Le 05/02/14 13:25, Jim Fulton a écrit :
Acquisition is added as a test dependency. Any hint how to replicate
>> >the bug without acquisition is welcome.
>>
>>Define a subclass of Persistent which emulates what Acquisition does, e.g.:
>>
>>   from persistent import Persistent
>>   class Foo(Persistent):
>>       @property
>>       def _p_jar(self): # or whatever attribute trggers
>>           return object()
>
>What if full replication requires a C extension module?
>
>(I hope that's not true and that it is possible to reproduce the bug
>using some fakes, but I haven't spent the time investigating this.)
I'm going to dig into this.  I'm baffled by the assertion that this has
anything to do with readCurrent.


For sure : the POSKeyError happens during connection.commit when checking oids stored in Connection._readCurrent mapping.

(see traceback at http://rpatterson.net/blog/poskeyerror-during-commit)

The _readCurrent mapping is populated only by calls to Connection.readCurrent method.

In the Plone code base, the only way I found to get that Connection.readCurrent method to be called is by adding a key value pair to a BTree.

_BTree_set C function is then called, which in turn calls readCurrent by inlining the PER_READCURRENT macro.

This calls the cPersistence.c readCurrent function, which in turn calls readCurrent method on the ZODB connection.

When setting a key value pair on a new (not already committed) instance of a standard BTree, readCurrent method is not called on the connection.

My understanding is that it is due to the fact that _p_jar and _p_oid are only set during transaction commit.

However, with a new BTree instance that also inherits from Acquisition.Implicit, readCurrent method is called on ZODB connection when setting key value pair. The only explanation I found is that this instance _p_jar attribute has a value (acquired in a way or another ?).

In this case, when readCurrent is called on an object created during a savepoint and this savepoint is rolled back, the oid is leftover in the Connection._readCurrent mapping. This leads to the POSKeyError when committing later as checkCurrentSerialInTransaction cannot check the object since it went away at rollback.

This brings us to the fix I propose: calls to readCurrent should not track objects with oid equal to z64.

AFAICS, this is inline with readCurrent API goal which is (if I understand well) to ensure that an object manipulated by two connections does not get into an incoherent state.

An object that has never been committed cannot be accessed by a second transaction hence does not need to participate in readCurrent dance.

This was a very long explanation which I hope will help to confirm the fix or to come up with a better one.

PS: keep in mind that english is not my mothertongue.
--
Godefroid Chapelle (aka __gotcha) http://bubblenet.be

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