Manage file locking using the 'with' statement

This is clearer and more exception-safe.

(imported from commit b67641b05da9dbf8e5a887f398bac81ab5985cf3)

zephyr/lib/context_managers.py

@@ -0,0 +1,36 @@
+"""
+Context managers, i.e. things you can use with the 'with' statement.
+"""
+
+import fcntl
+from os import path
+from contextlib import contextmanager
+
+@contextmanager
+def flock(lockfile, shared=False):
+    """Lock a file object using flock(2) for the duration of a 'with' statement.
+
+       If shared is True, use a LOCK_SH lock, otherwise LOCK_EX."""
+
+    fcntl.flock(lockfile, fcntl.LOCK_SH if shared else fcntl.LOCK_EX)
+    try:
+        yield
+    finally:
+        fcntl.flock(lockfile, fcntl.LOCK_UN)
+
+@contextmanager
+def lockfile(filename, shared=False):
+    """Lock a file using flock(2) for the duration of a 'with' statement.
+
+       If shared is True, use a LOCK_SH lock, otherwise LOCK_EX.
+
+       The file is given by name and will be created if it does not exist."""
+
+    if not path.exists(filename):
+        with open(filename, 'w') as lock:
+            lock.write('0')
+
+    # TODO: Can we just open the file for writing, and skip the above check?
+    with open(filename, 'r') as lock:
+        with flock(lock, shared=shared):
+            yield

zephyr/models.py

@@ -6,13 +6,13 @@ import base64
 import calendar
 from zephyr.lib.cache import cache_with_key
 from zephyr.lib.initial_password import initial_password, initial_api_key
-import fcntl
 import os
 import simplejson
 from django.db import transaction, IntegrityError
 from zephyr.lib import bugdown
 from zephyr.lib.bulk_create import batch_bulk_create
 from zephyr.lib.avatar import gravatar_hash
+from zephyr.lib.context_managers import lockfile
 import requests
 from django.contrib.auth.models import UserManager
 from django.utils import timezone
@@ -473,15 +473,9 @@ def get_user_profile_by_id(uid):
 # Store an event in the log for re-importing messages
 def log_event(event):
     assert("timestamp" in event)
-    if not os.path.exists(settings.MESSAGE_LOG + '.lock'):
+    with lockfile(settings.MESSAGE_LOG + '.lock'):
-        with open(settings.MESSAGE_LOG + '.lock', 'w') as lock:
-            lock.write('0')
-
-    with open(settings.MESSAGE_LOG + '.lock', 'r') as lock:
-        fcntl.flock(lock, fcntl.LOCK_EX)
         with open(settings.MESSAGE_LOG, 'a') as log:
             log.write(simplejson.dumps(event) + '\n')
-        fcntl.flock(lock, fcntl.LOCK_UN)
 
 def log_message(message):
     if not message.sending_client.name.startswith("test:"):