RFR: 8366154: Validate thread type requirements in debug commands

[Francesco Andreuzzi]

On Mon, 1 Sep 2025 11:36:21 GMT, Kerem Kat <krk at openjdk.org> wrote:

> Prevents segmentation faults during `gdb` sessions. The crashes were caused by the `ResourceMark` constructor being called on a native thread, which is not supported. This happened when invoking debug commands that require a `Thread` or `JavaThread` context from an incorrect thread type.
> 
> ### Solution
> 
> This change introduces `onThread()` and `onJavaThread()` helper methods to the `Command` class. These methods validate the thread context and ensure `ResourceMark` is only created when on a valid VM thread. All thread-dependent debug commands now use these guards to validate the context, printing a clear error and exiting gracefully upon failure.
> 
> ### Testing
> 
> Manually verified using `gdb` by calling the modified commands (`ps`, `universe`, `pns`, etc.) from different thread contexts (native, Java, and non-java threads) to ensure they fail gracefully with an error message instead of crashing the debug session.

src/hotspot/share/utilities/debug.cpp line 338:

> 336:     if (JavaThread::active() == nullptr) {
> 337:       tty->print_cr("Failed: Current thread is not a java thread or the vm thread");
> 338:       return  false;

Suggestion:

      return false;

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PR Review Comment: https://git.openjdk.org/jdk/pull/27033#discussion_r2313718023