feat: Implement SerialBatcher which helps with transforming single writes into batch writes.

google/cloud/pubsublite/internal/wire/serial_batcher.py

@@ -0,0 +1,50 @@
+from abc import ABC, abstractmethod
+from typing import Generic, List, Iterable
+import asyncio
+
+from google.cloud.pubsublite.internal.wire.connection import Request, Response
+from google.cloud.pubsublite.internal.wire.work_item import WorkItem
+
+
+class BatchTester(Generic[Request], ABC):
+  """A BatchTester determines whether a given batch of messages must be sent."""
+  @abstractmethod
+  def test(self, requests: Iterable[Request]) -> bool:
+    """
+    Args:
+      requests: The current outstanding batch.
+
+    Returns: Whether that batch must be sent.
+    """
+    raise NotImplementedError()
+
+
+class SerialBatcher(Generic[Request, Response]):
+  _tester: BatchTester[Request]
+  _requests: List[WorkItem[Request]]  # A list of outstanding requests
+
+  def __init__(self, tester: BatchTester[Request]):
+    self._tester = tester
+    self._requests = []
+
+  def add(self, request: Request) -> 'asyncio.Future[Response]':
+    """Add a new request to this batcher. Callers must always call should_flush() after add, and flush() if that returns
+    true.
+
+    Args:
+      request: The request to send.
+
+    Returns:
+      A future that will resolve to the response or a GoogleAPICallError.
+    """
+    item = WorkItem[Request](request)
+    self._requests.append(item)
+    return item.response_future
+
+  def should_flush(self) -> bool:
+    return self._tester.test(item.request for item in self._requests)
+
+  def flush(self) -> Iterable[WorkItem[Request]]:
+    requests = self._requests
+    self._requests = []
+    return requests