experimental.broken is working well for me. It greatly aided me in getting through a particularly nasty upgrade allowing me to cleanup the ZCA cruft left by poorly behaved add-ons. I'd like to proceed with adding the core of this to zope.interface and I need the developers/maintainers to weigh in. The first and most fundamental matter is changing interface pickles such that they can be unpickled into something that can fulfill the minimum interface contract and don't break the ZCA. To that end, I'd like to add the following to zope.interface.interface: ... try: from ZODB.broken import find_global from ZODB.broken import IBroken def find_interface(modulename, globalname, Broken=IBroken, type=InterfaceClass): """ Find a global interface, returning a broken interface if it can't be found. """ return find_global(modulename, globalname, Broken=IBroken, type=InterfaceClass) except ImportError: IBroken = Interface def find_interface(modulename, globalname, Broken=IBroken, type=InterfaceClass): """ Find a global interface, raising ImportError if it can't be found. """ # From pickle.Unpickler.find_class __import__(module) mod = sys.modules[module] klass = getattr(mod, name) return klass ... class InterfaceClass(Element, InterfaceBase, Specification): ... def __reduce__(self): if self is IBroken: return self.__name__ return (find_interface, (modulename, globalname)) ... With this change, previously existing interface pickles will be different from newly committed ones but both pickles would unpickle to the same object. Also, running zodbupdate would convert all pickles to the new format. Is this the correct approach? If not, how should it be changed or what might be the correct way? Is there a better way to put the broken object support in ZODB and still have the same interface pickles when using ZODB? This still leaves the problem of instance provides declarations and component registrations which contain previously existing interface pickles for missing interfaces. Without addressing these, previously existing instance pickles cannot be unpickled and all component registry operations fail. experimental.broken addresses these by adding handling for broken interfaces when provides declaration are unpickled (in the ProvidesClass.__init__ method) and when component registries are unpickled (in the __setstate__ method of persistentregistry.PersistentAdapterRegistry and persistentregistry.PersistentComponents). Again, with these patches, missing interfaces don't break instances or registries and allow running zodbupdate to fix all existing pickles. Where should this support live? Should I keep it in a separate package, maybe just rename experimental.broken to z3c.broken or somesuch? Should it be merged into zope.interface and zope.component? Will the developers/maintainers of zope.interface, zope.component and/or ZODB please weigh in on this and give me feedback towards getting this finished? Thanks! Ross Ross Patterson <me@rpatterson.net> writes:
I've done some more work on this and I've gotten the component registrations fully working now with one exception that I'm having real trouble with. I'd like some help with that, more below. I'm also a bit more clear on what might be appropriate to bring back into zope.interface and I'd like feedback on that.
Currently interfaces are pickled as globals. Given their centrality in any ZCA application, I think they should be pickled using a function:
def __reduce__(self): return (find_interface, (modulename, globalname))
where find_interface, if ZODB.broken.find_global can be imported, in turn calls:
ZODB.broken.find_global(modulename, globalname, Broken=IBroken, type=InterfaceClass)
since find_global already has nicely abstracted graceful missing global handling.
If this were added to zope.interface, and changed ZODB objects with marker interfaces or persistent registries where all the code for the interfaces is still available will then be updated with pickles that will gracefully handle missing interfaces in the future. Thus you could use zodbupdate to make sure that the interfaces in your ZODB will fail gracefully in the future. Do you agree/disagree that this belongs in zope.interface?
What this will not address are existing interfaces-as-globals pickles where the original interface is now missing. That's where the other patches in experimental.broken come in, they intercept the two contexts where we can infer that a missing global should be an interface: instance provides declarations and persistent component registries. By hooking into the unpickling of those objects, we can replace broken classes with broken interfaces as appropriate for those structures. With these patches installed it should be possible to use applications with missing interfaces and to use zodbupdate to make sure that even *already missing* interfaces will fail gracefully both now and in the future. I think these patches don't belong in zope.interface/component and if they work I would likely move them to five.localsitemanager along with a ZCML file that is not loaded by default. Does that sound right?
Then one thing I haven't been able to get working is making it possible to commit a changed persistent registry when it includes a component registration for a non-persistent component whose class/type is missing. This works just fine for a *persistent* component whose class/type is missing but fails for a non-persistent component. The error is:
AttributeError: 'Bar' object has no attribute '__Broken_newargs__'
More specifically, '__Broken_newargs__' is set by the Broken.__new__ method and I've confirmed that this isn't being called by instrumenting __new__, __init__, and __setstate__. Only __setstate__ is called when unpickling the non-persistent broken component not __new__ as should be. Below is the full traceback. I've also left the package repo in the broken state if you want to examine it, the egg checkout is also a buildout:
https://github.com/rpatterson/experimental.broken/commit/a4b648dc78e53c7303e...
Any help with this last issue would be appreciated.
Ross
Ross Patterson <me@rpatterson.net> writes:
Please take a look at experimental.broken:
https://github.com/rpatterson/experimental.broken http://pypi.python.org/pypi/experimental.broken
The handling of broken objects by the ZODB can make an application with add-ons that use zope.interface far too fragile. If marker interfaces from an add-on are used on objects in the ZODB, removing that add-on can make any zope.interface operation on that object fail. Even worse, if an add-on registers any components in a registry in the ZODB, that entire registry will become unusable for any ZCA operations which pretty much breaks everything, including admin interfaces.
Since the interfaces and the ZCA are often core parts of an application using the ZODB, it may be appropriate to add special handling for broken objects to those services. The experimental.broken patches are my attempt to prototype such special handling.
For objects in the ZODB which directly provide a marker interface, these patches allow that object to behave as without the application of the marker interface if the interface is no longer available. If the interface is made available again, the full behavior of that interface is restored. Similarly, if a component whose class, provided interface, or required interfaces are missing, these patches allow the registry to perform lookups it would have been able to do without the broken component registration. If the component class, provided interface, and required interfaces are restored, then the component registration is fully restored.
If an object or registry in the ZODB is committed to the ZODB with broken interfaces or components, the commit will succeed and it is still possible to fully restore previous behavior if the missing classes and interfaces are restored. Unfortunately, because interfaces are pickled as globals, there's no good way to have the same pickle written on commit for the interface as was in the original pickle, but it should behave exactly the same.
The intention of this package is to see if the implementation of broken object handling is correct and robust enough to merge into zope.interface and zope.component themselves. Is this the right approach? If not why and what would be better? How might this approach be improved?
Ross
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