Make sure our command line calls always exit

sarge's "wait_events" is unreliable. If an asynchronous
job is started but stops immediately and raises a sarge
Exception (inside the async thread), the associated
command's event will never be set event though the
process finished. So we'd wait indefinitely here.

We circumvent this by first waiting until the commands
are parsed and processed (p.commands contains
elements), then until said commands are started and then
making sure the command's process is actually set. Only
then do we actually have a background process running
that we'll be able to monitor further down, otherwise
the command immediately failed.

src/octoprint/util/commandline.py

@@ -9,6 +9,7 @@ __copyright__ = "Copyright (C) 2015 The OctoPrint Project - Released under terms
 import sarge
 import logging
 import re
+import time
 
 from . import to_unicode
 
@@ -80,7 +81,21 @@ class CommandlineCaller(object):
 		kwargs.update(dict(async=True, stdout=sarge.Capture(), stderr=sarge.Capture()))
 
 		p = sarge.run(command, **kwargs)
-		p.wait_events()
+		while len(p.commands) == 0:
+			# somewhat ugly... we can't use wait_events because
+			# the events might not be all set if an exception
+			# by sarge is triggered within the async process
+			# thread
+			time.sleep(0.01)
+
+		# by now we should have a command, let's wait for its
+		# process to have been prepared
+		p.commands[0].process_ready.wait()
+
+		if not p.commands[0].process:
+			# the process might have been set to None in case of any exception
+			self._logger.error(u"Error while trying to run command {}".format(joined_command))
+			return None, [], []
 
 		all_stdout = []
 		all_stderr = []