Callback function invoked just before the non-DATA frame |frame| is
sent.
The implementation of this function must return true if it succeeds.
If this callback returns false, it is treated as fatal error and
Session.recv and Session.send functions
immediately return ErrorCode.CALLBACK_FAILURE.
Callback function invoked just before the non-DATA frame |frame| is sent.
The implementation of this function must return true if it succeeds. If this callback returns false, it is treated as fatal error and Session.recv and Session.send functions immediately return ErrorCode.CALLBACK_FAILURE.