RFR: 8252657: JVMTI agent is not unloaded when Agent_OnAttach is failed

[Yasumasa Suenaga]

On Thu, 12 Nov 2020 01:34:33 GMT, Yasumasa Suenaga <ysuenaga at openjdk.org> wrote:

>> If we can change the spec that agent library would not be unloaded when `Agent_OnAttach()` failed, we can change like [webrev.00](https://cr.openjdk.java.net/~ysuenaga/JDK-8252657/webrev.00/). It is simple, and similar behavior with `Agent_OnLoad()`. It might be prefer for JVMTI agent developers.
>
> In case of `Agent_OnLoad()`, if it is failed (it returns other than zero), JVM is aborted and `Agent_OnUnload()` is not called. I think it is compliant with [JVMTI spec of Agent_OnUnload()](https://docs.oracle.com/en/java/javase/15/docs/specs/jvmti.html#onunload) which says uncontrolled shutdown (aborting JVM) is an exception to this rule.
> 
> I will add CSR for this fix, but I want to discuss what we should do before. I like that `Agent_OnUnload()` wouldn't be called when `Agent_OnAttach()` is failed if we can change the spec - it is consistent and friendly with `Agent_OnLoad()`.

I added [CSR for this PR](https://wiki.openjdk.java.net/display/csr/Main) and updated patch. Could you review it?
I think it is reasonable to clarify the spec not to call `Agent_OnUnload()` when `Agent_OnLoad()` or `Agent_OnAttach()` are failed. It is nature for me, and it does not change current behavior in HotSpot.

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PR: https://git.openjdk.java.net/jdk/pull/19