test: Use asyncio.SelectorEventLoop() over deprecated asyncio.WindowsSelectorEventLoopPolicy(), move loop creation #34820

1. 
   maflcko commented at 11:11 am on March 13, 2026: member

   It is deprecated according to https://docs.python.org/3.14/library/asyncio-policy.html#asyncio.WindowsSelectorEventLoopPolicy The replacement exists since python 3.7

   Also, move the event loop creation to happen in the thread that runs the loop.

2. DrahtBot renamed this:
   test: Use asyncio.SelectorEventLoop() over deprecated asyncio.SelectorEventLoop()
   test: Use asyncio.SelectorEventLoop() over deprecated asyncio.SelectorEventLoop()
   on Mar 13, 2026
3. DrahtBot added the label Tests on Mar 13, 2026
4. 
   DrahtBot commented at 11:11 am on March 13, 2026: contributor

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5. test: Use asyncio.SelectorEventLoop() over deprecated asyncio.WindowsSelectorEventLoopPolicy() fa9168ffcd
6. maflcko renamed this:
   test: Use asyncio.SelectorEventLoop() over deprecated asyncio.SelectorEventLoop()
   test: Use asyncio.SelectorEventLoop() over deprecated asyncio.WindowsSelectorEventLoopPolicy()
   on Mar 13, 2026
7. maflcko force-pushed on Mar 13, 2026
8. DrahtBot added the label CI failed on Mar 13, 2026
9. in test/functional/test_framework/p2p.py:743 in fa9168ffcd

   739@@ -740,9 +740,7 @@ def __init__(self):
   740 
   741         NetworkThread.listeners = {}
   742         NetworkThread.protos = {}
   743-        if platform.system() == 'Windows':
   744-            asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
   745-        NetworkThread.network_event_loop = asyncio.new_event_loop()
   746+        NetworkThread.network_event_loop = asyncio.SelectorEventLoop() if platform.system() == "Windows" else asyncio.new_event_loop()
   

   ---

   ---

   

   l0rinc commented at 12:15 pm on March 13, 2026:

   Removing the deprecation seems like a good start, but my understanding is that __init__ happens before thread.start(), and run() after it, so the event loop would still be created on the main thread and then run on the worker thread. My understanding is that this potential cross-thread handoff could still be the suspected source of flakiness.

   ---

   

   maflcko commented at 12:49 pm on March 13, 2026:

   Hmm, ok, I’ll move it as well.
10. DrahtBot removed the label CI failed on Mar 13, 2026
11. test: Move event loop creation to network thread

    This should fix https://github.com/bitcoin/bitcoin/issues/34367
    
    I am not familiar with Windows sockets thread-safety, but creating the
    event loop on the main thread, and running it in the network thread
    could lead to a fast abort in Python on Windows (without any stderr):
    
    ```
    77/276 - wallet_txn_clone.py failed, Duration: 1 s
    
    stdout:
    2025-12-10T08:04:27.500134Z TestFramework (INFO): PRNG seed is: 4018092284830106117
    
    stderr:
    
    Combine the logs and print the last 99999999 lines ...
    ============
    Combined log for D:\a\_temp/test_runner_₿_🏃_20251210_075632/wallet_txn_clone_196:
    ============
     test  2025-12-10T08:04:27.500134Z TestFramework (INFO): PRNG seed is: 4018092284830106117
     test  2025-12-10T08:04:27.500433Z TestFramework (DEBUG): Setting up network thread
    ```
    
    Also, I couldn't find any docs that require the loop must be created on
    the thread that runs them:
    
    * https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.new_event_loop
    * https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.loop.run_forever
    
    However, the patch seems trivial to review, harmless, and easy to
    revert, so it may be a good try to fix the intermittent Windows Python
    crash.

    fa050da980
12. maflcko renamed this:
    test: Use asyncio.SelectorEventLoop() over deprecated asyncio.WindowsSelectorEventLoopPolicy()
    test: Use asyncio.SelectorEventLoop() over deprecated asyncio.WindowsSelectorEventLoopPolicy(), move loop creation
    on Mar 13, 2026
13. 
    l0rinc commented at 8:31 pm on March 13, 2026: contributor

    ACK fa050da9805d5388ec74e2e0f92e59e861f11918

    The change seems low-risk and makes sense based on my limited understanding of the situation. Checked the related tests and they’re passing on Windows and Mac.

14. fanquake requested review from willcl-ark on Mar 15, 2026
15. 
    maflcko commented at 1:31 pm on March 16, 2026: member

    cc @hebasto Was there a reason for you to pick asyncio.WindowsSelectorEventLoopPolicy() over asyncio.SelectorEventLoop(), when initially adding this? Just curious, fine if you don’t recall.
16. 
    hebasto commented at 2:13 pm on March 16, 2026: member

    cc @hebasto Was there a reason for you to pick asyncio.WindowsSelectorEventLoopPolicy() over asyncio.SelectorEventLoop(), when initially adding this? Just curious, fine if you don’t recall.

    I genuinely don’t recall.

17. sedited approved
18. 
    sedited commented at 5:18 pm on March 18, 2026: contributor

    ACK fa050da9805d5388ec74e2e0f92e59e861f11918
19. sedited merged this on Mar 18, 2026
20. sedited closed this on Mar 18, 2026

    ---

21. maflcko deleted the branch on Mar 19, 2026
22. fanquake referenced this in commit c7127f21df on Mar 19, 2026
23. fanquake referenced this in commit 72d6c88165 on Mar 19, 2026
24. 
    fanquake commented at 5:36 am on March 19, 2026: member

    Backported to 31.x in #34800.

Contributors
maflcko DrahtBot l0rinc hebasto sedited fanquake

Review Requested
willcl-ark

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