Fix writing to stdout/stderr in pool worker processes on windows not working

src/calibre/utils/ipc/launch.py

@@ -16,7 +16,7 @@ from calibre.ptempfile import PersistentTemporaryFile, base_dir
 if iswindows:
     import win32process
     try:
-        _windows_null_file = open(os.devnull, 'wb')
+        windows_null_file = open(os.devnull, 'wb')
     except:
         raise RuntimeError('NUL file missing in windows. This indicates a'
                 ' corrupted windows. You should contact Microsoft'
@@ -212,7 +212,7 @@ class Worker(object):
             # On windows when using the pythonw interpreter,
             # stdout, stderr and stdin may not be valid
             args['stdin'] = subprocess.PIPE
-            args['stdout'] = _windows_null_file
+            args['stdout'] = windows_null_file
             args['stderr'] = subprocess.STDOUT
 
         if not iswindows:

src/calibre/utils/ipc/pool.py

@@ -12,6 +12,7 @@ from collections import namedtuple
 from Queue import Queue
 
 from calibre import detect_ncpus, as_unicode, prints
+from calibre.constants import iswindows
 from calibre.ptempfile import PersistentTemporaryFile
 from calibre.utils import join_with_timeout
 from calibre.utils.ipc import eintr_retry_call
@@ -24,6 +25,11 @@ File = namedtuple('File', 'name')
 
 MAX_SIZE = 30 * 1024 * 1024  # max size of data to send over the connection (old versions of windows cannot handle arbitrary data lengths)
 
+worker_kwargs = {'stdout':None}
+if iswindows and getattr(sys, 'gui_app', False):
+    from calibre.utils.ipc.launch import windows_null_file
+    worker_kwargs['stdout'] = worker_kwargs['stderr'] = windows_null_file
+
 class Failure(Exception):
 
     def __init__(self, tf):
@@ -132,7 +138,7 @@ class Pool(Thread):
     def create_worker(self):
         from calibre.utils.ipc.simple_worker import start_pipe_worker
         p = start_pipe_worker(
-            'from {0} import run_main, {1}; run_main({1})'.format(self.__class__.__module__, 'worker_main'), stdout=None)
+            'from {0} import run_main, {1}; run_main({1})'.format(self.__class__.__module__, 'worker_main'), **worker_kwargs)
         sys.stdout.flush()
         eintr_retry_call(p.stdin.write, self.worker_data)
         p.stdin.flush(), p.stdin.close()