bpo-44645: Check for interrupts on any potentially backwards edge (GH-27216) (GH-27235)

(cherry picked from commit d09c134)

Co-authored-by: Mark Shannon <mark@hotpy.org>

Author: miss-islington

Lib/test/test_threading.py

@@ -1604,6 +1604,31 @@ def test_interrupt_main_invalid_signal(self):
         self.assertRaises(ValueError, _thread.interrupt_main, signal.NSIG)
         self.assertRaises(ValueError, _thread.interrupt_main, 1000000)
 
+    @threading_helper.reap_threads
+    def test_can_interrupt_tight_loops(self):
+        cont = [True]
+        started = [False]
+        interrupted = [False]
+
+        def worker(started, cont, interrupted):
+            iterations = 100_000_000
+            started[0] = True
+            while cont[0]:
+                if iterations:
+                    iterations -= 1
+                else:
+                    return
+                pass
+            interrupted[0] = True
+
+        t = threading.Thread(target=worker,args=(started, cont, interrupted))
+        t.start()
+        while not started[0]:
+            pass
+        cont[0] = False
+        t.join()
+        self.assertTrue(interrupted[0])
+
 
 class AtexitTests(unittest.TestCase):
 

Python/ceval.c

@@ -3759,14 +3759,17 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int throwflag)
             if (Py_IsFalse(cond)) {
                 Py_DECREF(cond);
                 JUMPTO(oparg);
+                CHECK_EVAL_BREAKER();
                 DISPATCH();
             }
             err = PyObject_IsTrue(cond);
             Py_DECREF(cond);
             if (err > 0)
                 ;
-            else if (err == 0)
+            else if (err == 0) {
                 JUMPTO(oparg);
+                CHECK_EVAL_BREAKER();
+            }
             else
                 goto error;
             DISPATCH();
@@ -3783,12 +3786,14 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int throwflag)
             if (Py_IsTrue(cond)) {
                 Py_DECREF(cond);
                 JUMPTO(oparg);
+                CHECK_EVAL_BREAKER();
                 DISPATCH();
             }
             err = PyObject_IsTrue(cond);
             Py_DECREF(cond);
             if (err > 0) {
                 JUMPTO(oparg);
+                CHECK_EVAL_BREAKER();
             }
             else if (err == 0)
                 ;