Merge pull request #20306 from babsingh/main9

Return NULL from getStackTraceForThread for unsteady virtual threads

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;