Document async capabilities

=pod

=head1 NAME

ASYNC_init_pool, ASYNC_free_pool, ASYNC_start_job, ASYNC_pause_job,

ASYNC_in_job, ASYNC_get_wait_fd, ASYNC_get_current_job, ASYNC_wake,

ASYNC_clear_wake - asynchronous job management functions

=head1 SYNOPSIS

 #include <openssl/async.h>

 int ASYNC_init_pool(size_t max_size, size_t init_size);

 void ASYNC_free_pool(void);

 int ASYNC_start_job(ASYNC_JOB **job, int *ret, int (*func)(void *),

                     void *args, size_t size);

 int ASYNC_pause_job(void);

 int ASYNC_get_wait_fd(ASYNC_JOB *job);

 ASYNC_JOB *ASYNC_get_current_job(void);

 void ASYNC_wake(ASYNC_JOB *job);

 void ASYNC_clear_wake(ASYNC_JOB *job);

=head1 DESCRIPTION

OpenSSL implements asynchronous capabilities through an ASYNC_JOB. This

represents code that can be started and executes until some event occurs. At

that point the code can be paused and control returns to user code until some

subsequent event indicates that the job can be resumed.

The creation of an ASYNC_JOB is a relatively expensive operation. Therefore, for

efficiency reasons, jobs can be created up front and reused many times. They are

held in a pool until they are needed, at which point they are removed from the

pool, used, and then returned to the pool when the job completes. Before using

any of the asynchronous job functions, user code should first call

ASYNC_init_pool(). If the user application is multi-threaded, then this should

be done for each thread that will initiate asynchronous jobs. Before user code

exits it should free the pool up (for each thread where a pool was initialised)

using ASYNC_free_pool(). The B<max_size> argument limits the number of

ASYNC_JOBs that will be held in the pool. If B<max_size> is set to 0 then no

upper limit is set. When an ASYNC_JOB is needed but there are none available in

the pool already then one will be automatically created, as long as the total

of ASYNC_JOBs managed by the pool does not exceed B<max_size>. When the pool is

first initialised B<init_size> ASYNC_JOBs will be created immediately. If

ASYNC_init_pool() is not called before the pool is first used then it will be

called automatically with a B<max_size> of 0 (no upper limit) and an

B<init_size> of 0 (no ASYNC_JOBs created up front). If a pool is created in this

way it must still be cleaned up with an explicit call to ASYNC_free_pool().

An asynchronous job is started by calling the ASYNC_start_job() function.

Initially B<*job> should be NULL. B<ret> should point to a location where the

return value of the asynchronous function should be stored on completion of the

job. B<func> represents the function that should be started asynchronously. The

data pointed to by B<args> and of size B<size> will be copied and then passed as

an argument to B<func> when the job starts. ASYNC_start_job will return one of

the following values:

=over 4

=item B<ASYNC_ERR>

An error occurred trying to start the job. Check the OpenSSL error queue (e.g.

see L<ERR_print_errors(3)>) for more details.

=item B<ASYNC_NO_JOBS>

There are no jobs currently available in the pool. This call can be retried

again at a later time.

=item B<ASYNC_PAUSE>

The job was successfully started but was "paused" before it completed (see

ASYNC_pause_job() below). A handle to the job is placed in B<*job>. Other work

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.

=item B<ASYNC_FINISH>

The job completed. B<*job> will be NULL and the return value from B<func> will

be placed in B<*ret>.

=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.

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())

then ASYNC_pause_job() will immediately return control to the calling

application with ASYNC_PAUSE returned from the ASYNC_start_job() call. A

subsequent call to ASYNC_start_job passing in the relevant ASYNC_JOB in the

B<*job> parameter will resume execution from the ASYNC_pause_job() call. If

ASYNC_pause_job() is called whilst not within the context of a job then no

action is taken and ASYNC_pause_job() returns immediately.

Every ASYNC_JOB has a "wait" file descriptor associated with it. Calling

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().

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

initiate cryptographic operations. The engine would initiate those operations

asynchronously and then call ASYNC_pause_job() to return control to the user

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_clear_wake().

=head1 RETURN VALUES

ASYNC_init_pool returns 1 on success or 0 otherwise.

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_get_wait_fd returns the "wait" file descriptor associated with the

ASYNC_JOB provided as an argument.

ASYNC_get_current_job returns a pointer to the currently executing ASYNC_JOB or

NULL if not within the context of a job.

=head1 EXAMPLE

The following example demonstrates how to use most of the core async APIs:

 #include <stdio.h>

 #include <openssl/async.h>

 int jobfunc(void *arg)

 {

     ASYNC_JOB *currjob;

     unsigned char *msg;

     currjob = ASYNC_get_current_job();

     if (currjob != NULL) {

         printf("Executing within a job\n");

     } else {

         printf("Not executing within a job - should not happen\n");

         return 0;

     }

     msg = (unsigned char *)arg;

     printf("Passed in message is: %s\n", msg);

     /*

      * Normally some external event would cause this to happen at some

      * later point - but we do it here for demo purposes, i.e.

      * immediately signalling that the job is ready to be woken up after

      * we return to main via ASYNC_pause_job().

      */

     ASYNC_wake(currjob);

     /* Return control back to main */

     ASYNC_pause_job();

     /* Clear the wake signal */

     ASYNC_clear_wake(currjob);

     printf ("Resumed the job after a pause\n");

     return 1;

 }

 int main(void)

 {

     ASYNC_JOB *job = NULL;

     int ret, waitfd;

     fd_set waitfdset;

     unsigned char msg[13] = "Hello world!";

     /*

      * We're only expecting 1 job to be used here so we're only creating

      * a pool of 1

      */

     if (!ASYNC_init_pool(1, 1)) {

         printf("Error creating pool\n");

         goto end;

     }

     printf("Starting...\n");

     for (;;) {

         switch(ASYNC_start_job(&job, &ret, jobfunc, msg, sizeof(msg))) {

         case ASYNC_ERR:

         case ASYNC_NO_JOBS:

                 printf("An error occurred\n");

                 goto end;

         case ASYNC_PAUSE:

                 printf("Job was paused\n");

                 break;

         case ASYNC_FINISH:

                 printf("Job finished with return value %d\n", ret);

                 goto end;

         }

         /* Wait for the job to be woken */

         printf("Waiting for the job to be woken up\n");

         waitfd = ASYNC_get_wait_fd(job);

         FD_ZERO(&waitfdset);

         FD_SET(waitfd, &waitfdset);

         select(waitfd + 1, &waitfdset, NULL, NULL, NULL);

     }

 end:

     printf("Finishing\n");

     ASYNC_free_pool();

     return 0;

 }

The expected output from executing the above example program is:

 Starting...

 Executing within a job

 Passed in message is: Hello world!

 Job was paused

 Waiting for the job to be woken up

 Resumed the job after a pause

 Job finished with return value 1

 Finishing

=head1 SEE ALSO

L<crypto(3)>, L<ERR_print_errors(3)>

=head1 HISTORY

ASYNC_init_pool, ASYNC_free_pool, ASYNC_start_job, ASYNC_pause_job,

ASYNC_get_wait_fd, ASYNC_get_current_job, ASYNC_wake, ASYNC_clear_wake were

first added to OpenSSL 1.1.0.

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