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


Ignore:
Timestamp:
09/21/2009 08:46:11 AM (6 years ago)
Author:
mrts
Comment:

--

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  • DjangoSpecifications/Core/Threading

    v75 v76  
    9090         tmp.append(x)
    9191      foo = tuple(tmp) # assignment is "atomic", tuple is const-correct and more efficient
     92}}}
     93
     94== Lock handling ==
     95
     96Locks should always be acquired in exception-safe manner, otherwise the system will deadlock when an exception is raised while a lock is held.
     97
     98Consider the following example (#11193):
     99
     100'''WRONG'''
     101{{{
     102        if self._request_middleware is None:
     103            self.initLock.acquire()
     104            # Check that middleware is still uninitialised.
     105            if self._request_middleware is None:
     106                self.load_middleware()
     107            self.initLock.release()
     108}}}
     109
     110This will deadlock when `self.load_middleware()` raises an exception (the lock will never be released).
     111
     112'''RIGHT'''
     113{{{
     114        if self._request_middleware is None:
     115            self.initLock.acquire()
     116            try:
     117                # Check that middleware is still uninitialised.
     118                if self._request_middleware is None:
     119                    self.load_middleware()
     120            except:
     121                # possibly unload whatever middleware didn't fail here
     122                # set the guard back to uninitialized state
     123                self._request_middleware = None
     124                raise
     125            finally:
     126                self.initLock.release()
    92127}}}
    93128
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