XML datastore: new directory locking procedure

This new procedure use fcntl functions to lock the file during the
	life of the datastore instance.

	Now, locked directory error is not displayed if if nemubot is not
	correctly closed.

nemubot/datastore/xml.py

@@ -14,7 +14,7 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-import os
+import fcntl, os
 
 from nemubot.datastore.abstract import Abstract
 
@@ -32,22 +32,35 @@ class XML(Abstract):
         if not os.path.isdir(self.basedir):
             os.mkdir(self.basedir)
 
-        lock_file = os.path.join(self.basedir, ".used_by_nemubot")
-        if os.path.exists(lock_file):
-            with open(lock_file, 'r') as lf:
+        lock_path = os.path.join(self.basedir, ".used_by_nemubot")
+
+        self.lock_file = open(lock_path, 'a+')
+        ok = True
+        try:
+            fcntl.lockf(self.lock_file, fcntl.LOCK_EX | fcntl.LOCK_NB)
+        except OSError:
+            ok = False
+
+        if not ok:
+            with open(lock_path, 'r') as lf:
                 pid = lf.readline()
             raise Exception("Data dir already locked, by PID %s" % pid)
 
-        with open(lock_file, 'w') as lf:
-            lf.write(str(os.getpid()))
+        self.lock_file.truncate()
+        self.lock_file.write(str(os.getpid()))
+        self.lock_file.flush()
+
         return True
 
     def close(self):
         """Release a locked path"""
 
-        lock_file = os.path.join(self.basedir, ".used_by_nemubot")
-        if os.path.isdir(self.basedir) and os.path.exists(lock_file):
-            os.unlink(lock_file)
+        if hasattr(self, "lock_file"):
+            self.lock_file.close()
+            lock_path = os.path.join(self.basedir, ".used_by_nemubot")
+            if os.path.isdir(self.basedir) and os.path.exists(lock_path):
+                os.unlink(lock_path)
+            del self.lock_file
             return True
         return False