RFR: 8320525: G1: G1UpdateRemSetTrackingBeforeRebuild::distribute_marked_bytes accesses partially unloaded klass

[Thomas Schatzl]

Hi all,

  please review this fix that removes the access to a partially unloaded (i.e. unlinked only) `Klass` used for debug code in `G1UpdateRemSetTrackingBeforeRebuild::distribute_marked_bytes`.

This starts to fail if metadata purging happens before the call to this methods (as https://bugs.openjdk.org/browse/JDK-8317809 suggests). The test gc/g1/humongousObjects/TestHumongousClassLoader.java starts to crash on linux-x86 with 100% reproduction because it more aggressively uncommits memory when purging metaspace.

The fix fixes the asserts to only access the klass when it should not be unloaded yet.

Testing: failing test case not failing any more, gha

Thanks,
  Thomas

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Commit messages:
 - 8320525 initial implementation

Changes: https://git.openjdk.org/jdk/pull/16766/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16766&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320525
  Stats: 9 lines in 1 file changed: 1 ins; 4 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/16766.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16766/head:pull/16766

PR: https://git.openjdk.org/jdk/pull/16766