dinos
51 lines
2.2 KiB
Python
51 lines
2.2 KiB
Python
"""Windows asyncio event-loop policy compatibility.
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On Windows, MCP stdio servers are launched as subprocesses by the MCP SDK's
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stdio transport, which uses ``anyio.open_process`` → ``asyncio`` async
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subprocess support. ``asyncio``'s *Selector* event loop does **not** implement
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async subprocess creation, so on a Selector loop the stdio transport falls back
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to a synchronous ``subprocess.Popen`` inside an ``async`` function. Under
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``langgraph dev`` (which enables ``blockbuster`` by default to police blocking
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I/O) that synchronous call is flagged as a ``BlockingError`` — see issue #283.
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The *Proactor* loop supports async subprocesses natively, so the fallback never
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happens and ``blockbuster`` allows the (now genuinely async) spawn.
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Windows has defaulted to ``WindowsProactorEventLoopPolicy`` since Python 3.8, so
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this is normally a no-op. We set it explicitly anyway as a safeguard: a
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dependency, IDE, or notebook host may have installed a Selector policy earlier
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in the process, and the ``langgraph dev`` subprocess in particular runs code we
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don't fully control. Calling this at each process entrypoint — **before any
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event loop is created** — guarantees the MCP subprocess path stays async.
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This must run at import/startup time, ahead of the first ``asyncio.run`` /
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``new_event_loop`` call; once a loop exists, swapping the policy does not change
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the already-running loop.
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"""
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from __future__ import annotations
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import sys
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def ensure_proactor_event_loop_policy() -> bool:
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"""Install ``WindowsProactorEventLoopPolicy`` on Windows if needed.
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Returns ``True`` if a Proactor policy is in effect afterwards (always
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``False`` off Windows, where the concept doesn't apply). Safe and idempotent
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to call multiple times; a no-op on non-Windows platforms.
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"""
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if sys.platform != "win32":
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return False
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import asyncio
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proactor_policy = getattr(asyncio, "WindowsProactorEventLoopPolicy", None)
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if proactor_policy is None: # pragma: no cover - non-Windows / stripped build
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return False
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current = asyncio.get_event_loop_policy()
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if not isinstance(current, proactor_policy):
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asyncio.set_event_loop_policy(proactor_policy())
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return True
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