Tweak async documentation based on feedback

     further details. Libssl has also had this capability integrated with the

     introduction of the new mode SSL_MODE_ASYNC and associated error

     SSL_ERROR_WANT_ASYNC. See the SSL_CTX_set_mode() and SSL_get_error() man

     pages.

     pages. This work was developed in partnership with Intel Corp.

     [Matt Caswell]

  *) State machine rewrite. The state machine code has been significantly

    if(!async_get_ctx() || !async_get_ctx()->currjob) {

        /*

         * Could be we've deliberately not been started within a job so we

         * don't put an error on the error queue here.

         * Could be we've deliberately not been started within a job so this is

         * counted as success.

         */

        return 0;

        return 1;

    }

    job = async_get_ctx()->currjob;

can be performed (if desired) and the job restarted at a later time. To restart

a job call ASYNC_start_job() again passing the job handle in B<*job>. The

B<func>, B<args> and B<size> parameters will be ignored when restarting a job.

When restarting a job ASYNC_start_job() B<must> be called from the same thread

that the job was originally started from.

=item B<ASYNC_FINISH>

=back

ASYNC_get_current_job() can be used to get a pointer to the currently executing

ASYNC_JOB. If no job is currently executing then this will return NULL.

At any one time there can be a maximum of one job actively running per thread

(you can have many that are paused). ASYNC_get_current_job() can be used to get

a pointer to the currently executing ASYNC_JOB. If no job is currently executing

then this will return NULL.

If executing within the context of a job (i.e. having been called directly or

indirectly by the function "func" passed as an argument to ASYNC_start_job())

ASYNC_get_wait_fd() and passing in a pointer to an ASYNC_JOB in the B<job>

parameter will return the wait file descriptor associated with that job. This

file descriptor can be used to signal that the job should be resumed.

Applications can wait on the file descriptor using a system function call

such as select or poll. Applications can signal that a job is ready to resume

using ASYNC_wake() or clear an existing signal using ASYNC_clear_wake().

Applications can wait for the file descriptor to be ready for "read" using a

system function call such as select or poll (being ready for "read" indicates

that the job should be resumed). Applications can signal that a job is ready to

resume using ASYNC_wake() or clear an existing signal using ASYNC_clear_wake().

An example of typical usage might be an async capable engine. User code would

initiate cryptographic operations. The engine would initiate those operations

code. The user code can then perform other tasks or wait for the job to be ready

by calling "select" or other similar function on the wait file descriptor. The

engine can signal to the user code that the job should be resumed using

ASYNC_wait(). Once resumed the engine would clear the wake signal by calling

ASYNC_wake(). Once resumed the engine would clear the wake signal by calling

ASYNC_clear_wake().

ASYNC_start_job returns one of ASYNC_ERR, ASYNC_NO_JOBS, ASYNC_PAUSE or

ASYNC_FINISH as described above.

ASYNC_pause_job returns 0 if an error occured (including if called when not

within the context of an ASYNC_JOB), or 1 on success.

ASYNC_pause_job returns 0 if an error occured or 1 on success. If called when

not within the context of an ASYNC_JOB then this is counted as success so 1 is

returned.

ASYNC_get_wait_fd returns the "wait" file descriptor associated with the

ASYNC_JOB provided as an argument.

SSL_get_async_wait_fd() returns a file descriptor which can be used in a call to

select() or poll() to determine whether the current asynchronous operation has

completed or not. A completed operation will result in data appearing as

available on the file descriptor (no actual data should be read from the file

descriptor). This function should only be called if the SSL object is currently

waiting for asynchronous work to complete (i.e. SSL_ERROR_WANT_ASYNC has been

received - see L<SSL_get_error(3)>).

"read ready" on the file descriptor (no actual data should be read from the

file descriptor). This function should only be called if the SSL object is

currently waiting for asynchronous work to complete (i.e. SSL_ERROR_WANT_ASYNC

has been received - see L<SSL_get_error(3)>).

=head1 RETURN VALUES

completed its processing using select() or poll() on the asynchronous wait file

descriptor. This file descriptor is available by calling

L<SSL_get_async_wait_fd(3)>. The TLS/SSL I/O function should be called again

later.

later. The function B<must> be called from the same thread that the original

call was made from.

=item SSL_ERROR_SYSCALL