[LNT] Add signal handler on main thread - by doing it at module load time.

[kristof.beyls]

Summary

We've seen tracebacks like the one below on our LNT server, indicating
that previously, the code that adds the signal handler could be executed
by a thread that's not the main thread. Fix this by installing the
signal handler at module load time, which is assumed to always run in
the main thread.

File "/lnt/lnt/server/ui/views.py", line 124, in submit_run
  current_app.old_config, g.db_name, db, path, '<auto>', commit)
File "/lnt/lnt/util/ImportData.py", line 120, in import_and_report
  async_ops.async_fieldchange_calc(db_name, ts, run)
File "/lnt/lnt/util/async_ops.py", line 66, in async_fieldchange_calc
  func_args)
File "/lnt/lnt/util/async_ops.py", line 90, in async_run_job
  launch_workers()
File "/lnt/lnt/util/async_ops.py", line 47, in launch_workers
  signal.signal(signal.SIGTERM, sigHandler)

ValueError: signal only works in main thread

[cmatthews]

LGTM. Thanks!

[kristof.beyls]

Committed as r263416

lnt/util/async_ops.py

 from threading import Lock
 from lnt.testing.util.commands import note, warning, timed, error
 NUM_WORKERS = 4  # The number of subprocesses to spawn per LNT process.
 WORKERS = None  # The worker pool.
 
 JOBS = []
 
 
-
 def launch_workers():
     """Make sure we have a worker pool ready to queue."""
     global WORKERS
     if not WORKERS:
         note("Starting workers")
-        manager = Manager()         
+        manager = Manager()
         try:
-            current_app.config['mem_logger'].buffer = manager.list(current_app.config['mem_logger'].buffer)
+            current_app.config['mem_logger'].buffer = \
+                manager.list(current_app.config['mem_logger'].buffer)
         except RuntimeError:
             #  It might be the case that we are not running in the app.
             #  In this case, don't bother memory logging, stdout should
             #  sufficent for console mode.
             pass
-        atexit.register(cleanup)
-        signal.signal(signal.SIGTERM, sigHandler)
+
 
 def sigHandler(signo, frame):
     sys.exit(0)
 
 
 def cleanup():
     print "Running process cleanup."
     for p in JOBS:
         print "Waiting for", p.name, p.pid
         if p.is_alive:
             p.join()
 
+
+atexit.register(cleanup)
+signal.signal(signal.SIGTERM, sigHandler)
+
+
 def async_fieldchange_calc(db_name, ts, run):
     """Run regenerate field changes in the background."""
     func_args = {'run_id': run.id}
     #  Make sure this run is in the database!
     async_run_job(fieldchange.post_submit_tasks,
                   db_name, ts,
                   func_args)