Also rewrote the guts of asyncio.Semaphore (patch by manipopopo).

Lib/asyncio/locks.py

@@ -411,6 +411,13 @@ class Semaphore(_ContextManagerMixin):
             extra = '{},waiters:{}'.format(extra, len(self._waiters))
         return '<{} [{}]>'.format(res[1:-1], extra)
 
+    def _wake_up_next(self):
+        while self._waiters:
+            waiter = self._waiters.popleft()
+            if not waiter.done():
+                waiter.set_result(None)
+                return
+
     def locked(self):
         """Returns True if semaphore can not be acquired immediately."""
         return self._value == 0
@@ -425,18 +432,19 @@ class Semaphore(_ContextManagerMixin):
         called release() to make it larger than 0, and then return
         True.
         """
-        if not self._waiters and self._value > 0:
-            self._value -= 1
-            return True
-
+        while self._value <= 0:
             fut = futures.Future(loop=self._loop)
             self._waiters.append(fut)
             try:
                 yield from fut
+            except:
+                # See the similar code in Queue.get.
+                fut.cancel()
+                if self._value > 0 and not fut.cancelled():
+                    self._wake_up_next()
+                raise
         self._value -= 1
         return True
-        finally:
-            self._waiters.remove(fut)
 
     def release(self):
         """Release a semaphore, incrementing the internal counter by one.
@@ -444,10 +452,7 @@ class Semaphore(_ContextManagerMixin):
         become larger than zero again, wake up that coroutine.
         """
         self._value += 1
-        for waiter in self._waiters:
-            if not waiter.done():
-                waiter.set_result(True)
-                break
+        self._wake_up_next()
 
 
 class BoundedSemaphore(Semaphore):

Lib/test/test_asyncio/test_locks.py

@@ -7,7 +7,6 @@ import re
 import asyncio
 from asyncio import test_utils
 
-
 STR_RGX_REPR = (
     r'^<(?P<class>.*?) object at (?P<address>.*?)'
     r'\[(?P<extras>'
@@ -783,22 +782,20 @@ class SemaphoreTests(test_utils.TestCase):
 
         test_utils.run_briefly(self.loop)
         self.assertEqual(0, sem._value)
-        self.assertEqual([1, 2, 3], result)
+        self.assertEqual(3, len(result))
         self.assertTrue(sem.locked())
         self.assertEqual(1, len(sem._waiters))
         self.assertEqual(0, sem._value)
 
         self.assertTrue(t1.done())
         self.assertTrue(t1.result())
-        self.assertTrue(t2.done())
-        self.assertTrue(t2.result())
-        self.assertTrue(t3.done())
-        self.assertTrue(t3.result())
-        self.assertFalse(t4.done())
+        race_tasks = [t2, t3, t4]
+        done_tasks = [t for t in race_tasks if t.done() and t.result()]
+        self.assertTrue(2, len(done_tasks))
 
         # cleanup locked semaphore
         sem.release()
-        self.loop.run_until_complete(t4)
+        self.loop.run_until_complete(asyncio.gather(*race_tasks))
 
     def test_acquire_cancel(self):
         sem = asyncio.Semaphore(loop=self.loop)
@@ -809,7 +806,44 @@ class SemaphoreTests(test_utils.TestCase):
         self.assertRaises(
             asyncio.CancelledError,
             self.loop.run_until_complete, acquire)
-        self.assertFalse(sem._waiters)
+        self.assertTrue((not sem._waiters) or
+                        all(waiter.done() for waiter in sem._waiters))
+
+    def test_acquire_cancel_before_awoken(self):
+        sem = asyncio.Semaphore(value=0, loop=self.loop)
+
+        t1 = asyncio.Task(sem.acquire(), loop=self.loop)
+        t2 = asyncio.Task(sem.acquire(), loop=self.loop)
+        t3 = asyncio.Task(sem.acquire(), loop=self.loop)
+        t4 = asyncio.Task(sem.acquire(), loop=self.loop)
+
+        test_utils.run_briefly(self.loop)
+
+        sem.release()
+        t1.cancel()
+        t2.cancel()
+
+        test_utils.run_briefly(self.loop)
+        num_done = sum(t.done() for t in [t3, t4])
+        self.assertEqual(num_done, 1)
+
+        t3.cancel()
+        t4.cancel()
+        test_utils.run_briefly(self.loop)
+
+    def test_acquire_hang(self):
+        sem = asyncio.Semaphore(value=0, loop=self.loop)
+
+        t1 = asyncio.Task(sem.acquire(), loop=self.loop)
+        t2 = asyncio.Task(sem.acquire(), loop=self.loop)
+
+        test_utils.run_briefly(self.loop)
+
+        sem.release()
+        t1.cancel()
+
+        test_utils.run_briefly(self.loop)
+        self.assertTrue(sem.locked())
 
     def test_release_not_acquired(self):
         sem = asyncio.BoundedSemaphore(loop=self.loop)

Misc/NEWS

@@ -81,7 +81,8 @@ Library
 - Issue #25034: Fix string.Formatter problem with auto-numbering and
   nested format_specs. Patch by Anthon van der Neut.
 
-- Issue #25233: Rewrite the guts of asyncio.Queue to be more understandable and correct.
+- Issue #25233: Rewrite the guts of asyncio.Queue and
+  asyncio.Semaphore to be more understandable and correct.
 
 - Issue #23600: Default implementation of tzinfo.fromutc() was returning
   wrong results in some cases.