8371083: FollowReferences reports non-class objects as JVMTI_HEAP_REFERENCE_SYSTEM_CLASS

[alexmenkov]

FollowReferences with null initial_object starts heap walking from "heap roots", which include system classes.
All oops from ClassLoaderDataGraph are reported with JVMTI_HEAP_REFERENCE_SYSTEM_CLASS kind, but some of the objects are not classes.
The fix updates FollowReferences to report non-class objects from ClassLoaderDataGraph as JVMTI_HEAP_REFERENCE_OTHER

Testing: tier1..4,hs-tier5-svc

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Issue

• JDK-8371083: FollowReferences reports non-class objects as JVMTI_HEAP_REFERENCE_SYSTEM_CLASS (Bug - P4)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/28224/head:pull/28224
$ git checkout pull/28224

Update a local copy of the PR:
$ git checkout pull/28224
$ git pull https://git.openjdk.org/jdk.git pull/28224/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 28224

View PR using the GUI difftool:
$ git pr show -t 28224

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/28224.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

👋 Welcome back amenkov! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

@alexmenkov The following labels will be automatically applied to this pull request:

• hotspot
• serviceability

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing lists. If you would like to change these labels, use the /label pull request command.

[mlbridge]

Webrevs

• 00: Full (1ee62064)