[Zope-dev] Adding broken/missing support to zope.interface? (was: experimental.broken - Graceful handling of broken interfaces and components in the ZODB)

Sun Apr 8 20:04:37 UTC 2012

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 at 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/a4b648dc78e53c7303ea2d417d2d7c5e7fea24ac
>
> Any help with this last issue would be appreciated.
>
> Ross
>
> Ross Patterson <me at 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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>
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