RFR: 8222422: vmTestbase/nsk/jdi/ClassLoaderReference/definedClasses tests failed with Unexpected Exception: null

[Daniel D. Daugherty]

On 5/11/19 6:39 AM, David Holmes wrote:
> On 11/05/2019 2:14 pm, Jean Christophe Beyler wrote:
>> Isn't that the point? The list returned could have unloaded classes  
>> and we can catch it via this exception (from the comment above 
>> the ReferenceType interface):
>>
>>   * Any method on <code>ReferenceType</code> or which directly or 
>> indirectly takes
>>   * <code>ReferenceType</code> as parameter may throw
>>   * {@link ObjectCollectedException} if the mirrored type has been 
>> unloaded.
>>
>> Turns out that even in the definedClasses, we can get that exception 
>> so we should check for it while walking the reference types as well?
>
> I understand that the list returned to the "debugger" process may 
> contain ReferenceTypes for types that have actually been unloaded by 
> the time it queries them (unless the debuggee is suspended of course). 
> But I don't see how we can encounter those types while compiling the 
> list in the debuggee in the first place.
>
> Something seems amiss here ... possibly just my understanding ...

JC quoted the right bits of spec:

>  * Any method on <code>ReferenceType</code> or which directly or 
> indirectly takes
>  * <code>ReferenceType</code> as parameter may throw
>  * {@link ObjectCollectedException} if the mirrored type has been 
> unloaded.

The JDI client is working with mirrored type references. The debuggee
has the real type references. If the JDI client has not suspended the
debuggee, then the JDI client must guard against ObjectCollectedException.

At one point, we talked about keeping a reference (global JNI handle)
in the debuggee agent, but we quickly came to the conclusion that we
would never know when it would be safe to release those references.

Dan

>
> David
>
>> Jc
>>
>> *From: *Chris Plummer <chris.plummer at oracle.com 
>> <mailto:chris.plummer at oracle.com>>
>> *Date: *Fri, May 10, 2019 at 9:09 PM
>> *To: *David Holmes, Daniil Titov, OpenJDK Serviceability
>>
>>     On 5/10/19 9:03 PM, Chris Plummer wrote:
>>      > On 5/10/19 8:59 PM, David Holmes wrote:
>>      >> Hi Daniil,
>>      >>
>>      >> On 11/05/2019 12:10 pm, Daniil Titov wrote:
>>      >>> Please review the change that fixes an intermittent failure 
>> of the
>>      >>> test.
>>      >>>
>>      >>> The tests checks the implementation of  the
>>      >>> com.sun.tools.jdi.ClassLoaderReference class. The problem 
>> here is
>>      >>> that while
>>      >>> com.sun.tools.jdi.ClassLoaderReferenceImpl.definedClasses()
>>     iterates
>>      >>> over all loaded classes to retrieve a classloader and compares
>>     it to
>>      >>> the current one, some of the classes might become unloaded and
>>      >>> garbage collected (e.g.
>>      >>> org.graalvm.compiler.nodes.InliningLog$$Lambda$41.899832640 or
>>      >>> jdk.internal.reflect.GeneratedConstructorAccessor1, etc.). 
>> If this
>>      >>> happens then the attempt to retrieve a classloader for the
>>     collected
>>      >>> class results in com.sun.jdi.ObjectCollectedException being 
>> thrown.
>>      >>
>>      >> That seems odd to me. If you have a reference to the Class 
>> then it
>>      >> can't be unloaded. I would not expect allClasses() to have
>>      >> weak-references, so a class should not be unloadable while 
>> you are
>>      >> examining it. Unless it is finding VM anonymous classes 
>> (which it
>>      >> should not!).
>>      >>
>>      > I was just typing up something similar. Shouldn't the test do a
>>      > vm.suspend() and then call disableCollection() on each class
>>     returned
>>      > by vm.allClasses()?
>>     Oh wait, this isn't a test. It's part of JDI. I guess it would be 
>> up to
>>     the user of ClassLoaderReference.definedClasses() to do the 
>> suspend. In
>>     fact I'm not sure there's much purpose in calling
>>     ClassLoaderReference.definedClasses() without suspending first. Even
>>     with your changes, the list returned can end up with references to
>>     unloaded classes.
>>
>>     Chris
>>      >
>>      > Chris
>>      >> David
>>      >> -----
>>      >>
>>      >>> The fix catches this com.sun.jdi.ObjectCollectedException and
>>      >>> continues iterating over the rest of the classes.
>>      >>>
>>      >>> Webrev: http://cr.openjdk.java.net/~dtitov/8222422/webrev.01
>>      >>> Bug: https://bugs.openjdk.java.net/browse/JDK-8222422
>>      >>>
>>      >>> Thanks!
>>      >>> --Daniil
>>      >>>
>>      >>>
>>      >
>>      >
>>
>>
>>
>>
>> -- 
>>
>> Thanks,
>> Jc
>