Changes between Version 75 and Version 76 of DjangoSpecifications/Core/Threading

Timestamp:

Sep 21, 2009, 8:46:11 AM (16 years ago)

Author:

mrts

Comment:

--

DjangoSpecifications/Core/Threading

@@ -90 +90 @@
          tmp.append(x)
       foo = tuple(tmp) # assignment is "atomic", tuple is const-correct and more efficient
+}}}
+
+== Lock handling ==
+
+Locks should always be acquired in exception-safe manner, otherwise the system will deadlock when an exception is raised while a lock is held.
+
+Consider the following example (#11193):
+
+'''WRONG'''
+{{{
+        if self._request_middleware is None:
+            self.initLock.acquire()
+            # Check that middleware is still uninitialised.
+            if self._request_middleware is None:
+                self.load_middleware()
+            self.initLock.release()
+}}}
+
+This will deadlock when `self.load_middleware()` raises an exception (the lock will never be released).
+
+'''RIGHT'''
+{{{
+        if self._request_middleware is None:
+            self.initLock.acquire()
+            try:
+                # Check that middleware is still uninitialised.
+                if self._request_middleware is None:
+                    self.load_middleware()
+            except:
+                # possibly unload whatever middleware didn't fail here
+                # set the guard back to uninitialized state
+                self._request_middleware = None
+                raise
+            finally:
+                self.initLock.release()
 }}}