gh-141786 Fix missing parent executor during trace

An executor's trace can point to another executor, forming a graph
of traces / executors. Sometimes it is possible while recording a
trace that the parent executor is freed / invalidated halfway. This
leads to the bug described in the issue gh-141786, which if left
unfixed could cause a memory leak.

This patch checks for the validity of the parent executor as well
as allowing JIT from the cold executor. While the cold executor is
not linked, it is the executor responsible for creating side traces
and we still want to JIT from it.

Author: caje731

Python/ceval.c

@@ -1408,7 +1408,9 @@ stop_tracing_and_jit(PyThreadState *tstate, _PyInterpreterFrame *frame)
         // Likewise, we hold a strong reference to the executor containing this exit, so the exit is guaranteed
         // to be valid to access.
         if (err <= 0) {
-            exit->temperature = restart_backoff_counter(exit->temperature);
+            if (exit->executor->vm_data.linked || exit->executor->vm_data.valid) {
+                exit->temperature = restart_backoff_counter(exit->temperature);
+            }
         }
         else {
             exit->temperature = initial_temperature_backoff_counter();

Python/optimizer.c

@@ -140,6 +140,14 @@ _PyOptimizer_Optimize(
     }
     assert(!interp->compiling);
     assert(_tstate->jit_tracer_state.initial_state.stack_depth >= 0);
+    _PyExitData *exit = _tstate->jit_tracer_state.initial_state.exit;
+    _PyExecutorObject *cold_executor = _PyExecutor_GetColdExecutor();
+    if (exit != NULL &&
+        (!exit->executor->vm_data.linked || !exit->executor->vm_data.valid) &&
+        exit->executor != cold_executor) {
+        // gh-141786 Parent executor is either unlinked or invalid - cannot optimize.
+        return 0;
+    }
 #ifndef Py_GIL_DISABLED
     assert(_tstate->jit_tracer_state.initial_state.func != NULL);
     interp->compiling = true;
@@ -185,7 +193,6 @@ _PyOptimizer_Optimize(
     else {
         executor->vm_data.code = NULL;
     }
-    _PyExitData *exit = _tstate->jit_tracer_state.initial_state.exit;
     if (exit != NULL) {
         exit->executor = executor;
     }