Add tracing probes to isc_job unit:
* libisc:job_cb_before - before the job callback is called
* libisc:job_cb_after - after the job callback is called
The isc_async API was using lock-free stack (where enqueue operation was
not wait-free). Change the isc_async to use cds_wfcqueue internally -
enqueue and splice (move the queue members from one list to another) is
nonblocking and wait-free.
Change the isc_job_run() to not-make any allocations. The caller must
make sure that it allocates isc_job_t - usually as part of the argument
passed to the callback.
For simple jobs, using isc_async_run() is advised as it allocates its
own separate isc_job_t.
Previously, the async job queue would use a locked-list (ISC_LIST).
With introduction of atomic stack (that has to be drained at once), we
could use it to remove some contention between the threads and simplify
the async queue.
Fortunately, the reverse order still works for us - instead of append
and tail/prev operation on the list, we are now using prepend and
head/next operation on the atomic stack.
This commit introduces new APIs for applications and signal handling,
intended to replace isc_app for applications built on top of libisc.
* isc_app will be replaced with isc_loopmgr, which handles the
starting and stopping of applications. In isc_loopmgr, the main
thread is not blocked, but is part of the working thread set.
The loop manager will start a number of threads, each with a
uv_loop event loop running. Setup and teardown functions can be
assigned which will run when the loop starts and stops, and
jobs can be scheduled to run in the meantime. When
isc_loopmgr_shutdown() is run from any the loops, all loops
will shut down and the application can terminate.
* signal handling will now be handled with a separate isc_signal unit.
isc_loopmgr only handles SIGTERM and SIGINT for application
termination, but the application may install additional signal
handlers, such as SIGHUP as a signal to reload configuration.
* new job running primitives, isc_job and isc_async, have been added.
Both units schedule callbacks (specifying a callback function and
argument) on an event loop. The difference is that isc_job unit is
unlocked and not thread-safe, so it can be used to efficiently
run jobs in the same thread, while isc_async is thread-safe and
uses locking, so it can be used to pass jobs from one thread to
another.
* isc_tid will be used to track the thread ID in isc_loop worker
threads.
* unit tests have been added for the new APIs.
