JDK-8275509: (jlink) SystemModulesPlugin generates a jdk.internal.module.SystemModules$all.class which isn't reproducible

[Ioi Lam]

On 10/21/21 5:25 AM, Alan Bateman wrote:
> On 21/10/2021 10:49, Jaikiran Pai wrote:
>> :
>>
>> Digging into it, it appears that since the ModuleDescriptor#equals() 
>> calls equals() on enum types (in this specific case on 
>> ModuleDescriptor.Requires.Modifier) and since enum type equality is 
>> implemented as identity checks, those identity checks are 
>> surprisingly failing. More specifically 
>> ModuleDescriptor.Requires.Modifier.MANDATED == 
>> ModuleDescriptor.Requires.Modifier.MANDATED is equating to false 
>> because at runtime I see that two different instances of 
>> ModuleDescriptor.Requires.Modifier.MANDATED have been loaded (by the 
>> same boot module classloader). Although I use 
>> ModuleDescriptor.Requires.Modifier.MANDATED as an example, the same 
>> is reproducible with other enum values like 
>> ModuleDescriptor.Requires.Modifier.TRANSITIVE.
>>
>> This appears to be specific to CDS since running the above program with:
>>
>> java -Xshare:off EnumEquality
>>
>> succeeds and the ModuleDescriptor equality check passes.
>>
>> In short, it looks like there is some general issue with CDS and 
>> equality checks with enums and perhaps deserves a separate JBS issue?
> I've asked Ioi Lam to comment on this, off-hand I'm not aware of any 
> issues with CDS here but it may be related to the archiving of object 
> graphs.
>
> -Alan

Hi Jaikiran and Alan,

Thanks for reporting this issue. It's a bug in CDS. I have filed 
https://bugs.openjdk.java.net/browse/JDK-8275731 and am working on a fix.

This is my initial analysis of the problem.

====>>> Can anyone think of similar problems that may happen elsewhere?

The static constructors of enum classes are executed at both CDS dump 
time and run time. E.g.,

     public enum Modifier {
         OPEN
     }

The <clinit> method essentially does this:

     public static final Modifier OPEN = new Modifier("OPEN");

If a reference of Modifier.OPEN is stored inside the CDS archived heap 
during dump time, it will be different than the value of Modifier.OPEN 
that is re-created at runtime by the execution of Modifier.<clinit>

Thanks
- Ioi