RFR: 8318026: jcmd should provide access to detailed JVM object information [v13]

[Kevin Walls]

On Thu, 4 Apr 2024 12:46:26 GMT, Kevin Walls <kevinw at openjdk.org> wrote:

>> Introduce the jcmd "VM.inspect" to implement access to detailed JVM object information.
>> 
>> Not recommended for live production use.  Requires UnlockDiagnosticVMOptions and not included in jcmd help output, to remind us this is not a general-purpose customer-facing tool.
>
> Kevin Walls has updated the pull request incrementally with one additional commit since the last revision:
> 
>   VMInspectTest update

Updating to change the "guard" flag.

I was proposing using a -XX "guard" flag at startup time, to enable jcmd VM.inspect, because it allows inspection of arbitrary pointers given by the the user.  With the improved dbg_is_good_oop I have not managed to cause any crashes, but using the guard flag is a cautious approach.

Will not use UnlockDiagnosticVMOptions as that is intended for command-line option processing.

Therefore a new -XX product flag: -XX:+UnlockDiagnosticVMFeatures which defaults to false and is true in debug builds.

While named similarly to UnlockDiagnosticVMOptions, that option is quite long established and well-documented with many search results, so we should not cause confusion.  The logic is that that option relates to enabling further command-line options, and the new option relates to diagnostic features in the VM at runtime.

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PR Comment: https://git.openjdk.org/jdk/pull/17655#issuecomment-2043037830