RFR: 8257967: JFR: Events for loaded agents [v16]

[Markus Grönlund]

On Thu, 13 Apr 2023 10:15:02 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

> Your fix introduced a hidden dependency of this new structure on the JPLISEnvironment structure and some Java agents implementation details:
> 
> ```
> 202 struct JPLISEnvironmentMirror {
> 203   jvmtiEnv* mJVMTIEnv; // the JVMTI environment
> 204   const void* mAgent;  // corresponding agent
> 205   jboolean mIsRetransformer; // indicates if special environment
> 206 };
> ```
> 
> It does not look good to me but I can't suggest any other approach at the moment. How important is this part? Have you considered other ways to achieve what is needed?

Yes. It is the key to locating which JavaAgent maps to which JvmtiEnv. I tried some other variants, but those would change the layout of the exported structs in jplisAgent.h, and I don't know if people depend on that layout, implicitly or explicitly. So I choose not go down that route.

This seemed the best alternative since we own jdk.instrument and the implementation on the JDK side is unlikely to change very much.

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PR Comment: https://git.openjdk.org/jdk/pull/12923#issuecomment-1506830279