A callback for rustls_connection_write_tls_vectored.
An implementation of this callback should attempt to write the bytes in
the given count iovecs to the network. If any bytes were written,
the implementation should set out_n to the number of bytes written and return 0.
If there was an error, the implementation should return a nonzero rustls_io_result,
which will be passed through to the caller. On POSIX systems, returning errno is convenient.
On other systems, any appropriate error code works.
It's best to make one write attempt to the network per call. Additional write will
be triggered by subsequent calls to one of the _write_tls methods.
userdata is set to the value provided to rustls_*_session_set_userdata. In most
cases that should be a struct that contains, at a minimum, a file descriptor.
The buf and out_n pointers are borrowed and should not be retained across calls.
A callback for rustls_connection_write_tls_vectored. An implementation of this callback should attempt to write the bytes in the given count iovecs to the network. If any bytes were written, the implementation should set out_n to the number of bytes written and return 0. If there was an error, the implementation should return a nonzero rustls_io_result, which will be passed through to the caller. On POSIX systems, returning errno is convenient. On other systems, any appropriate error code works. It's best to make one write attempt to the network per call. Additional write will be triggered by subsequent calls to one of the _write_tls methods. userdata is set to the value provided to rustls_*_session_set_userdata. In most cases that should be a struct that contains, at a minimum, a file descriptor. The buf and out_n pointers are borrowed and should not be retained across calls.