fix EOFError when execute targetcli commands concurrently

1.fcntl.LOCK_UN means unlock, but save file must own the exclusive lock.
  So, we must use fcntl.LOCK_EX(acquire an exclusive lock) flag in here.
2.Then, fcntl.lockf is different from fcntl.flock:
  a)fcntl.flock always lock for the whole file
  b)fcntl.lockf can only lock part of the file
  c)see https://docs.python.org/2.7/library/fcntl.html for details
  I think we need to call fcntl.flock in here to avoid EOFError.

Signed-off-by: Zou Mingzhe <[email protected]>

configshell/prefs.py

@@ -130,10 +130,12 @@ def save(self, filename=None):
 
         if filename is not None:
             fsock = open(filename, 'wb')
-            fcntl.lockf(fsock, fcntl.LOCK_UN)
             try:
+                fcntl.flock(fsock, fcntl.LOCK_EX)
                 six.moves.cPickle.dump(self._prefs, fsock, 2)
+                fsock.flush()
             finally:
+                fcntl.flock(fsock, fcntl.LOCK_UN)
                 fsock.close()
 
     def load(self, filename=None):
@@ -146,8 +148,9 @@ def load(self, filename=None):
 
         if filename is not None and os.path.exists(filename):
             fsock = open(filename, 'rb')
-            fcntl.lockf(fsock, fcntl.LOCK_SH)
             try:
+                fcntl.flock(fsock, fcntl.LOCK_SH)
                 self._prefs = six.moves.cPickle.load(fsock)
             finally:
+                fcntl.flock(fsock, fcntl.LOCK_UN)
                 fsock.close()