RFR: 8262376: ReplaceCriticalClassesForSubgraphs.java fails if --with-build-jdk is used

[Ioi Lam]

On Thu, 29 Apr 2021 17:17:20 GMT, Calvin Cheung <ccheung at openjdk.org> wrote:

>> When CDS tries to load sun.util.resources.cldr.provider.CLDRLocaleDataMetaInfo from the classlist, the VM has already finished the "early" bootup phase. To load this class, the module system tries to create an instance of jdk.internal.module.SystemModuleFinders$SystemModuleReader and store that into the ArchivedBootLayer. However, the SystemModuleReader class is also not yet loaded, so the VM loads it on demand. As a result, SystemModuleReader is treated as a "non-early class".
>> 
>> The problem is now ArchivedBootLayer refers to a non-early class. At runtime, if JVMTI ClassLoadHook is enabled, HeapShared will refuse to load ArchivedBootLayer from the archived heap. The reason is explained by this in heapShared.cpp:
>> 
>> 
>> // Note: if a ArchivedKlassSubGraphInfoRecord contains non-early classes, and JVMTI
>> // ClassFileLoadHook is enabled, it's possible for this class to be dynamically replaced. In
>> // this case, we will not load the ArchivedKlassSubGraphInfoRecord and will clear its roots.
>> 
>> 
>> However, this doesn't make sense for ArchivedBootLayer -- it's loaded in the early phase at runtime, so any classes referenced by ArchivedBootLayer will also be loaded in the early phase. This means JVMTI ClassLoadHook cannot replace SystemModuleReader.
>> 
>> Hence, the fix is simple -- for any classes referenced by the 3 "full-module-graph" subgraphs, we always treat them as "early" classes, regardless of when they are loaded during dump time.
>> 
>> 
>> // Entry fields for subgraphs archived in the open archive heap region (full module graph).
>> static ArchivableStaticFieldInfo fmg_open_archive_subgraph_entry_fields[] = {
>>   {"jdk/internal/loader/ArchivedClassLoaders", "archivedClassLoaders"},
>>   {"jdk/internal/module/ArchivedBootLayer", "archivedBootLayer"},
>>   {"java/lang/Module$ArchivedData", "archivedData"},
>> };
>> 
>> 
>> The validate the fix, I modified the test case to always load CLDRLocaleDataMetaInfo during dump time. The test failed without the fix and passed with the fix.
>
> Looks good.

Thanks @calvinccheung @dholmes-ora @yminqi for the review!

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PR: https://git.openjdk.java.net/jdk/pull/3780