utils/misc: Cleanup check_output()

* Remove check_output_lock as the issue has been fixed in Python 3.4
* Use Popen process as a context manager. Technically,
  Popen.communicate() already achieves those but the context manager
  will ensure this is done even if an exception happens at any point.

devlib/utils/misc.py

@@ -151,22 +151,16 @@ def preexec_function():
 
 
 check_output_logger = logging.getLogger('check_output')
-# Popen is not thread safe. If two threads attempt to call it at the same time,
-# one may lock up. See https://bugs.python.org/issue12739.
-check_output_lock = threading.RLock()
-
 
 def get_subprocess(command, **kwargs):
     if 'stdout' in kwargs:
         raise ValueError('stdout argument not allowed, it will be overridden.')
-    with check_output_lock:
-        process = subprocess.Popen(command,
+    return subprocess.Popen(command,
                                 stdout=subprocess.PIPE,
                                 stderr=subprocess.PIPE,
                                 stdin=subprocess.PIPE,
                                 preexec_fn=preexec_function,
                                 **kwargs)
-    return process
 
 
 def check_subprocess_output(process, timeout=None, ignore=None, inputtext=None):
@@ -181,6 +175,7 @@ def check_subprocess_output(process, timeout=None, ignore=None, inputtext=None):
         message = 'Invalid value for ignore parameter: "{}"; must be an int or a list'
         raise ValueError(message.format(ignore))
 
+    with process:
         try:
             output, error = process.communicate(inputtext, timeout=timeout)
         except subprocess.TimeoutExpired as e:
@@ -195,7 +190,7 @@ def check_subprocess_output(process, timeout=None, ignore=None, inputtext=None):
         if timeout_expired:
             raise TimeoutError(process.args, output='\n'.join([output, error]))
 
-    retcode = process.poll()
+    retcode = process.returncode
     if retcode and ignore != 'all' and retcode not in ignore:
         raise subprocess.CalledProcessError(retcode, process.args, output, error)