fix(stream): don't block event loop in EventQueue

src/a2a/server/events/event_queue.py

@@ -17,6 +17,8 @@
 Event = Message | Task | TaskStatusUpdateEvent | TaskArtifactUpdateEvent
 """Type alias for events that can be enqueued."""
 
+DEFAULT_MAX_QUEUE_SIZE = 1024
+
 
 @trace_class(kind=SpanKind.SERVER)
 class EventQueue:
@@ -27,27 +29,38 @@ class EventQueue:
     to create child queues that receive the same events.
     """
 
-    def __init__(self) -> None:
+    def __init__(self, max_queue_size=DEFAULT_MAX_QUEUE_SIZE) -> None:
         """Initializes the EventQueue."""
-        self.queue: asyncio.Queue[Event] = asyncio.Queue()
+
+        # Make sure the `asyncio.Queue` is bounded.
+        # If it's unbounded (maxsize=0), then `queue.put()` never needs to wait,
+        # and so the streaming won't work correctly.
+        if max_queue_size <= 0:
+            raise ValueError('max_queue_size must be greater than 0')
+
+        self.queue: asyncio.Queue[Event] = asyncio.Queue(maxsize=max_queue_size)
         self._children: list[EventQueue] = []
         self._is_closed = False
         self._lock = asyncio.Lock()
         logger.debug('EventQueue initialized.')
 
-    def enqueue_event(self, event: Event):
+    async def enqueue_event(self, event: Event):
         """Enqueues an event to this queue and all its children.
 
         Args:
             event: The event object to enqueue.
         """
-        if self._is_closed:
-            logger.warning('Queue is closed. Event will not be enqueued.')
-            return
+        async with self._lock:
+            if self._is_closed:
+                logger.warning('Queue is closed. Event will not be enqueued.')
+                return
+
         logger.debug(f'Enqueuing event of type: {type(event)}')
-        self.queue.put_nowait(event)
+
+        # Make sure to use put instead of put_nowait to avoid blocking the event loop.
+        await self.queue.put(event)
         for child in self._children:
-            child.enqueue_event(event)
+            await child.enqueue_event(event)
 
     async def dequeue_event(self, no_wait: bool = False) -> Event:
         """Dequeues an event from the queue.

src/a2a/server/tasks/task_updater.py

@@ -34,7 +34,7 @@ def __init__(self, event_queue: EventQueue, task_id: str, context_id: str):
         self.task_id = task_id
         self.context_id = context_id
 
-    def update_status(
+    async def update_status(
         self,
         state: TaskState,
         message: Message | None = None,
@@ -52,7 +52,7 @@ def update_status(
         current_timestamp = (
             timestamp if timestamp else datetime.now(timezone.utc).isoformat()
         )
-        self.event_queue.enqueue_event(
+        await self.event_queue.enqueue_event(
             TaskStatusUpdateEvent(
                 taskId=self.task_id,
                 contextId=self.context_id,
@@ -65,7 +65,7 @@ def update_status(
             )
         )
 
-    def add_artifact(
+    async def add_artifact(
         self,
         parts: list[Part],
         artifact_id: str = str(uuid.uuid4()),
@@ -82,7 +82,7 @@ def add_artifact(
             append: Optional boolean indicating if this chunk appends to a previous one.
             last_chunk: Optional boolean indicating if this is the last chunk.
         """
-        self.event_queue.enqueue_event(
+        await self.event_queue.enqueue_event(
             TaskArtifactUpdateEvent(
                 taskId=self.task_id,
                 contextId=self.context_id,
@@ -95,32 +95,32 @@ def add_artifact(
             )
         )
 
-    def complete(self, message: Message | None = None):
+    async def complete(self, message: Message | None = None):
         """Marks the task as completed and publishes a final status update."""
-        self.update_status(
+        await self.update_status(
             TaskState.completed,
             message=message,
             final=True,
         )
 
-    def failed(self, message: Message | None = None):
+    async def failed(self, message: Message | None = None):
         """Marks the task as failed and publishes a final status update."""
-        self.update_status(TaskState.failed, message=message, final=True)
+        await self.update_status(TaskState.failed, message=message, final=True)
 
-    def reject(self, message: Message | None = None):
+    async def reject(self, message: Message | None = None):
         """Marks the task as rejected and publishes a final status update."""
-        self.update_status(TaskState.rejected, message=message, final=True)
+        await self.update_status(TaskState.rejected, message=message, final=True)
 
-    def submit(self, message: Message | None = None):
+    async def submit(self, message: Message | None = None):
         """Marks the task as submitted and publishes a status update."""
-        self.update_status(
+        await self.update_status(
             TaskState.submitted,
             message=message,
         )
 
-    def start_work(self, message: Message | None = None):
+    async def start_work(self, message: Message | None = None):
         """Marks the task as working and publishes a status update."""
-        self.update_status(
+        await self.update_status(
             TaskState.working,
             message=message,
         )

tests/server/events/test_event_queue.py

@@ -39,7 +39,7 @@ def event_queue() -> EventQueue:
 async def test_enqueue_and_dequeue_event(event_queue: EventQueue) -> None:
     """Test that an event can be enqueued and dequeued."""
     event = Message(**MESSAGE_PAYLOAD)
-    event_queue.enqueue_event(event)
+    await event_queue.enqueue_event(event)
     dequeued_event = await event_queue.dequeue_event()
     assert dequeued_event == event
 
@@ -48,7 +48,7 @@ async def test_enqueue_and_dequeue_event(event_queue: EventQueue) -> None:
 async def test_dequeue_event_no_wait(event_queue: EventQueue) -> None:
     """Test dequeue_event with no_wait=True."""
     event = Task(**MINIMAL_TASK)
-    event_queue.enqueue_event(event)
+    await event_queue.enqueue_event(event)
     dequeued_event = await event_queue.dequeue_event(no_wait=True)
     assert dequeued_event == event
 
@@ -71,7 +71,7 @@ async def test_dequeue_event_wait(event_queue: EventQueue) -> None:
         status=TaskStatus(state=TaskState.working),
         final=True,
     )
-    event_queue.enqueue_event(event)
+    await event_queue.enqueue_event(event)
     dequeued_event = await event_queue.dequeue_event()
     assert dequeued_event == event
 
@@ -84,7 +84,7 @@ async def test_task_done(event_queue: EventQueue) -> None:
         contextId='session-xyz',
         artifact=Artifact(artifactId='11', parts=[Part(TextPart(text='text'))]),
     )
-    event_queue.enqueue_event(event)
+    await event_queue.enqueue_event(event)
     _ = await event_queue.dequeue_event()
     event_queue.task_done()
 
@@ -99,6 +99,6 @@ async def test_enqueue_different_event_types(
         JSONRPCError(code=111, message='rpc error'),
     ]
     for event in events:
-        event_queue.enqueue_event(event)
+        await event_queue.enqueue_event(event)
         dequeued_event = await event_queue.dequeue_event()
         assert dequeued_event == event

tests/server/request_handlers/test_default_request_handler.py

@@ -0,0 +1,77 @@
+import time
+
+import pytest
+
+from a2a.server.agent_execution import AgentExecutor, RequestContext
+from a2a.server.events import EventQueue
+from a2a.server.request_handlers import DefaultRequestHandler
+from a2a.server.tasks import InMemoryTaskStore, TaskUpdater
+from a2a.types import (
+    Message,
+    MessageSendParams,
+    Part,
+    Role,
+    TaskState,
+    TextPart,
+)
+
+
+class DummyAgentExecutor(AgentExecutor):
+    async def execute(self, context: RequestContext, event_queue: EventQueue):
+        task_updater = TaskUpdater(
+            event_queue, context.task_id, context.context_id
+        )
+        async for i in self._run():
+            parts = [Part(root=TextPart(text=f'Event {i}'))]
+            try:
+                await task_updater.update_status(
+                    TaskState.working,
+                    message=task_updater.new_agent_message(parts),
+                )
+            except RuntimeError:
+                # Stop processing when the event loop is closed
+                break
+
+    async def _run(self):
+        for i in range(1_000_000):  # Simulate a long-running stream
+            yield i
+
+    async def cancel(self, context: RequestContext, event_queue: EventQueue):
+        pass
+
+
+@pytest.mark.asyncio
+async def test_on_message_send_stream():
+    request_handler = DefaultRequestHandler(
+        DummyAgentExecutor(), InMemoryTaskStore()
+    )
+    message_params = MessageSendParams(
+        message=Message(
+            role=Role.user,
+            messageId='msg-123',
+            parts=[Part(root=TextPart(text='How are you?'))],
+        ),
+    )
+
+    async def consume_stream():
+        events = []
+        async for event in request_handler.on_message_send_stream(
+            message_params
+        ):
+            events.append(event)
+            if len(events) >= 3:
+                break  # Stop after a few events
+
+        return events
+
+    # Consume first 3 events from the stream and measure time
+    start = time.perf_counter()
+    events = await consume_stream()
+    elapsed = time.perf_counter() - start
+
+    # Assert we received events quickly
+    assert len(events) == 3
+    assert elapsed < 0.5
+
+    texts = [p.root.text for e in events for p in e.status.message.parts]
+    assert texts == ['Event 0', 'Event 1', 'Event 2']