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Deadlock under concurrent multithreaded use when DEBUG logging is enabled via setup_logging()

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Seit 10.7.2026.

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Beschreibung

area: performance bug inADO triage done under development
Describe the bug

When driver logging is enabled via mssql_python.setup_logging(output="file") (DEBUG level) and multiple threads concurrently open connections and execute statements (for example a ThreadPoolExecutor with 4 workers), the process permanently deadlocks. It sits at 0% CPU — fully frozen, not merely slow.

The same code with setup_logging not called does not deadlock. It also does not deadlock with a single worker thread. The trigger is therefore the combination of DEBUG logging + concurrency (workers > 1).

Environment
  • mssql-python version: 1.10.0
  • Python version: 3.12.11
  • Operating system: macOS (arm64)
  • Authentication: ActiveDirectoryDefault (Entra ID) against a Microsoft Fabric Warehouse. The deadlock is in the logging path and is very likely authentication-independent.
Observed thread stacks

Captured with Python's faulthandler. The dumps were identical across 7 dumps over roughly 7 minutes, which confirms a permanent deadlock rather than slowness.

Thread A (opening a connection):

File ".../mssql_python/connection.py", line 571 in setautocommit
File ".../mssql_python/connection.py", line 410 in __init__
File ".../mssql_python/db_connection.py", line 55 in connect
-> (calls into Python logging) ...
File ".../logging/handlers.py", line 75 in emit
File ".../logging/__init__.py", line 1144 in flush   # blocked here

Thread B (executing a statement, a COPY INTO):

File ".../mssql_python/cursor.py", line 1545 in execute   # blocked in native execute
Server-side check

While the client is frozen, the database server shows no query running for this session. This confirms the block is entirely client-side and is not waiting on the server.

Hypothesized mechanism

mssql-python emits DEBUG log records from within native code paths (for example setautocommit during connect, and cursor.execute) that hold the GIL / internal locks. Under concurrency this creates a lock-acquisition cycle with Python's logging Handler locks across threads:

  • Thread A holds the logging handler lock and is blocked in flush.
  • Thread B is blocked in native execute and cannot proceed.

This is a classic logging-from-native-under-the-GIL deadlock. It only reproduces with workers > 1.

Steps to reproduce (minimal sketch)
  1. Call mssql_python.setup_logging(output="file") to enable DEBUG logging.
  2. From N >= 2 threads, concurrently open connections (autocommit=True) and run statements against the same server.

The process deadlocks within seconds to a couple of minutes.

Expected behavior

Enabling DEBUG logging should not affect correctness. Concurrent, multithreaded workloads should run to completion whether or not setup_logging is enabled.

Workaround

Do not enable setup_logging for concurrent / multithreaded workloads.

Impact

This makes the DEBUG logging feature unusable for any multithreaded application (for example parallel data-loading), which is precisely the scenario where enabling DEBUG logging is most valuable for diagnosis.

Possibly related issues

The following existing issues appear related but are distinct from this report:

  • #540 — GIL held for the entire duration of cursor.execute() (describes the native-call-holds-GIL behaviour that is part of the hypothesized mechanism here, but is about starvation rather than a permanent deadlock and does not involve logging).
  • #321 — multithreaded hang/deadlock with 3+ threads (does not involve logging; closed as non-reproducible).
  • #421 — hang with setup_logging enabled (single-threaded, diagnosed as a driver-load hang on Linux rather than a concurrency logging-lock deadlock).
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