feat: raise error for tasks in terminal states (#215)

- make tasks to be not restartable after they reach a terminal state.
- Based on discussion in
a2aproject/A2A#723

Author: swapydapy

src/a2a/server/request_handlers/default_request_handler.py

@@ -29,6 +29,7 @@
 from a2a.types import (
     GetTaskPushNotificationConfigParams,
     InternalError,
+    InvalidParamsError,
     Message,
     MessageSendConfiguration,
     MessageSendParams,
@@ -38,6 +39,7 @@
     TaskNotFoundError,
     TaskPushNotificationConfig,
     TaskQueryParams,
+    TaskState,
     UnsupportedOperationError,
 )
 from a2a.utils.errors import ServerError
@@ -46,6 +48,12 @@
 
 logger = logging.getLogger(__name__)
 
+TERMINAL_TASK_STATES = {
+    TaskState.completed,
+    TaskState.canceled,
+    TaskState.failed,
+    TaskState.rejected,
+}
 
 @trace_class(kind=SpanKind.SERVER)
 class DefaultRequestHandler(RequestHandler):
@@ -178,6 +186,13 @@ async def on_message_send(
         )
         task: Task | None = await task_manager.get_task()
         if task:
+            if task.status.state in TERMINAL_TASK_STATES:
+                raise ServerError(
+                    error=InvalidParamsError(
+                        message=f'Task {task.id} is in terminal state: {task.status.state}'
+                    )
+                )
+
             task = task_manager.update_with_message(params.message, task)
             if self.should_add_push_info(params):
                 assert isinstance(self._push_notifier, PushNotifier)
@@ -264,8 +279,14 @@ async def on_message_send_stream(
         task: Task | None = await task_manager.get_task()
 
         if task:
-            task = task_manager.update_with_message(params.message, task)
+            if task.status.state in TERMINAL_TASK_STATES:
+                raise ServerError(
+                    error=InvalidParamsError(
+                        message=f'Task {task.id} is in terminal state: {task.status.state}'
+                    )
+                )
 
+            task = task_manager.update_with_message(params.message, task)
             if self.should_add_push_info(params):
                 assert isinstance(self._push_notifier, PushNotifier)
                 assert isinstance(
@@ -413,6 +434,13 @@ async def on_resubscribe_to_task(
         if not task:
             raise ServerError(error=TaskNotFoundError())
 
+        if task.status.state in TERMINAL_TASK_STATES:
+            raise ServerError(
+                error=InvalidParamsError(
+                    message=f'Task {task.id} is in terminal state: {task.status.state}'
+                )
+            )
+
         task_manager = TaskManager(
             task_id=task.id,
             context_id=task.contextId,

tests/server/request_handlers/test_default_request_handler.py

@@ -28,6 +28,7 @@
 )
 from a2a.types import (
     InternalError,
+    InvalidParamsError,
     Message,
     MessageSendConfiguration,
     MessageSendParams,
@@ -1137,3 +1138,136 @@ async def consume_stream():
 
     texts = [p.root.text for e in events for p in e.status.message.parts]
     assert texts == ['Event 0', 'Event 1', 'Event 2']
+
+TERMINAL_TASK_STATES = {
+    TaskState.completed,
+    TaskState.canceled,
+    TaskState.failed,
+    TaskState.rejected,
+} 
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize('terminal_state', TERMINAL_TASK_STATES)
+async def test_on_message_send_task_in_terminal_state(terminal_state):
+    """Test on_message_send when task is already in a terminal state."""
+    task_id = f'terminal_task_{terminal_state.value}'
+    terminal_task = create_sample_task(
+        task_id=task_id, status_state=terminal_state
+    )
+
+    mock_task_store = AsyncMock(spec=TaskStore)
+    # The get method of TaskManager calls task_store.get.
+    # We mock TaskManager.get_task which is an async method.
+    # So we should patch that instead.
+
+    request_handler = DefaultRequestHandler(
+        agent_executor=DummyAgentExecutor(), task_store=mock_task_store
+    )
+
+    params = MessageSendParams(
+        message=Message(
+            role=Role.user,
+            messageId='msg_terminal',
+            parts=[],
+            taskId=task_id,
+        )
+    )
+
+    from a2a.utils.errors import ServerError
+
+    # Patch the TaskManager's get_task method to return our terminal task
+    with patch(
+        'a2a.server.request_handlers.default_request_handler.TaskManager.get_task',
+        return_value=terminal_task,
+    ):
+        with pytest.raises(ServerError) as exc_info:
+            await request_handler.on_message_send(
+                params, create_server_call_context()
+            )
+
+    assert isinstance(exc_info.value.error, InvalidParamsError)
+    assert exc_info.value.error.message
+    assert (
+        f'Task {task_id} is in terminal state: {terminal_state.value}'
+        in exc_info.value.error.message
+    )
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize('terminal_state', TERMINAL_TASK_STATES)
+async def test_on_message_send_stream_task_in_terminal_state(terminal_state):
+    """Test on_message_send_stream when task is already in a terminal state."""
+    task_id = f'terminal_stream_task_{terminal_state.value}'
+    terminal_task = create_sample_task(
+        task_id=task_id, status_state=terminal_state
+    )
+
+    mock_task_store = AsyncMock(spec=TaskStore)
+
+    request_handler = DefaultRequestHandler(
+        agent_executor=DummyAgentExecutor(), task_store=mock_task_store
+    )
+
+    params = MessageSendParams(
+        message=Message(
+            role=Role.user,
+            messageId='msg_terminal_stream',
+            parts=[],
+            taskId=task_id,
+        )
+    )
+
+    from a2a.utils.errors import ServerError
+
+    with patch(
+        'a2a.server.request_handlers.default_request_handler.TaskManager.get_task',
+        return_value=terminal_task,
+    ):
+        with pytest.raises(ServerError) as exc_info:
+            async for _ in request_handler.on_message_send_stream(
+                params, create_server_call_context()
+            ):
+                pass  # pragma: no cover
+
+    assert isinstance(exc_info.value.error, InvalidParamsError)
+    assert exc_info.value.error.message
+    assert (
+        f'Task {task_id} is in terminal state: {terminal_state.value}'
+        in exc_info.value.error.message
+    )
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize('terminal_state', TERMINAL_TASK_STATES)
+async def test_on_resubscribe_to_task_in_terminal_state(terminal_state):
+    """Test on_resubscribe_to_task when task is in a terminal state."""
+    task_id = f'resub_terminal_task_{terminal_state.value}'
+    terminal_task = create_sample_task(
+        task_id=task_id, status_state=terminal_state
+    )
+
+    mock_task_store = AsyncMock(spec=TaskStore)
+    mock_task_store.get.return_value = terminal_task
+
+    request_handler = DefaultRequestHandler(
+        agent_executor=DummyAgentExecutor(),
+        task_store=mock_task_store,
+        queue_manager=AsyncMock(spec=QueueManager),
+    )
+    params = TaskIdParams(id=task_id)
+
+    from a2a.utils.errors import ServerError
+
+    with pytest.raises(ServerError) as exc_info:
+        async for _ in request_handler.on_resubscribe_to_task(
+            params, create_server_call_context()
+        ):
+            pass  # pragma: no cover
+
+    assert isinstance(exc_info.value.error, InvalidParamsError)
+    assert exc_info.value.error.message
+    assert (
+        f'Task {task_id} is in terminal state: {terminal_state.value}'
+        in exc_info.value.error.message
+    )
+    mock_task_store.get.assert_awaited_once_with(task_id)