Return NULL from getStackTraceForThread for unsteady virtual threads

If inspectorCount is -1, then the virtual thread is in an unsteady
state (mounting or unmounting). In such cases, NULL should be returned
and the JCL code should retry in order to avoid unexpected behavior.

Related: #18910

Signed-off-by: Babneet Singh <[email protected]>

runtime/jcl/common/getstacktrace.c

@@ -80,9 +80,16 @@ getStackTraceForThread(J9VMThread *currentThread, J9VMThread *targetThread, UDAT
  */
  walkState.skipCount = 0;
  rc = vmfns->walkContinuationStackFrames(currentThread, targetThread->currentContinuation, threadObject, &walkState);
- } else if (isVirtual && (threadObject != targetThread->threadObject)) {
+ } else if (isVirtual
+ && ((threadObject != targetThread->threadObject)
+ || (-1 == J9OBJECT_I64_LOAD(currentThread, threadObject, vm->virtualThreadInspectorCountOffset)))
+ ) {
  /* If the virtual thread object doesn't match the current thread object, it must have unmounted
  * from this carrier thread, return NULL and the JCL code will handle the retry.
+ *
+ * If inspectorCount is -1, then the virtual thread is in an unsteady state (mounting or unmounting).
+ * In such cases, NULL should be returned and the JCL code should retry in order to avoid unexpected
+ * behavior.
  */
  vmfns->resumeThreadForInspection(currentThread, targetThread);
  goto done;