Ignore threads reporting a fatal error during GC.

Author: peter-hofer

substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/GCImpl.java

@@ -783,6 +783,17 @@ private void blackenStackRoots() {
                          */
                         continue;
                     }
+                    if (VMThreads.StatusSupport.isStatusIgnoreSafepoints(vmThread)) {
+                        /*
+                         * When a thread is ignoring safepoints, it is reporting a fatal VM error.
+                         * The thread is not paused and we cannot safely walk its stack (we don't
+                         * even know where it starts). We simply ignore it and keep collecting until
+                         * it terminates the VM rather than crashing here. The thread might access
+                         * the heap to print diagnostics, but it must do so with due caution and
+                         * should not crash because of our ongoing GC.
+                         */
+                        continue;
+                    }
                     if (JavaStackWalker.initWalk(walk, vmThread)) {
                         walkStack(walk);
                     }