tests: fix and work around some race conditions in tests

These race conditions exist even with the GIL but are unlikely to be
triggered unless the GIL is disabled.

Lib/test/test_concurrent_futures.py

@@ -918,9 +918,9 @@ def acquire_lock(lock):
 
     def test_idle_thread_reuse(self):
         executor = self.executor_type()
-        executor.submit(mul, 21, 2).result()
-        executor.submit(mul, 6, 7).result()
-        executor.submit(mul, 3, 14).result()
+        for args in [(21, 2), (6, 7), (3, 14)]:
+            executor.submit(mul, *args).result()
+            time.sleep(0.01)  # wait a tiny bit for the thread to be idle
         self.assertEqual(len(executor._threads), 1)
         executor.shutdown(wait=True)
 

Lib/test/test_faulthandler.py

@@ -580,7 +580,7 @@ def run(self):
         regex = r"""
             ^Thread 0x[0-9a-f]+ \(most recent call first\):
             (?:  File ".*threading.py", line [0-9]+ in [_a-z]+
-            ){{1,3}}  File "<string>", line 23 in run
+            ){{1,3}}  File "<string>", line (22|23) in run
               File ".*threading.py", line [0-9]+ in _bootstrap_inner
               File ".*threading.py", line [0-9]+ in _bootstrap
 

Lib/test/test_sys.py

@@ -477,6 +477,7 @@ def test_current_exceptions(self):
         # thread does sys._current_frames(), and verifies that the frames
         # returned make sense.
         entered_g = threading.Event()
+        raised_err = threading.Event()
         leave_g = threading.Event()
         thread_info = []  # the thread's id
 
@@ -490,6 +491,7 @@ def g456():
                 try:
                     raise ValueError("oops")
                 except ValueError:
+                    raised_err.set()
                     if leave_g.wait(timeout=support.LONG_TIMEOUT):
                         break
 
@@ -503,6 +505,8 @@ def g456():
         self.assertEqual(len(thread_info), 1)
         thread_id = thread_info[0]
 
+        raised_err.wait()
+
         d = sys._current_exceptions()
         for tid in d:
             self.assertIsInstance(tid, int)
@@ -529,7 +533,8 @@ def g456():
         # And the next record must be for g456().
         filename, lineno, funcname, sourceline = stack[i+1]
         self.assertEqual(funcname, "g456")
-        self.assertTrue(sourceline.startswith("if leave_g.wait("))
+        self.assertTrue(sourceline.startswith("if leave_g.wait(") or
+                        sourceline.startswith("raised_err.set("))
 
         # Reap the spawned thread.
         leave_g.set()