Add SubscriptionAPI

And `TrioSubscriptionAPI`, to make subscription io-agnostic.

libp2p/pubsub/subscription.py

@@ -0,0 +1,39 @@
+from types import TracebackType
+from typing import AsyncIterator, Optional, Type
+
+import trio
+
+from .abc import ISubscriptionAPI
+from .pb import rpc_pb2
+
+
+class BaseSubscriptionAPI(ISubscriptionAPI):
+    async def __aenter__(self) -> "BaseSubscriptionAPI":
+        await trio.hazmat.checkpoint()
+        return self
+
+    async def __aexit__(
+        self,
+        exc_type: "Optional[Type[BaseException]]",
+        exc_value: "Optional[BaseException]",
+        traceback: "Optional[TracebackType]",
+    ) -> None:
+        await self.cancel()
+
+
+class TrioSubscriptionAPI(BaseSubscriptionAPI):
+    receive_channel: "trio.MemoryReceiveChannel[rpc_pb2.Message]"
+
+    def __init__(
+        self, receive_channel: "trio.MemoryReceiveChannel[rpc_pb2.Message]"
+    ) -> None:
+        self.receive_channel = receive_channel
+
+    async def cancel(self) -> None:
+        await self.receive_channel.aclose()
+
+    def __aiter__(self) -> AsyncIterator[rpc_pb2.Message]:
+        return self.receive_channel.__aiter__()
+
+    async def get(self) -> rpc_pb2.Message:
+        return await self.receive_channel.receive()