avoid holding on a not alive Thread in GilReleaseScheduler

Author: qunaibit

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/runtime/AsyncHandler.java

@@ -59,8 +59,8 @@
 import com.oracle.graal.python.builtins.objects.function.PArguments;
 import com.oracle.graal.python.builtins.objects.function.Signature;
 import com.oracle.graal.python.nodes.PRootNode;
-import com.oracle.graal.python.nodes.call.CallNode;
 import com.oracle.graal.python.nodes.call.CallDispatchers;
+import com.oracle.graal.python.nodes.call.CallNode;
 import com.oracle.graal.python.nodes.frame.ReadCallerFrameNode;
 import com.oracle.graal.python.runtime.ExecutionContext.CalleeContext;
 import com.oracle.graal.python.runtime.exception.ExceptionUtils;
@@ -388,15 +388,23 @@ protected void perform(ThreadLocalAction.Access access) {
                                 }
                             }
                         });
-                    } else if (gilOwner != lastGilOwner) {
+                    } else if (gilOwner != lastGilOwner && gilOwner.isAlive()) {
                         /*
                          * If the gil changed owner since the last time we observed it, clear the
                          * flag to make sure we don't get stuck if the last owner exits before
                          * executing the safepoint.
                          */
                         gilReleaseRequested = false;
                     }
-                    lastGilOwner = gilOwner;
+                    if (gilOwner.isAlive()) {
+                        lastGilOwner = gilOwner;
+                    } else {
+                        /*
+                         * we should only store the thread if the thread is still alive, otherwise,
+                         * we will be referring to an object that should have been collected.
+                         */
+                        lastGilOwner = null;
+                    }
                 }
             }
         }