RFR: 8296089: Remove debug agent code for special handling of Thread.resume()

[Chris Plummer]

On Mon, 31 Oct 2022 19:08:14 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

> The debug agent sets a breakpoint in Thread.resume() so it can prevent the debugger from suspending threads while in the resume call:
> 
>              /*
>               * Track the resuming thread by marking it as being within
>               * a resume and by setting up for notification on
>               * a frame pop or exception. We won't allow the debugger
>               * to suspend threads while any thread is within a
>               * call to resume. This (along with the block below)
>               * ensures that when the debugger
>               * suspends a thread it will remain suspended.
>               */
>              trackAppResume(resumer);
> 
> Now that Thread.resume() is unsupported and just throws UnsupportedOperationException, all debug agent code related to this support can be removed. It's at least a couple of hundred lines of code, and with a fair amount of confusing synchronization. It will be nice to see it go. 
> 
> Also, there is a little bit of code to deal with the app calling Thread.suspend(). I'm a bit unsure of it's removal, because I'm not certain if we also need to consider another JVMTI agent doing a suspend. However, we don't seem to defend against another JVMTI agent resuming a thread, so maybe the debug agent is just not expected to work if another JVMTI agent interferes.  I would appreciate some insight on this possibility. I've called out this code in the inlined review comments.

src/jdk.jdwp.agent/share/native/libjdwp/threadControl.c line 741:

> 739:         node->toBeResumed = JNI_TRUE;
> 740:     }
> 741:     JDI_ASSERT(error != JVMTI_ERROR_THREAD_SUSPENDED);

Should the original code remain in place because another JVMTI agent may have suspended the thread?

src/jdk.jdwp.agent/share/native/libjdwp/threadControl.c line 952:

> 950:      * won't suspend this thread after we are done with the resumeAll.
> 951:      */
> 952:     if (node->suspendCount == 1 && node->suspendOnStart) {

This is also related to Thread.suspend() and the previous comment I made above.

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PR: https://git.openjdk.org/jdk/pull/10922