bpo-38091: Import deadlock detection causes deadlock (GH-17518)

Automerge-Triggered-By: @brettcannon

Lib/importlib/_bootstrap.py

@@ -67,6 +67,7 @@ class _ModuleLock:
         # Deadlock avoidance for concurrent circular imports.
         me = _thread.get_ident()
         tid = self.owner
+        seen = set()
         while True:
             lock = _blocking_on.get(tid)
             if lock is None:
@@ -74,6 +75,14 @@ class _ModuleLock:
             tid = lock.owner
             if tid == me:
                 return True
+            if tid in seen:
+                # bpo 38091: the chain of tid's we encounter here
+                # eventually leads to a fixpoint or a cycle, but
+                # does not reach 'me'.  This means we would not
+                # actually deadlock.  This can happen if other
+                # threads are at the beginning of acquire() below.
+                return False
+            seen.add(tid)
 
     def acquire(self):
         """

Lib/test/test_import/__init__.py

@@ -436,16 +436,24 @@ class ImportTests(unittest.TestCase):
                 os.does_not_exist
 
     def test_concurrency(self):
+        # bpo 38091: this is a hack to slow down the code that calls
+        # has_deadlock(); the logic was itself sometimes deadlocking.
+        def delay_has_deadlock(frame, event, arg):
+            if event == 'call' and frame.f_code.co_name == 'has_deadlock':
+                time.sleep(0.1)
+
         sys.path.insert(0, os.path.join(os.path.dirname(__file__), 'data'))
         try:
             exc = None
             def run():
+                sys.settrace(delay_has_deadlock)
                 event.wait()
                 try:
                     import package
                 except BaseException as e:
                     nonlocal exc
                     exc = e
+                sys.settrace(None)
 
             for i in range(10):
                 event = threading.Event()

Misc/NEWS.d/next/Core and Builtins/2020-02-14-23-10-07.bpo-38091.pwR0K7.rst

@@ -0,0 +1 @@
+Tweak import deadlock detection code to not deadlock itself.