RFR: JDK-8319784: VM crash during heap dump after JDK-8287061

Tue Nov 14 00:27:52 UTC 2023

This fixes an incorrect assert in `debugInfo.hpp`. The assert is incorrect because this method may be called before rematerialization of the ObjectValue takes place, and therefore, `_selected` will be nullptr. This pull request patches the method to just return `Handle() => nullptr` if the rematerialization of the object hasn't taken place yet. This is the same behavior used by scalar-replaced objects not participating in merges.

I added a test to reproduce a scenario where the assert was causing a problem.

Tested on Ubuntu/MacOS/Windows x86_64 tier1-4.

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Commit messages:
 - Return null handle if _selected == nullptr.

Changes: https://git.openjdk.org/jdk/pull/16622/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16622&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319784
  Stats: 83 lines in 3 files changed: 82 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16622.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16622/head:pull/16622

PR: https://git.openjdk.org/jdk/pull/16622