Session Cache API Change

This affects users who use mbedtls_ssl_conf_session_cache() to configure a custom session cache implementation different from the one Mbed TLS implements in library/ssl_cache.c.

Those users will need to modify the API of their session cache implementation to that of a key-value store with keys being session IDs and values being instances of mbedtls_ssl_session:

typedef int mbedtls_ssl_cache_get_t( void *data,
                                     unsigned char const *session_id,
                                     size_t session_id_len,
                                     mbedtls_ssl_session *session );
typedef int mbedtls_ssl_cache_set_t( void *data,
                                     unsigned char const *session_id,
                                     size_t session_id_len,
                                     const mbedtls_ssl_session *session );

Since the structure of mbedtls_ssl_session is no longer public from 3.0 onwards, portable session cache implementations must not access fields of mbedtls_ssl_session. See the corresponding migration guide. Users that find themselves unable to migrate their session cache functionality without accessing fields of mbedtls_ssl_session should describe their usecase on the Mbed TLS mailing list.