RFR: 8295303: cleanup debug agent's confusing use of EI_GC_FINISH

[Chris Plummer]

This PR renamed EI_GC_FINISH to EI_CLASS_UNLOAD and does some other minor cleanups related to EI_GC_FINISH.

The debug agent deals with 3 types of events: JVMTI (per the spec), JDWP (per the spec), and an event indexing that the debug agent refers to as EI (Event Index) events. We have the following EI_GC_FINISH event. The indexing is into the array of jdwp event handlers which are created when the debugger sends an EventRequest command.

Note there is no EI_CLASS_UNLOAD event that maps to the JDWP CLASS_UNLOAD event, but instead we have:

`        EI_GC_FINISH = 8,`

And these are the mappings between EI_GC_FINISH and JDWP and JVMTI events


  case JVMTI_EVENT_GARBAGE_COLLECTION_FINISH:
      return EI_GC_FINISH;

  case JDWP_EVENT(CLASS_UNLOAD):
      return EI_GC_FINISH;

  index2jvmti[EI_GC_FINISH - EI_min] = JVMTI_EVENT_GARBAGE_COLLECTION_FINISH;

  index2jdwp[EI_GC_FINISH - EI_min] = JDWP_EVENT(CLASS_UNLOAD);


So there is this odd relationship between EI_GC_FINISH and the JDWP CLASS_UNLOAD event. Note that JVMTI does not have a CLASS_UNLOAD (except for unused support in the extension mechanism), and JDWP has no GC_FINISH event.

This relationship between EI_GC_FINISH and the JDWP CLASS_UNLOAD events stems from the fact that at one point JVMTI_EVENT_GARBAGE_COLLECTION_FINISH was used to trigger the synthesizing all of JDWP CLASS_UNLOAD events for classes that unloaded during the last GC. That is no longer the case, and instead each time a JVMTI OBJECT_FREE event is triggered for a Class instance, the JDWP CLASS_UNLOAD is generated.

Since JDWP CLASS_UNLOAD maps to EI_GC_FINISH, we have the following:

`    node = getHandlerChain(EI_GC_FINISH)->first;`

By looking at this line of code, you would never guess that this is how you fetch the event handler chain for JDWP CLASS_UNLOAD EventRequests, but it is.

To clean this up I renamed EI_GC_FINISH to EI_CLASS_UNLOAD. However, that still leaves the EI_GC_FINISH to JVMTI_EVENT_GARBAGE_COLLECTION_FINISH mapping to deal with. It's not needed anymore. When we get a JVMTI_EVENT_GARBAGE_COLLECTION_FINISH, this is all we ever do with it:


static void JNICALL
cbGarbageCollectionFinish(jvmtiEnv *jvmti_env)
{
    LOG_CB(("cbGarbageCollectionFinish"));
    ++garbageCollected;
    LOG_MISC(("END cbGarbageCollectionFinish"));
}


So the event is not passed around at all, and doesn't trigger other events or mapping to a JDWP event. It is never used as an "event index". This means jvmti2EventIndex should assert if it is ever passed in, since following mapping should never be needed:


        case JVMTI_EVENT_GARBAGE_COLLECTION_FINISH:
            return EI_GC_FINISH; 


This is accomplished by simply deleting the above code, and failing through to the default error handling code.

Also no longer needed is the following entry:

    index2jvmti[EI_GC_FINISH          -EI_min] = JVMTI_EVENT_GARBAGE_COLLECTION_FINISH

For this we just assign the entry to 0, which will result in an error if the entry is ever referenced.

-------------

Commit messages:
 - fix extra whitespace
 - Rename EI_GC_FINISH to EI_CLASS_UNLOAD and other cleanups related to EI_GC_FINISH

Changes: https://git.openjdk.org/jdk/pull/10887/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10887&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295303
  Stats: 48 lines in 6 files changed: 19 ins; 3 del; 26 mod
  Patch: https://git.openjdk.org/jdk/pull/10887.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10887/head:pull/10887

PR: https://git.openjdk.org/jdk/pull/10887