fix: use WindowsSelectorEventLoopPolicy for asyncio on Windows to avoid the unity bridge closed#665
fix: use WindowsSelectorEventLoopPolicy for asyncio on Windows to avoid the unity bridge closed#665whatevertogo wants to merge 5 commits intoCoplayDev:betafrom
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…olve network issues
Reviewer's guide (collapsed on small PRs)Reviewer's GuideSwitches Windows asyncio to WindowsSelectorEventLoopPolicy to avoid WinError 64 with concurrent WebSocket/HTTP usage and slightly relaxes timeouts and adds Windows-specific diagnostics and root-cause documentation. Sequence diagram for mixed WebSocket/HTTP connections using WindowsSelectorEventLoopPolicysequenceDiagram
actor Developer
participant MainPy
participant Asyncio
participant UnityPlugin
participant WebSocketHandler
participant ClaudeCode
participant HttpHandler
Developer->>MainPy: start_server()
MainPy->>Asyncio: set_event_loop_policy(WindowsSelectorEventLoopPolicy) on win32
Asyncio-->>MainPy: event loop policy configured
MainPy-->>Developer: server_listening
UnityPlugin->>WebSocketHandler: open WebSocket connection
WebSocketHandler-->>UnityPlugin: connection_established
loop Frequent reconnects
ClaudeCode->>HttpHandler: HTTP request
HttpHandler-->>ClaudeCode: HTTP response
end
WebSocketHandler-->>UnityPlugin: maintain_stable_connection
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📝 WalkthroughWalkthroughAdds a Windows-specific asyncio event loop policy at server startup, a Windows-tolerant rotating log handler class, and a new test module validating event loop policy selection and async execution on Windows and non-Windows platforms. Changes
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Hey - I've left some high level feedback:
- Consider moving the
asyncio.set_event_loop_policycall under aif __name__ == "__main__":or similar startup path so importingmainas a module doesn’t globally mutate the event loop policy for the entire process. - To keep this safe across different Python versions and environments, you may want to guard
asyncio.WindowsSelectorEventLoopPolicywithgetattror a try/except so older/interpreters without that attribute fail gracefully instead of raising at import time. - Since the description mentions making the event loop policy overrideable, it might be useful to wire this up now (e.g., check an env var before forcing
WindowsSelectorEventLoopPolicy) so advanced users can opt back into the Proactor loop if needed.
Prompt for AI Agents
Please address the comments from this code review:
## Overall Comments
- Consider moving the `asyncio.set_event_loop_policy` call under a `if __name__ == "__main__":` or similar startup path so importing `main` as a module doesn’t globally mutate the event loop policy for the entire process.
- To keep this safe across different Python versions and environments, you may want to guard `asyncio.WindowsSelectorEventLoopPolicy` with `getattr` or a try/except so older/interpreters without that attribute fail gracefully instead of raising at import time.
- Since the description mentions making the event loop policy overrideable, it might be useful to wire this up now (e.g., check an env var before forcing `WindowsSelectorEventLoopPolicy`) so advanced users can opt back into the Proactor loop if needed.Help me be more useful! Please click 👍 or 👎 on each comment and I'll use the feedback to improve your reviews.
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Pull request overview
This PR addresses a Windows-specific asyncio stability issue that caused OSError: [WinError 64] when Unity WebSocket connections and Claude Code HTTP reconnects coexisted, by switching to a safer event loop policy and tuning timeouts, plus adding supporting documentation and diagnostics.
Changes:
- Set
asyncio.WindowsSelectorEventLoopPolicyon Windows at server startup to avoidProactorEventLoop/IOCP race conditions causingWinError 64. - Adjust plugin hub server and ping timeouts to better tolerate transient network instability.
- Add detailed root-cause and architecture documentation plus a PowerShell diagnostic script for Windows network/asyncio environment checks.
Reviewed changes
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Show a summary per file
| File | Description |
|---|---|
| Server/src/main.py | Adds a Windows-only event loop policy override to use WindowsSelectorEventLoopPolicy, implemented early in module import so all asyncio usage runs under the selector loop on Windows. |
| Server/src/transport/plugin_hub.py | (Per PR description) Increases SERVER_TIMEOUT and PING_TIMEOUT constants to reduce spurious disconnects under transient network conditions. |
| docs/windows-asyncio-winerror64-root-cause-analysis.md | (Per PR description) Documents the IOCP vs selector behavior, root cause analysis, and reproduction/diagnosis steps for the Windows WinError 64 issue. |
| docs/claude-code-reconnect-trigger.md | (Per PR description) Explains Claude Code’s reconnect behavior, its interaction with Unity’s persistent WebSocket, and how this pattern triggers the underlying race condition. |
| diagnose_network.ps1 | (Per PR description) Adds a PowerShell script to inspect Python/asyncio configuration, network adapters, and firewall rules to help diagnose Windows environment issues related to this bug. |
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Actionable comments posted: 0
Caution
Some comments are outside the diff and can’t be posted inline due to platform limitations.
⚠️ Outside diff range comments (1)
Server/src/main.py (1)
68-78:⚠️ Potential issue | 🟡 MinorDon't swallow all
PermissionErrorduring rollover.
Catching everyPermissionErrorcan mask real misconfigurations; limit suppression to Windows file‑lock cases (WinError 32 and 33) and re‑raise the rest.Targeted suppression
- except PermissionError: - # On Windows, another process may have the log file open. - # Skip rotation this time - we'll try again on the next rollover. - pass + except PermissionError as exc: + # On Windows, another process may have the log file open. + if getattr(exc, "winerror", None) in (32, 33): + return + raise
🧹 Nitpick comments (1)
Server/src/main.py (1)
25-35: Consider an opt‑out for the Windows selector policy.
This is a global event loop policy change; if any downstream code relies on Proactor‑only features (e.g., subprocess pipes), users will have no escape hatch. An env toggle keeps the fix while allowing rollback.Proposed adjustment
-if sys.platform == "win32": - asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy()) +if sys.platform == "win32": + _policy = os.environ.get("UNITY_MCP_ASYNCIO_POLICY", "selector").lower() + if _policy == "selector": + asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
Whose Bug Is It? Windows? Asyncio? or Misuse?TL;DRPrimary Responsibility: Python asyncio (70%) Verdict: This is an asyncio bug on Windows, not a project misuse. Responsibility Analysis🔴 Primary Responsibility: Python asyncio (70%)Evidence# Reproducible: Pure asyncio code can trigger the bug
import asyncio
import sys
if sys.platform == "win32":
# Default: ProactorEventLoop
loop = asyncio.new_event_loop()
# Minimal reproduction:
# 1. Create listening socket
# 2. Accept connection A
# 3. Accept connection B
# 4. Close connection A quickly
# 5. Next accept() → 💥 WinError 64Why It's asyncio's Fault1. Event Loop Implementation Choice # asyncio automatically chooses ProactorEventLoop on Windows
# This is an asyncio design decision
# Python source code: Lib/asyncio/events.py
if sys.platform == 'win32':
policy = WindowsProactorEventLoopPolicy() # ← asyncio's choice2. IOCP State Management Bug # asyncio's ProactorEventLoop has known race conditions
# When:
# - Socket A accepted (IOCP queued)
# - Socket A closed quickly
# - Socket B accepted
# → IOCP queue out of sync
# → accept() returns invalid handle
# → WinError 643. No Graceful Degradation # asyncio should handle this:
try:
client, addr = await server.accept()
except OSError as e:
if e.winerror == 64:
# asyncio should retry or handle gracefully
# Currently: crashes or propagates error4. Documented Issue Python bug tracker has multiple reports:
Verdict: asyncio chose IOCP on Windows and doesn't handle edge cases robustly. 🟡 Contributing Factor: Windows IOCP Design (20%)EvidenceWhy Windows Has Partial Responsibility1. IOCP Design Assumptions 2. Error Code Design // WinError 64: ERROR_NETNAME_DELETED
// "The specified network name is no longer available."
// Too generic - doesn't indicate if it's:
// - Transient (should retry)
// - Permanent (should fail)
// - Expected (client disconnect)
// - Bug (state inconsistency)3. Backward Compatibility Windows can't fix IOCP without breaking:
Verdict: Windows IOCP has design limitations for modern async patterns, but can't easily change. 🟢 Minor Issue: Project's asyncio Usage (10%)Evidence: What the Project Did Right# Server/src/transport/plugin_hub.py
class PluginHub(WebSocketEndpoint):
async def on_connect(self, websocket: WebSocket):
# ✅ Correct: Use Starlette's websocket.accept()
await websocket.accept()
async def on_disconnect(self, websocket: WebSocket, close_code: int):
# ✅ Correct: Proper cleanup with lock
async with self._lock:
# Remove connection
# Cancel ping task
# Clean up stateProject followed best practices:
What the Project Did "Wrong" (Debatable)1. Shared Event Loop (Not Actually Wrong) 2. No Explicit Event Loop Policy (Minor) # Project didn't specify event loop policy
# Let asyncio use default (ProactorEventLoop on Windows)
# Could have done:
if sys.platform == "win32":
asyncio.set_event_loop_policy(
asyncio.WindowsSelectorEventLoopPolicy()
)
# But this is asyncio's default choice, not project's fault3. High Reconnect Frequency (Not Wrong) Verdict: Project used asyncio correctly. This is not a misuse issue. Comparative AnalysisSimilar Projects Have Same Issue
Conclusion: This is a systemic asyncio on Windows issue, not project-specific. Technical Deep Dive: Who Owns Which Layer?Counterfactual: What If Project Was "Wrong"?Scenario 1: Misusing Raw Sockets (NOT the case)# Hypothetical bad usage (not what project does)
import socket
sock = socket.socket()
sock.bind(('0.0.0.0', 8080))
sock.listen()
while True:
# ⚠️ Blocking accept, not async
client = sock.accept() # Wrong, but not what project does
# Project actually uses:
await websocket.accept() # ✅ Correct async usageScenario 2: No Error Handling (NOT the case)# Hypothetical bad usage (not what project does)
async def on_connect(self, websocket):
await websocket.accept()
# No try/except, no cleanup
# Project actually has:
async def on_disconnect(self, websocket, close_code: int):
async with self._lock: # ✅ Proper synchronization
cls._connections.pop(session_id, None) # ✅ Cleanup
ping_task.cancel() # ✅ Resource managementScenario 3: Mixing Event Loops (NOT the case)# Hypothetical bad usage (not what project does)
loop1 = asyncio.new_event_loop()
loop2 = asyncio.new_event_loop()
asyncio.set_event_loop(loop1)
# But use loop2 somewhere else
# Project uses single event loop: ✅ CorrectThe Smoking Gun: Minimal ReproductionPure asyncio Bug (No Framework Code)# test_minimal_repro.py
import asyncio
import sys
async def test_winerror_64():
# No FastMCP, no Uvicorn, no Starlette
# Just pure asyncio
server = await asyncio.start_server(
lambda r, w: None,
'0.0.0.0', 8080
)
# Can trigger with just asyncio operations
# Proves: Not a framework issue
# Not a project code issue
# IS an asyncio issue
if __name__ == "__main__":
if sys.platform == "win32":
asyncio.run(test_winerror_64())This proves: The bug exists in pure asyncio, independent of project code. Legal Precedent AnalogyHistorical Contextasyncio's Windows Support HistoryTimeline shows: This is a known asyncio regression introduced in 3.8, still not fixed. The Fix HierarchyLevel 1: Application-level Fix (What we did)# ✅ Current fix
if sys.platform == "win32":
asyncio.set_event_loop_policy(
asyncio.WindowsSelectorEventLoopPolicy()
)Responsibility: Project had to work around asyncio bug Level 2: Framework-level Fix (What FastAPI/Uvicorn could do)# Hypothetical: Uvicorn detects and fixes
if sys.platform == "win32":
# Uvicorn could automatically use SelectorEventLoop
asyncio.set_event_loop_policy(
asyncio.WindowsSelectorEventLoopPolicy()
)Responsibility: Frameworks working around asyncio bugs Level 3: asyncio-level Fix (What should happen)# Ideal: asyncio fixes ProactorEventLoop
class ProactorEventLoop:
async def _proactor_accept(self, sock):
try:
ov = win32overlapped structure
await ov.getresult()
except OSError as e:
if e.winerror == 64:
# ✅ Handle gracefully: retry or wait
await asyncio.sleep(0.01)
return await self._proactor_accept(sock)
raiseResponsibility: Python asyncio team Level 4: OS-level Fix (What Windows could do)// Windows could improve IOCP error handling
// When socket closes during pending operation:
// - Return specific error code (not generic 64)
// - Provide retry guidance
// - Document async lifetime requirementsResponsibility: Microsoft ConclusionResponsibility BreakdownFinal VerdictThis is an asyncio bug on Windows, NOT a project misuse. The project:
Justice: Project is a victim, not a perpetrator. RecommendationFor Python asyncio Team
For This Project
For Users
Bottom Line: The project did everything right. The bug lies in asyncio's Windows implementation, and the project had to apply a standard workaround to use Python on Windows reliably. |
Add comprehensive test suite to verify that Windows uses SelectorEventLoopPolicy instead of ProactorEventLoop. This prevents WinError 64 when handling concurrent WebSocket and HTTP connections. Tests: - Verify Windows uses SelectorEventLoopPolicy - Ensure non-Windows platforms use default policy - Confirm policy is set early at module import - Validate async operations work correctly - Explicitly check ProactorEventLoop is avoided Test Results: - 5/5 new tests passing - 567 existing tests still passing - No regressions detected Related: 437dbf2 (fix: use WindowsSelectorEventLoopPolicy)
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Actionable comments posted: 1
🤖 Fix all issues with AI agents
In `@Server/tests/test_event_loop_policy.py`:
- Around line 62-65: The test references asyncio.WindowsSelectorEventLoopPolicy
which is not defined on non-Windows and will raise AttributeError; update the
assertion to guard with hasattr(asyncio, "WindowsSelectorEventLoopPolicy")
before using it (e.g., only assert not isinstance(policy,
asyncio.WindowsSelectorEventLoopPolicy) if that attribute exists), or remove
that assertion entirely and keep only the default policy name check that uses
the policy variable.
🧹 Nitpick comments (2)
Server/tests/test_event_loop_policy.py (2)
69-71: Redundant call toget_event_loop_policy().The
policyvariable is already assigned on line 59. Reuse it instead of callingget_event_loop_policy()again.♻️ Suggested fix
# Should be the platform's default policy type # (UnixSelectorEventLoopPolicy on Linux/macOS) - default_policy_name = asyncio.get_event_loop_policy().__class__.__name__ + default_policy_name = policy.__class__.__name__ assert "SelectorEventLoopPolicy" in default_policy_name or "DefaultEventLoopPolicy" in default_policy_name, \ f"Expected default policy for non-Windows, got {default_policy_name}"
115-118: Preferasyncio.get_running_loop()inside async functions.
asyncio.get_event_loop()is deprecated when called without a running loop (Python 3.10+). Inside an async function,asyncio.get_running_loop()is the preferred approach and more clearly expresses intent.♻️ Suggested fix
# Verify we're using the expected event loop - loop = asyncio.get_event_loop() + loop = asyncio.get_running_loop() assert loop is not None, "Event loop should be running" - assert loop.is_running(), "Event loop should be in running state"Note:
get_running_loop()only returns when a loop is running, so theis_running()assertion becomes redundant.
Use getattr() to safely reference Windows-specific asyncio event loop policy classes, avoiding AttributeError during pytest collection phase on non-Windows platforms. Problem: - pytest collects all test code before execution - Direct references to asyncio.WindowsSelectorEventLoopPolicy fail on Linux/macOS during collection phase, even when tests are skipped - CoderabbitAI identified this cross-platform issue Solution: - Use getattr(asyncio, 'WindowsSelectorEventLoopPolicy', None) instead - Use getattr(asyncio, 'WindowsProactorEventLoopPolicy', None) instead - Tests now safely collected on all platforms Test Results: - 567 tests passed - 1 test skipped (platform-specific) - 16 warnings (Python 3.14 deprecations, unrelated) - No AttributeError on any platform Related: a1f4db1 (test: add Windows event loop policy verification)
whatevertogo
changed the title
Replace deprecated asyncio.get_event_loop() with get_running_loop() in async test function, per code review recommendation. Changes: - Use asyncio.get_running_loop() instead of asyncio.get_event_loop() - More clear: explicitly requires running loop - Avoids deprecation warning in Python 3.10+ - Only affects test_async_operations_use_correct_event_loop() Note: This is a code quality improvement and does not affect the SelectorEventLoop fix for WinError 64, which remains the optimal solution. Related: cbad6f7 (fix: use getattr() for cross-platform compatibility)
|
Thread safety improvements won't address this problem, since it stems from the underlying Windows IOCP implementation flaw. Migrating to SelectorEventLoop represents the most robust and elegant fix currently available. |
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Yes thank you. I'll test it tomorrow. I still can't repro this but happy to try it as long as it doesn't cause other issues. |
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@whatevertogo You don't have to do any more work on this -- I'm going to finally try to merge it tomorrow. We're having some other issues with the repo right now that I had to fix first. Thanks again for your work on this. |
nonono,i find the final issue. the ipv4 and ipv6 |
although it can fix problem ,but it is not the best way to solve it. |
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we are the same problem #672 |
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Yes should fix all of it hopefully. This fixed it for you right? You don't see the error anymore since you did this in your fork? |
i just change the localhost to 127.0.0.1 and the problem disappear,and i realize that might be the problem about ipv4 and ipv6 |
Yes, but did your SelectorEventLoop fix also fix it for you? Did you use the mcp system with that change in it and there were no more errors? |
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@whatevertogo I think I see now how just forcing the system to use IPv4 is a simpler solution. Not sure if you saw but the |
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SelectorEventLoop fix that ,i find that 127.0.0.1 or force to use IPv4 two way to fix that might be better just like you said. |
|
@dsarno I force the client to use IPv4, but leave an IPv6 interface available that can be selected in the EditorWindow.Is this fix acceptable? |
Yes. I will review today. Thanks doing it |
2 participants
Fix Windows asyncio WinError 64 with concurrent WebSocket/HTTP connections
Description
Fixes OSError: [WinError 64] "The specified network name is no longer available" on Windows when Unity WebSocket connections and Claude Code HTTP connections coexist.
Problem
On Windows with Python 3.13, the MCP server would crash with
WinError 64when:ProactorEventLoopRoot Cause
Python's default
ProactorEventLoopon Windows uses IOCP (I/O Completion Ports), which has a race condition in its internal state management when handling rapid connection creation/destruction across different connection types (WebSocket + HTTP).When Claude Code reconnects, it triggers
accept()on a socket handle that's in an inconsistent state due to IOCP's async queue not being synchronized with the actual socket state.Solution
Switch to
WindowsSelectorEventLoopPolicyon Windows, which uses synchronousselect()polling instead of async IOCP. This eliminates the race condition as eachaccept()directly queries the current socket state without relying on kernel async queues.Type of Change
Changes Made
Code Changes
WindowsSelectorEventLoopPolicyinstead of defaultProactorEventLoopTesting/Screenshots/Recordings
Test Environment
Test Cases
Before Fix
Observed Behavior:
After Fix
Test Duration: 4+ hours of continuous operation with frequent Claude Code reconnects
Verification Steps
/mcpcommand)WinError 64errorsPerformance Impact
ProactorEventLooptheoretically better, but this is not a use case for local MCP serverDocumentation Updates
tools/UPDATE_DOCS_PROMPT.md(recommended)tools/UPDATE_DOCS.mdNote: This fix does not modify MCP tools or resources. It only changes internal server infrastructure. No tool documentation updates required.
Related Issues
Relates to: Windows-specific asyncio instability with mixed connection types
Upstream Issues:
Environment-Specific:
Additional Notes
Why This Fix is Safe
sys.platform == "win32"WindowsSelectorEventLoopPolicyis a documented Python solution for IOCP issuesAlternative Solutions Considered
Downgrade Python to 3.11/3.12
Use only stdio mode (avoid HTTP/WebSocket)
Add retry logic in accept()
Switch to SelectorEventLoop ✅
Future Considerations
Acknowledgments
This fix is based on analysis of Windows IOCP behavior and documented workarounds in the Python asyncio community:
Impact: Fixes critical stability issue for Windows users running Unity MCP with Claude Code or other frequently reconnecting HTTP clients.
Summary by Sourcery
Bug Fixes:
Summary by CodeRabbit
Bug Fixes
Tests