[FIX] race condition in session directory creation

try to create the directory and handle the possible exception instead of doing
an unsafe 2 step check and creation. 

The issues related to the naming of the directory mentionned in the bug report
are not handled.

lp bug: https://launchpad.net/bugs/1157102 fixed

bzr revid: alexandre.fayolle@camptocamp.com-20130319102008-omtaka8dtq9v7m1l

addons/web/http.py

@@ -20,6 +20,7 @@ import traceback
 import urlparse
 import uuid
 import xmlrpclib
+import errno
 
 import babel.core
 import simplejson
@@ -477,8 +478,15 @@ def session_path():
     except Exception:
         username = "unknown"
     path = os.path.join(tempfile.gettempdir(), "oe-sessions-" + username)
-    if not os.path.exists(path):
+    try:
         os.mkdir(path, 0700)
+    except OSError as exc:
+        if exc.errno == errno.EEXIST:
+            # directory exists: ensure it has the correct permissions
+            # this will fail if the directory is not owned by the current user
+            os.chmod(path, 0700)
+        else:
+            raise
     return path
 
 class Root(object):