RFR: 8281615: Deadlock caused by jdwp agent [v2]

[Chris Plummer]

On Wed, 16 Feb 2022 00:17:46 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> I am not sure if it is possible, but checking bagSize(deletedSignatures) == 0 seems to race against classTrack_reset() where it does not take handlerLock lock.

I had thought of that too, but I think the way `classTrack_reset()` is called, it is likely not possible for there to be a `classTrack_processUnloads()` also coming in because everything is shut down:

``` 
    threadControl_onDisconnect();
    standardHandlers_onDisconnect();

    /*
     * Cut off the transport immediately. This has the effect of
     * cutting off any events that the eventHelper thread might
     * be trying to send.
     */
    transport_close();
    debugMonitorDestroy(cmdQueueLock);

    /* Reset for a new connection to this VM if it's still alive */
    if ( ! gdata->vmDead ) {
        debugInit_reset(getEnv());    <--- calls classTrack_reset()
    }

-------------

PR: https://git.openjdk.java.net/jdk/pull/7461