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Pooled connection checkout has no timeout: connect() hangs ~16 min on a dead socket

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Seit 22.10.2026.

  • #859 von @jahnvi480 — offen

Bewertung

Schwierigkeit
4/5
Geschätzter Aufwand
3-5 Tage
Anfängerfreundlichkeit
48/100
Issue-Typ
Bug
Klarheit
Klar beschrieben
Aktivitätsstatus
Aktiv
Tech-Stack
python
Bereich
backend, database

Rechercherichtung

Start by tracing pooled connection checkout in the driver, especially isAlive() and reset(); the issue identifies SQL_ATTR_CONNECTION_DEAD and SQL_ATTR_TXN_ISOLATION as relevant. Reproduce with the provided Python script against SQL Server, then inspect how connection validation and login timeouts are applied. Done means pooled checkout is bounded or the behavior is documented, as the issue requests.

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Beschreibung

area: connectivity-auth bug inADO triage done under development
Describe the bug

With pooling enabled, if a pooled connection's TCP socket has been silently dropped (no FIN/RST), the next connect() that reuses it blocks until the OS gives up on the socket. On Linux with the default net.ipv4.tcp_retries2=15 that is ~16 minutes. The login timeout doesn't apply. Once the socket errors, the pool opens a new connection and connect() returns successfully.

If several pooled connections are dead, one connect() checks them one after another.

From the source, isAlive() only reads the cached SQL_ATTR_CONNECTION_DEAD flag, so a dead socket passes it. reset() then sets SQL_ATTR_TXN_ISOLATION, which appears to be the call that waits on the server, with no timeout.

To reproduce

Client: python:3.12-slim-trixie container (linux/amd64), mssql-python==1.16.0, run with --cap-add NET_ADMIN. Server: mcr.microsoft.com/mssql/server:2022-latest, reachable as sqlserver.

import os, subprocess, time
import mssql_python

mssql_python.pooling(idle_timeout=600)
kw = dict(Server="sqlserver", UID=os.environ["DB_USER"], PWD=os.environ["DB_PASSWORD"], Encrypt="yes",
          TrustServerCertificate="yes", timeout=15, autocommit=True)

conn = mssql_python.connect(**kw)
cur = conn.cursor()
cur.execute("select client_tcp_port from sys.dm_exec_connections where session_id = @@SPID")
port = cur.fetchone()[0]
conn.close()  # back to the pool

# silently drop that connection's traffic (DROP, so no RST is sent)
subprocess.run(["iptables", "-A", "OUTPUT", "-p", "tcp", "--sport", str(port), "-j", "DROP"], check=True)
subprocess.run(["iptables", "-A", "INPUT", "-p", "tcp", "--dport", str(port), "-j", "DROP"], check=True)
time.sleep(8)

t = time.monotonic()
mssql_python.connect(**kw).close()  # reuses the dead pooled connection
print(f"connect() took {time.monotonic() - t:.1f}s")

Results:

Case connect() duration
tcp_retries2=15 (Linux default) 951.8 s
tcp_retries2=5 (--sysctl net.ipv4.tcp_retries2=5) ~13 s
3 dead pooled connections, tcp_retries2=5 29.5 s
Pooling disabled 0.02 s

The same happens on 1.15.0, and with autocommit=False. It doesn't happen if the socket was dead long enough for TCP keepalive to mark it dead first (~40 s with the default KeepAlive=30).

Expected behavior

Checking out a pooled connection should be bounded in time, for example by the login timeout or a separate validation timeout, with a timed-out candidate treated as dead. Alternatively, the checkout could avoid a server round trip. At minimum, the behaviour should be documented.

Further technical details
  • Python version: 3.12.15
  • mssql-python: 1.15.0 and 1.16.0
  • SQL Server version: SQL Server 2022 (repro); also seen against Azure SQL Managed Instance
  • Operating system: Debian 13 (trixie), linux/amd64
Additional context

The investigation, repro and analysis were done with the help of Claude Opus 5.5. The results above were measured; the root-cause reading of the source is our interpretation and may be wrong.

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