Allow subprocess to use posix_spawn()

This is both faster and needed on macOS where one cannot use fork()
when using Qt.

src/calibre/utils/ipc/launch.py

@@ -6,7 +6,6 @@ __copyright__ = '2009, Kovid Goyal <kovid@kovidgoyal.net>'
 __docformat__ = 'restructuredtext en'
 
 import subprocess, os, sys, time
-from functools import partial
 
 from calibre.constants import iswindows, isosx, isfrozen
 from calibre.utils.config import prefs
@@ -171,7 +170,7 @@ class Worker(object):
                     'low'    : 10,
                     'high'   : 20,
             }[priority]
-            args['preexec_fn'] = partial(renice, niceness)
+            args['env']['CALIBRE_WORKER_NICENESS'] = str(niceness)
         ret = None
         if redirect_output:
             self._file = PersistentTemporaryFile('_worker_redirect.log')
@@ -188,12 +187,6 @@ class Worker(object):
             args['stdout'] = windows_null_file
             args['stderr'] = subprocess.STDOUT
 
-        if not iswindows:
-            # Close inherited file descriptors in worker
-            # On windows, this is done in the worker process
-            # itself
-            args['close_fds'] = True
-
         self.child = subprocess.Popen(cmd, **args)
         if 'stdin' in args:
             self.child.stdin.close()

src/calibre/utils/ipc/simple_worker.py

@@ -143,7 +143,6 @@ def create_worker(env, priority='normal', cwd=None, func='main'):
 
 def start_pipe_worker(command, env=None, priority='normal', **process_args):
     import subprocess
-    from functools import partial
     w = Worker(env or {})
     args = {'stdout':subprocess.PIPE, 'stdin':subprocess.PIPE, 'env':w.env}
     args.update(process_args)
@@ -155,14 +154,8 @@ def start_pipe_worker(command, env=None, priority='normal', **process_args):
                 'low'    : win32process.IDLE_PRIORITY_CLASS}[priority]
         args['creationflags'] = win32process.CREATE_NO_WINDOW|priority
     else:
-        def renice(niceness):
-            try:
-                os.nice(niceness)
-            except:
-                pass
         niceness = {'normal' : 0, 'low'    : 10, 'high'   : 20}[priority]
-        args['preexec_fn'] = partial(renice, niceness)
-        args['close_fds'] = True
+        args['env']['CALIBRE_WORKER_NICENESS'] = str(niceness)
 
     exe = w.executable
     cmd = [exe] if isinstance(exe, string_or_bytes) else exe

src/calibre/utils/ipc/worker.py

@@ -177,6 +177,12 @@ def main():
         # so launch the gui as usual
         from calibre.gui2.main import main as gui_main
         return gui_main(['calibre'])
+    niceness = os.environ.pop('CALIBRE_WORKER_NICENESS', None)
+    if niceness:
+        try:
+            os.nice(int(niceness))
+        except Exception:
+            pass
     csw = os.environ.pop('CALIBRE_SIMPLE_WORKER', None)
     if csw:
         mod, _, func = csw.partition(':')