Add a context manager to the useful read/writer lock.

taskflow/utils.py

@@ -15,3 +15,81 @@
 #    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 #    License for the specific language governing permissions and limitations
 #    under the License.
+
+import contextlib
+import threading
+
+
+class ReaderWriterLock(object):
+    """A simple reader-writer lock.
+
+    Several readers can hold the lock simultaneously, and only one writer.
+    Write locks have priority over reads to prevent write starvation.
+
+    Public domain @ http://majid.info/blog/a-reader-writer-lock-for-python/
+    """
+
+    def __init__(self):
+        self.rwlock = 0
+        self.writers_waiting = 0
+        self.monitor = threading.Lock()
+        self.readers_ok = threading.Condition(self.monitor)
+        self.writers_ok = threading.Condition(self.monitor)
+
+    @contextlib.contextmanager
+    def acquire(self, read=True):
+        """Acquire a read or write lock in a context manager."""
+        try:
+            if read:
+                self.acquire_read()
+            else:
+                self.acquire_write()
+            yield self
+        finally:
+            self.release()
+
+    def acquire_read(self):
+        """Acquire a read lock.
+
+        Several threads can hold this typeof lock.
+        It is exclusive with write locks."""
+
+        self.monitor.acquire()
+        while self.rwlock < 0 or self.writers_waiting:
+            self.readers_ok.wait()
+        self.rwlock += 1
+        self.monitor.release()
+
+    def acquire_write(self):
+        """Acquire a write lock.
+
+        Only one thread can hold this lock, and only when no read locks
+        are also held."""
+
+        self.monitor.acquire()
+        while self.rwlock != 0:
+            self.writers_waiting += 1
+            self.writers_ok.wait()
+            self.writers_waiting -= 1
+        self.rwlock = -1
+        self.monitor.release()
+
+    def release(self):
+        """Release a lock, whether read or write."""
+
+        self.monitor.acquire()
+        if self.rwlock < 0:
+            self.rwlock = 0
+        else:
+            self.rwlock -= 1
+        wake_writers = self.writers_waiting and self.rwlock == 0
+        wake_readers = self.writers_waiting == 0
+        self.monitor.release()
+        if wake_writers:
+            self.writers_ok.acquire()
+            self.writers_ok.notify()
+            self.writers_ok.release()
+        elif wake_readers:
+            self.readers_ok.acquire()
+            self.readers_ok.notifyAll()
+            self.readers_ok.release()