bpo-45997: Fix asyncio.Semaphore waiters order

Lib/asyncio/locks.py

@@ -6,6 +6,7 @@
 
 from . import exceptions
 from . import mixins
+from . import tasks
 
 
 class _ContextManagerMixin:
@@ -350,6 +351,7 @@ def __init__(self, value=1, *, loop=mixins._marker):
             raise ValueError("Semaphore initial value must be >= 0")
         self._value = value
         self._waiters = collections.deque()
+        self._skip_loop_iteration = False
 
     def __repr__(self):
         res = super().__repr__()
@@ -365,6 +367,9 @@ def _wake_up_next(self):
                 waiter.set_result(None)
                 return
 
+        # Next acquire call should give a chance to other waiters acquire lock
+        self._skip_loop_iteration = True
+
     def locked(self):
         """Returns True if semaphore can not be acquired immediately."""
         return self._value == 0
@@ -378,6 +383,12 @@ async def acquire(self):
         called release() to make it larger than 0, and then return
         True.
         """
+
+        # Skip one loop iteration and give a change to next waiter to acquire
+        if self._skip_loop_iteration:
+            self._skip_loop_iteration = False
+            await tasks.sleep(0)
+
         while self._value <= 0:
             fut = self._get_loop().create_future()
             self._waiters.append(fut)

Lib/test/test_asyncio/test_locks.py

@@ -965,6 +965,45 @@ def test_release_no_waiters(self):
         sem.release()
         self.assertFalse(sem.locked())
 
+    def test_acquire_fifo_order(self):
+        sem = asyncio.Semaphore(1)
+        result = []
+
+        async def c1():
+            await sem.acquire()
+            result.append('c1_1')
+            sem.release()
+
+            await sem.acquire()
+            result.append('c1_2')
+            sem.release()
+
+        async def c2():
+            await sem.acquire()
+            result.append('c2_1')
+            sem.release()
+
+            await sem.acquire()
+            result.append('c2_2')
+            sem.release()
+
+        async def c3():
+            await sem.acquire()
+            result.append('c3_1')
+            sem.release()
+
+            await sem.acquire()
+            result.append('c3_2')
+            sem.release()
+
+        t1 = self.loop.create_task(c1())
+        t2 = self.loop.create_task(c2())
+        t3 = self.loop.create_task(c3())
+
+        race_tasks = [t1, t2, t3]
+        self.loop.run_until_complete(asyncio.gather(*race_tasks))
+        self.assertEqual(['c1_1', 'c2_1', 'c3_1', 'c1_2', 'c2_2', 'c3_2'], result)
+
 
 if __name__ == '__main__':
     unittest.main()