at the beginning of the communication, you do not need to call this function explicitly unless you want clearer error reporting.
Previously, calling handshake after the initial handshake would trigger a rehandshake; however, this usage was deprecated in GLib 2.60 because rehandshaking was removed from the TLS protocol in TLS 1.3. Since GLib 2.64, calling this function after the initial handshake will no longer do anything.
accept_certificate may be emitted during the handshake.
a Cancellable, or null
success or failure