SSL_CTX_set_async_callback, SSL_CTX_set_async_callback_arg,
  SSL_set_async_callback, SSL_set_async_callback_arg, SSL_get_async_status,
  SSL_async_callback_fn - manage asynchronous operations
 #include <openssl/ssl.h>
 typedef int (*SSL_async_callback_fn)(SSL *s, void *arg);
 int SSL_CTX_set_async_callback(SSL_CTX *ctx, SSL_async_callback_fn callback);
 int SSL_CTX_set_async_callback_arg(SSL_CTX *ctx, void *arg);
 int SSL_set_async_callback(SSL *s, SSL_async_callback_fn callback);
 int SSL_set_async_callback_arg(SSL *s, void *arg);
 int SSL_get_async_status(SSL *s, int *status);
SSL_CTX_set_async_callback() sets an asynchronous callback function. All
  SSL objects generated based on this SSL_CTX will get this
  callback. If an engine supports the callback mechanism, it will be
  automatically called if SSL_MODE_ASYNC has been set and an asynchronous
  capable engine completes a cryptography operation to notify the application to
  resume the paused work flow.
SSL_CTX_set_async_callback_arg() sets the callback
    argument.
SSL_set_async_callback() allows an application to set a
    callback in an asynchronous SSL object, so that when an engine
    completes a cryptography operation, the callback will be called to notify
    the application to resume the paused work flow.
SSL_set_async_callback_arg() sets an argument for the
    SSL object when the above callback is called.
SSL_get_async_status() returns the engine status. This
    function facilitates the communication from the engine to the application.
    During an SSL session, cryptographic operations are dispatched to an engine.
    The engine status is very useful for an application to know if the operation
    has been successfully dispatched. If the engine does not support this
    additional callback method, ASYNC_STATUS_UNSUPPORTED will be
    returned. See ASYNC_WAIT_CTX_set_status() for a description of all of
    the status values.
An example of the above functions would be the following:
  - 1.
- Application sets the async callback and callback data on an SSL connection
      by calling SSL_set_async_callback().
- 2.
- Application sets SSL_MODE_ASYNC and makes an asynchronous SSL
    call
- 3.
- OpenSSL submits the asynchronous request to the engine. If a retry occurs
      at this point then the status within the ASYNC_WAIT_CTX would be
      set and the async callback function would be called (goto Step 7).
- 4.
- The OpenSSL engine pauses the current job and returns, so that the
      application can continue processing other connections.
- 5.
- At a future point in time (probably via a polling mechanism or via an
      interrupt) the engine will become aware that the asynchronous request has
      finished processing.
- 6.
- The engine will call the application's callback passing the callback data
      as a parameter.
- 7.
- The callback function should then run. Note: it is a requirement that the
      callback function is small and nonblocking as it will be run in the
      context of a polling mechanism or an interrupt.
- 8.
- It is the application's responsibility via the callback function to
      schedule recalling the OpenSSL asynchronous function and to continue
      processing.
- 9.
- The callback function has the option to check the status returned via
      SSL_get_async_status() to determine whether a retry happened
      instead of the request being submitted, allowing different processing if
      required.
SSL_CTX_set_async_callback(), SSL_set_async_callback(),
  SSL_CTX_set_async_callback_arg(),
  SSL_CTX_set_async_callback_arg() and SSL_get_async_status()
  return 1 on success or 0 on error.
SSL_CTX_set_async_callback(), SSL_CTX_set_async_callback_arg(),
  SSL_set_async_callback(), SSL_set_async_callback_arg() and
  SSL_get_async_status() were first added to OpenSSL 3.0.
Copyright 2019-2021 The OpenSSL Project Authors. All Rights Reserved.Licensed under the Apache License 2.0 (the "License").
    You may not use this file except in compliance with the License. You can
    obtain a copy in the file LICENSE in the source distribution or at
    <https://www.openssl.org/source/license.html>.