asyncio: Locks refactor: use a separate context manager; remove Semaphore._locked.

Lib/asyncio/locks.py

@@ -9,6 +9,36 @@ from . import futures
 from . import tasks
 
 
+class _ContextManager:
+    """Context manager.
+
+    This enables the following idiom for acquiring and releasing a
+    lock around a block:
+
+        with (yield from lock):
+            <block>
+
+    while failing loudly when accidentally using:
+
+        with lock:
+            <block>
+    """
+
+    def __init__(self, lock):
+        self._lock = lock
+
+    def __enter__(self):
+        # We have no use for the "as ..."  clause in the with
+        # statement for locks.
+        return None
+
+    def __exit__(self, *args):
+        try:
+            self._lock.release()
+        finally:
+            self._lock = None  # Crudely prevent reuse.
+
+
 class Lock:
     """Primitive lock objects.
 
@@ -124,17 +154,29 @@ class Lock:
             raise RuntimeError('Lock is not acquired.')
 
     def __enter__(self):
-        if not self._locked:
         raise RuntimeError(
             '"yield from" should be used as context manager expression')
-        return True
 
     def __exit__(self, *args):
-        self.release()
+        # This must exist because __enter__ exists, even though that
+        # always raises; that's how the with-statement works.
+        pass
 
     def __iter__(self):
+        # This is not a coroutine.  It is meant to enable the idiom:
+        #
+        #     with (yield from lock):
+        #         <block>
+        #
+        # as an alternative to:
+        #
+        #     yield from lock.acquire()
+        #     try:
+        #         <block>
+        #     finally:
+        #         lock.release()
         yield from self.acquire()
-        return self
+        return _ContextManager(self)
 
 
 class Event:
@@ -311,14 +353,16 @@ class Condition:
         self.notify(len(self._waiters))
 
     def __enter__(self):
-        return self._lock.__enter__()
+        raise RuntimeError(
+            '"yield from" should be used as context manager expression')
 
     def __exit__(self, *args):
-        return self._lock.__exit__(*args)
+        pass
 
     def __iter__(self):
+        # See comment in Lock.__iter__().
         yield from self.acquire()
-        return self
+        return _ContextManager(self)
 
 
 class Semaphore:
@@ -341,7 +385,6 @@ class Semaphore:
             raise ValueError("Semaphore initial value must be >= 0")
         self._value = value
         self._waiters = collections.deque()
-        self._locked = (value == 0)
         if loop is not None:
             self._loop = loop
         else:
@@ -349,7 +392,7 @@ class Semaphore:
 
     def __repr__(self):
         res = super().__repr__()
-        extra = 'locked' if self._locked else 'unlocked,value:{}'.format(
+        extra = 'locked' if self.locked() else 'unlocked,value:{}'.format(
             self._value)
         if self._waiters:
             extra = '{},waiters:{}'.format(extra, len(self._waiters))
@@ -357,7 +400,7 @@ class Semaphore:
 
     def locked(self):
         """Returns True if semaphore can not be acquired immediately."""
-        return self._locked
+        return self._value == 0
 
     @tasks.coroutine
     def acquire(self):
@@ -371,8 +414,6 @@ class Semaphore:
         """
         if not self._waiters and self._value > 0:
             self._value -= 1
-            if self._value == 0:
-                self._locked = True
             return True
 
         fut = futures.Future(loop=self._loop)
@@ -380,8 +421,6 @@ class Semaphore:
         try:
             yield from fut
             self._value -= 1
-            if self._value == 0:
-                self._locked = True
             return True
         finally:
             self._waiters.remove(fut)
@@ -392,23 +431,22 @@ class Semaphore:
         become larger than zero again, wake up that coroutine.
         """
         self._value += 1
-        self._locked = False
         for waiter in self._waiters:
             if not waiter.done():
                 waiter.set_result(True)
                 break
 
     def __enter__(self):
-        # TODO: This is questionable.  How do we know the user actually
-        # wrote "with (yield from sema)" instead of "with sema"?
-        return True
+        raise RuntimeError(
+            '"yield from" should be used as context manager expression')
 
     def __exit__(self, *args):
-        self.release()
+        pass
 
     def __iter__(self):
+        # See comment in Lock.__iter__().
         yield from self.acquire()
-        return self
+        return _ContextManager(self)
 
 
 class BoundedSemaphore(Semaphore):

Lib/test/test_asyncio/test_locks.py

@@ -208,6 +208,24 @@ class LockTests(unittest.TestCase):
 
         self.assertFalse(lock.locked())
 
+    def test_context_manager_cant_reuse(self):
+        lock = asyncio.Lock(loop=self.loop)
+
+        @asyncio.coroutine
+        def acquire_lock():
+            return (yield from lock)
+
+        # This spells "yield from lock" outside a generator.
+        cm = self.loop.run_until_complete(acquire_lock())
+        with cm:
+            self.assertTrue(lock.locked())
+
+        self.assertFalse(lock.locked())
+
+        with self.assertRaises(AttributeError):
+            with cm:
+                pass
+
     def test_context_manager_no_yield(self):
         lock = asyncio.Lock(loop=self.loop)
 
@@ -219,6 +237,8 @@ class LockTests(unittest.TestCase):
                 str(err),
                 '"yield from" should be used as context manager expression')
 
+        self.assertFalse(lock.locked())
+
 
 class EventTests(unittest.TestCase):
 
@@ -655,6 +675,8 @@ class ConditionTests(unittest.TestCase):
                 str(err),
                 '"yield from" should be used as context manager expression')
 
+        self.assertFalse(cond.locked())
+
 
 class SemaphoreTests(unittest.TestCase):
 
@@ -830,6 +852,19 @@ class SemaphoreTests(unittest.TestCase):
 
         self.assertEqual(2, sem._value)
 
+    def test_context_manager_no_yield(self):
+        sem = asyncio.Semaphore(2, loop=self.loop)
+
+        try:
+            with sem:
+                self.fail('RuntimeError is not raised in with expression')
+        except RuntimeError as err:
+            self.assertEqual(
+                str(err),
+                '"yield from" should be used as context manager expression')
+
+        self.assertEqual(2, sem._value)
+
 
 if __name__ == '__main__':
     unittest.main()