[Bugfix][CI/Build] Fix test and improve code for `merge_async_iterators` (#5096)

tests/async_engine/test_merge_async_iterators.py

-import asyncio
-from typing import AsyncIterator, Tuple
-
-import pytest
-
-from vllm.utils import merge_async_iterators
-
-
-@pytest.mark.asyncio
-async def test_merge_async_iterators():
-
-    async def mock_async_iterator(idx: int) -> AsyncIterator[str]:
-        try:
-            while True:
-                yield f"item from iterator {idx}"
-                await asyncio.sleep(0.1)
-        except asyncio.CancelledError:
-            pass
-
-    iterators = [mock_async_iterator(i) for i in range(3)]
-    merged_iterator: AsyncIterator[Tuple[int, str]] = merge_async_iterators(
-        *iterators)
-
-    async def stream_output(generator: AsyncIterator[Tuple[int, str]]):
-        async for idx, output in generator:
-            print(f"idx: {idx}, output: {output}")
-
-    task = asyncio.create_task(stream_output(merged_iterator))
-    await asyncio.sleep(0.5)
-    task.cancel()
-    with pytest.raises(asyncio.CancelledError):
-        await task
-
-    for iterator in iterators:
-        try:
-            await asyncio.wait_for(anext(iterator), 1)
-        except StopAsyncIteration:
-            # All iterators should be cancelled and print this message.
-            print("Iterator was cancelled normally")
-        except (Exception, asyncio.CancelledError) as e:
-            raise AssertionError() from e

tests/test_utils.py

+import asyncio
+import sys
+from typing import (TYPE_CHECKING, Any, AsyncIterator, Awaitable, Protocol,
+                    Tuple, TypeVar)
+
 import pytest
 
-from vllm.utils import deprecate_kwargs
+from vllm.utils import deprecate_kwargs, merge_async_iterators
 
 from .utils import error_on_warning
 
+if sys.version_info < (3, 10):
+    if TYPE_CHECKING:
+        _AwaitableT = TypeVar("_AwaitableT", bound=Awaitable[Any])
+        _AwaitableT_co = TypeVar("_AwaitableT_co",
+                                 bound=Awaitable[Any],
+                                 covariant=True)
+
+        class _SupportsSynchronousAnext(Protocol[_AwaitableT_co]):
+
+            def __anext__(self) -> _AwaitableT_co:
+                ...
+
+    def anext(i: "_SupportsSynchronousAnext[_AwaitableT]", /) -> "_AwaitableT":
+        return i.__anext__()
+
+
+@pytest.mark.asyncio
+async def test_merge_async_iterators():
+
+    async def mock_async_iterator(idx: int) -> AsyncIterator[str]:
+        try:
+            while True:
+                yield f"item from iterator {idx}"
+                await asyncio.sleep(0.1)
+        except asyncio.CancelledError:
+            pass
+
+    iterators = [mock_async_iterator(i) for i in range(3)]
+    merged_iterator: AsyncIterator[Tuple[int, str]] = merge_async_iterators(
+        *iterators)
+
+    async def stream_output(generator: AsyncIterator[Tuple[int, str]]):
+        async for idx, output in generator:
+            print(f"idx: {idx}, output: {output}")
+
+    task = asyncio.create_task(stream_output(merged_iterator))
+    await asyncio.sleep(0.5)
+    task.cancel()
+    with pytest.raises(asyncio.CancelledError):
+        await task
+
+    for iterator in iterators:
+        try:
+            await asyncio.wait_for(anext(iterator), 1)
+        except StopAsyncIteration:
+            # All iterators should be cancelled and print this message.
+            print("Iterator was cancelled normally")
+        except (Exception, asyncio.CancelledError) as e:
+            raise AssertionError() from e
+
 
 def test_deprecate_kwargs_always():
 

vllm/utils.py

@@ -5,6 +5,7 @@ import gc
 import os
 import socket
 import subprocess
+import sys
 import tempfile
 import threading
 import uuid
@@ -234,9 +235,11 @@ def merge_async_iterators(
                 yield item
         except (Exception, asyncio.CancelledError) as e:
             for task in _tasks:
-                # NOTE: Pass the error msg in cancel()
-                # when only Python 3.9+ is supported.
-                task.cancel()
+                if sys.version_info >= (3, 9):
+                    # msg parameter only supported in Python 3.9+
+                    task.cancel(e)
+                else:
+                    task.cancel()
             raise e
         await asyncio.gather(*_tasks)