Re: [Jython-dev] PyType publication with better concurrency

[Jeff A. <ja...@fa...>]

Jim:

I figured out needsInners (I think!) and how I could make fromClass and 
fromClassSkippingInners into one method using the state of the registry 
to steer the action to be almost the same -- and reach the same 
conclusion -- with a net reduction in code.

https://bitbucket.org/tournesol/jython-fgtype/commits/f781dc1f52c9b3733841b375831f8a3c0d27a93e

I have added a test to prove the PyType (actually PyJavaType) is GC'd 
when we cease to reference it, using make_jar_classloader, but it is 
presently failing. Applied retrospectively to successive change sets it 
begins to fail exactly where I introduce ClassValue. :(

Maybe ClassValue doesn't quite work as I thought. Or I'm still not using 
it as John Rose intended. It seems wrong we should have to layer weak 
references on top or have some kind of removal queue.

I tried visualvm to see where it might be referenced, but I'm not sure 
how to interpret what it's telling me. However, this week, and maybe 
longer, other things have priority. Ideas welcome for when I'm back to this.

For exposed PyObjects, I'm pretty sure classToBuilder always was keeping 
a strong reference that would keep them alive now. (But not for the type 
under test.) I've removed this effect. But maybe we don't mind that?
https://bitbucket.org/tournesol/jython-fgtype/commits/f781dc1f52c9b3733841b375831f8a3c0d27a93e#Lsrc/org/python/core/PyType.javaT1208

Jeff

On 12/09/2017 01:37, Jim Baker wrote:
> [I'm been having some problem posting to the jython-dev mailing list, 
> let's see if this response gets through!]
>
> On Mon, Sep 11, 2017 at 1:47 PM, Jeff Allen <ja...@fa... 
> <mailto:ja...@fa...>> wrote:
>
>     Jim:
>
>     Thanks for the pointers. I agree with you on #2 that what I've got
>     *should* behave correctly with class loaders, but is there a test
>     you can suggest (or one I've missed) that conclusively
>     demonstrates it?
>
>
> So I would start with test_support.make_jar_classloader, just because 
> it's easier to test this behavior in Python. test_classpathimporter 
> seems to use this support functionality nicely.
>
> If we want to add testing of our expectations around ClassValue's 
> lifetime, we can also add some sort of weakref scheme, in conjunction 
> with a multi-round GC, to verify that the PyType gets collected as 
> expected. Of course we should expect ClassValue to behave as expected, 
> but this would ensure we are not holding anything, now or in the 
> future implementation, in the Jython runtime that would violate this 
> contract.
>
>
>     I'll press in on the question of inners. I tried just calling
>     fromClass, but it fails when the inner extends the outer
>     (test_import_jy:: test_selfreferential_classes fails). You have to
>     initialise the outer, then visit the inners. In a way, it's the
>     PyJavaType counterpart of the deferred initialisation of exposed
>     types that we control by identifying bootstrap types. But here it
>     has to be self-guiding.
>
>     I have an idea that if I can make fromClass and
>     fromClassSkippingInners the same thing, then that can become the
>     implementation classToValue.computeValue, and I won't need the
>     NotReady exception or findExistingType.
>
>
> So Java classes can form a general type graph (directed, with cycles). 
> It seems to me that the NotReady acts as some sort of coloring in the 
> graph exploration that fromClass is doing. But as I said, I find this 
> code for inner classes rather subtle! Good luck in your coding here, 
> it would be really nice to get some further simplification.
>
>
>     Jeff
>
>     On 10/09/2017 01:12, Jim Baker wrote:
>
>         Thanks for working on this change set, it's really good stuff.
>         In particular, that's a very nice handoff you do in the
>         PyType.Registry class, specifically moving computed types from
>         classToNewType to classToType; and managing the NotReady
>         state. Perhaps it needs some sort of state diagram to
>         illustrate, but as someone who has been in that code, it makes
>         perfect sense.
>
>         With respect to GC, the following should be the case:
>
>           * As long as the ClassLoader for the mapped Java class
>         instance C is
>             around, the corresponding PyType instance T will live. This
>             follows from the definition of ClassValue as you point out
>         in the
>             code comments. (Good!)
>           * As long as Python code holds a hard ref to T, the
>         corresponding C
>             will live. (Good!) (So we would see this in from
>         `some_java_pkg
>             import baz`; and it follows from the underlying_class field
>             holding a hard ref from T to C.)
>           * As long as Python code holds a hard ref to an instance of
>         C, the
>             corresponding T will also live. (Good!)
>
>         Otherwise, this mapping will allow for GC of T, and
>         subsequently of C, per the definition of ClassValue. I was
>         also looking at Groovy's usage of a similar mapping, and
>         corresponding bug,
>         https://bugs.openjdk.java.net/browse/JDK-8136353
>         <https://bugs.openjdk.java.net/browse/JDK-8136353>, and it
>         looks like the problem reported there doesn't hold. This is
>         because there are no strong refs outside of PyType itself.
>
>         So everything looks good for getting this change in.
>
>         On Sat, Sep 9, 2017 at 2:12 PM, Jeff Allen <ja...@fa...
>         <mailto:ja...@fa...> <mailto:ja...@fa...
>         <mailto:ja...@fa...>>> wrote:
>
>             Jim, all:
>
>             By a series of small steps I've come quite a long way in this
>             aspect of PyType. It might be worth a look.
>         https://bitbucket.org/tournesol/jython-fgtype/commits/3fc5d8ff48bf5d20cb57169cd3f43a3cd90df0cd
>         <https://bitbucket.org/tournesol/jython-fgtype/commits/3fc5d8ff48bf5d20cb57169cd3f43a3cd90df0cd>
>            
>         <https://bitbucket.org/tournesol/jython-fgtype/commits/3fc5d8ff48bf5d20cb57169cd3f43a3cd90df0cd
>         <https://bitbucket.org/tournesol/jython-fgtype/commits/3fc5d8ff48bf5d20cb57169cd3f43a3cd90df0cd>>
>
>             In this last commit I've entirely dispensed with any long-term
>             reference to the created PyType in the type registry: only the
>             ClassValue holds one. The explicit management of
>         reachability goes
>             away and we have just the behaviour given us by
>         ClassValue. The
>             only down-side I see is that a PyType created for a Java
>         class you
>             handle briefly from Python will not be collectable until
>         the class is.
>
>             The old test for weakness, which I've been running in parallel
>             with the new, requires PyType.getClassToType(), and can no
>         longer
>             be supported. (You can't enumerate the ClassValue.) I
>         believe the
>             replacement tests the same thing -- they used to pass and fail
>             together.
>
>
>         Right, it makes sense to remove this test. This is because we
>         don't want to support the necessary mapping, even though it
>         could be readily supported with an appropriate concurrent weak
>         mapping - the only semantics it would be supporting is for
>         this very test.
>
>
>             I've moved some actions around, and removed some of the
>         checks. We
>             seemingly expected the desired PyType to be created at any
>         moment
>             by a side effect. Reason and regression testing tell me that
>             doesn't happen in the new arrangement. I think there is
>         slightly
>             less code to execute, but masses more comment and debug --
>         it's my
>             way of figuring it out. It's all very educational.
>
>             I still don't understand:
>
>             1. Whether we find the downside (PyType not GC'd until the
>         class is)
>                important.
>
>
>         So we do know that Jython will continue to work if we remove a
>         mapping entry, since it happens all the time currently in some
>         code like json, re, and socket, where classes are not directly
>         imported :) We could always do some sort of LRU and then call
>         ClassValue#remove, but I don't think it makes sense. There are
>         many other other optimizations in the runtime first.
>
>             2. Whether I have broken something that the regression
>         tests don't
>                cover (like use with isolating class loaders).
>
>
>         These are different Java classes if in different class
>         loaders. For Python usage of these Java classes, it's all the
>         same, thanks to duck typing, but we need to maintain a
>         different mapping entry. Again #1 demonstrates we have been
>         handling this correctly in the current codebase.
>
>             3. What needsInners is really up to. I see *what* it does,
>         but not
>                *why* well enough to consider alternatives. It would
>         simplify
>                computeValue if there were no fromClassSkippingInners:
>         could
>                needsInners be a registry global? And why do we skip
>         processing
>                them, when we do?
>
>
>         That codepath is exceedingly subtle, at least to me. But it
>         would be an interesting experiment to see if it could be made
>         global, vs this potentially expanding context that is passed
>         through.
>
>         - Jim
>
>
>