1 files changed, 42 insertions, 22 deletions

diff --git a/Lib/asyncio/locks.py b/Lib/asyncio/locks.py
index 7b81c25..fc03830 100644
--- a/Lib/asyncio/locks.py
+++ b/Lib/asyncio/locks.py
@@ -349,9 +349,8 @@ class Semaphore(_ContextManagerMixin, mixins._LoopBoundMixin):
         super().__init__(loop=loop)
         if value < 0:
             raise ValueError("Semaphore initial value must be >= 0")
+        self._waiters = None
         self._value = value
-        self._waiters = collections.deque()
-        self._wakeup_scheduled = False
 
     def __repr__(self):
         res = super().__repr__()
@@ -360,16 +359,8 @@ class Semaphore(_ContextManagerMixin, mixins._LoopBoundMixin):
             extra = f'{extra}, waiters:{len(self._waiters)}'
         return f'<{res[1:-1]} [{extra}]>'
 
-    def _wake_up_next(self):
-        while self._waiters:
-            waiter = self._waiters.popleft()
-            if not waiter.done():
-                waiter.set_result(None)
-                self._wakeup_scheduled = True
-                return
-
     def locked(self):
-        """Returns True if semaphore can not be acquired immediately."""
+        """Returns True if semaphore counter is zero."""
         return self._value == 0
 
     async def acquire(self):
@@ -381,28 +372,57 @@ class Semaphore(_ContextManagerMixin, mixins._LoopBoundMixin):
         called release() to make it larger than 0, and then return
         True.
         """
-        # _wakeup_scheduled is set if *another* task is scheduled to wakeup
-        # but its acquire() is not resumed yet
-        while self._wakeup_scheduled or self._value <= 0:
-            fut = self._get_loop().create_future()
-            self._waiters.append(fut)
+        if (not self.locked() and (self._waiters is None or
+                all(w.cancelled() for w in self._waiters))):
+            self._value -= 1
+            return True
+
+        if self._waiters is None:
+            self._waiters = collections.deque()
+        fut = self._get_loop().create_future()
+        self._waiters.append(fut)
+
+        # Finally block should be called before the CancelledError
+        # handling as we don't want CancelledError to call
+        # _wake_up_first() and attempt to wake up itself.
+        try:
             try:
                 await fut
-                # reset _wakeup_scheduled *after* waiting for a future
-                self._wakeup_scheduled = False
-            except exceptions.CancelledError:
-                self._wake_up_next()
-                raise
+            finally:
+                self._waiters.remove(fut)
+        except exceptions.CancelledError:
+            if not self.locked():
+                self._wake_up_first()
+            raise
+
         self._value -= 1
+        if not self.locked():
+            self._wake_up_first()
         return True
 
     def release(self):
         """Release a semaphore, incrementing the internal counter by one.
+
         When it was zero on entry and another coroutine is waiting for it to
         become larger than zero again, wake up that coroutine.
         """
         self._value += 1
-        self._wake_up_next()
+        self._wake_up_first()
+
+    def _wake_up_first(self):
+        """Wake up the first waiter if it isn't done."""
+        if not self._waiters:
+            return
+        try:
+            fut = next(iter(self._waiters))
+        except StopIteration:
+            return
+
+        # .done() necessarily means that a waiter will wake up later on and
+        # either take the lock, or, if it was cancelled and lock wasn't
+        # taken already, will hit this again and wake up a new waiter.
+        if not fut.done():
+            fut.set_result(True)
 
 
 class BoundedSemaphore(Semaphore):