Bug 1139722 - Allow use of nested job objects in mozprocess on windows versions that support them.;r=ahal

This patch detects when breaking away from the parent job isn't strictly necessary to manage processses in a new job to allow using nested job objects to manage processes and their children. Loss of job object functionality is handled in mozprocess as non-fatal, however mozrunner and others doing things like restarting firefox require it.

testing/mozbase/mozprocess/mozprocess/processhandler.py

@@ -211,11 +211,12 @@ class ProcessHandlerMixin(object):
                     comspec = os.environ.get("COMSPEC", "cmd.exe")
                     args = comspec + " /c " + args
 
-                # determine if we can create create a job
-                canCreateJob = winprocess.CanCreateJobObject()
+                # Determine if we can create a job or create nested jobs.
+                can_create_job = winprocess.CanCreateJobObject()
+                can_nest_jobs = self._can_nest_jobs()
 
                 # Ensure we write a warning message if we are falling back
-                if not canCreateJob and not self._ignore_children:
+                if not (can_create_job or can_nest_jobs) and not self._ignore_children:
                     # We can't create job objects AND the user wanted us to
                     # Warn the user about this.
                     print >> sys.stderr, "ProcessManager UNABLE to use job objects to manage child processes"
@@ -223,9 +224,9 @@ class ProcessHandlerMixin(object):
                 # set process creation flags
                 creationflags |= winprocess.CREATE_SUSPENDED
                 creationflags |= winprocess.CREATE_UNICODE_ENVIRONMENT
-                if canCreateJob:
+                if can_create_job:
                     creationflags |= winprocess.CREATE_BREAKAWAY_FROM_JOB
-                else:
+                if not (can_create_job or can_nest_jobs):
                     # Since we've warned, we just log info here to inform you
                     # of the consequence of setting ignore_children = True
                     print "ProcessManager NOT managing child processes"
@@ -244,7 +245,7 @@ class ProcessHandlerMixin(object):
                 self.pid = pid
                 self.tid = tid
 
-                if not self._ignore_children and canCreateJob:
+                if not self._ignore_children and (can_create_job or can_nest_jobs):
                     try:
                         # We create a new job for this process, so that we can kill
                         # the process and any sub-processes
@@ -318,6 +319,17 @@ falling back to not using job objects for managing child processes"""
                     if i is not None:
                         i.Close()
 
+            def _can_nest_jobs(self):
+                # Per:
+                # https://msdn.microsoft.com/en-us/library/windows/desktop/hh448388%28v=vs.85%29.aspx
+                # Nesting jobs came in with windows versions starting with 6.2 according to the table
+                # on this page:
+                # https://msdn.microsoft.com/en-us/library/ms724834%28v=vs.85%29.aspx
+                winver = sys.getwindowsversion()
+                return (winver.major > 6 or
+                        winver.major == 6 and winver.minor >= 2)
+
+
             # Windows Process Manager - watches the IO Completion Port and
             # keeps track of child processes
             def _procmgr(self):