RFR: 8267879: ClassLoaderMetaspace destructor asserts on !_frozen

[Ioi Lam]

On Wed, 2 Jun 2021 15:27:10 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> ZGC is doing concurrent class unloading while CDS is dumping the archive but it must be unloading some hidden class loader and not what CDS is trying to archive, right?

Yes, ZGC can be unloading non-alive classes while CDS is dumping. I think that's what happened during the crash in this bug report.

Your question led me to finding this bug: [JDK-8268139](https://bugs.openjdk.java.net/browse/JDK-8268139), where CDS could access classes that have been unloaded. I'll try to fix it in JDK 17, too.

Thanks @coleenp and @dholmes-ora for the review.

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PR: https://git.openjdk.java.net/jdk/pull/4286