cjerdonek · June 27, 2017 00:55 · Jun 27, 2017
diff --git a/test-rw.py b/test-rw.py
@@ -0,0 +1,85 @@
+import asyncio
+import random
+
+
+NO_READERS_EVENT = asyncio.Event()
+NO_WRITERS_EVENT = asyncio.Event()
+WRITE_LOCK = asyncio.Lock()
+
+
+class State:
+    reader_count = 0
+    mock_file_data = 'initial'
+
+
+async def read_file():
+    data = State.mock_file_data
+    print(f'read: {data}')
+
+
+async def write_file(data):
+    print(f'writing: {data}')
+    State.mock_file_data = data
+    await asyncio.sleep(0.5)
+
+
+async def write(data):
+    async with WRITE_LOCK:
+        NO_WRITERS_EVENT.clear()
+        # Wait for the readers to finish.
+        await NO_READERS_EVENT.wait()
+        # Do the file write.
+        await write_file(data)
+
+    # Awaken waiting readers.
+    NO_WRITERS_EVENT.set()
+
+
+async def read():
+    while True:
+        await NO_WRITERS_EVENT.wait()
+        # Check the writer_lock again in case a new writer has
+        # started writing.
+        if WRITE_LOCK.locked():
+            print(f'cannot read: still writing: {State.mock_file_data!r}')
+        else:
+            # Otherwise, we can do the read.
+            break
+
+    State.reader_count += 1
+    if State.reader_count == 1:
+        NO_READERS_EVENT.clear()
+    # Do the file read.
+    await read_file()
+    State.reader_count -= 1
+    if State.reader_count == 0:
+        # Awaken any waiting writer.
+        NO_READERS_EVENT.set()
+
+
+async def delayed(coro):
+    await asyncio.sleep(random.random())
+    await coro
+
+
+async def test_synchronization():
+    NO_READERS_EVENT.set()
+    NO_WRITERS_EVENT.set()
+
+    coros = [
+        read(),
+        read(),
+        read(),
+        read(),
+        read(),
+        read(),
+        write('apple'),
+        write('banana'),
+    ]
+    # Add a delay before each coroutine for variety.
+    coros = [delayed(coro) for coro in coros]
+    await asyncio.gather(*coros)
+
+
+if __name__ == '__main__':
+    asyncio.get_event_loop().run_until_complete(test_synchronization())