SIGSEGV in SQLFetchScroll (libmsodbcsql.18) under concurrent threaded fetch on macOS arm64

Summary

mssql-python 1.10.0 crashes with a SIGSEGV during concurrent multi-threaded fetching on macOS arm64. The process is killed by the signal with no Python traceback (exit 139). The macOS crash report puts the faulting frame inside libmsodbcsql.18.dylib at SQLFetchScroll, reached through the driver's ddbc_bindings (FetchBatchData / FetchMany_wrap).

This looks like the same underlying problem as #668 (TLS stream corruption under concurrent threaded reads on macOS arm64), but here the failure is a hard crash instead of corrupted rows or a link error, so I am reporting it separately with the native stack. More on that connection at the end.

Environment

• macOS 26.5.2 (build 25F84), Apple Silicon (arm64)
• Python 3.12.11
• mssql-python 1.10.0 (bundled libmsodbcsql.18.dylib, ddbc_bindings.cp312-universal2.so)
• SQLAlchemy 2.1.0b3, mssql+mssqlpython dialect
• Connection: Encrypt=yes, TrustServerCertificate=yes, built-in connection pooling on, transaction isolation SNAPSHOT
• Remote SQL Server, reached over a VPN

How it shows up

The workload is a dlt sql_database extract on the pyarrow backend. Eight worker threads read different tables at the same time, each running a SELECT and pulling rows with fetchmany. The crash lands during the fetch phase, early in a schema that has many tables loading concurrently. It is intermittent and happens under sustained concurrent reads, not on one specific row. A short synthetic stress script did not force it, which is in line with what others noted on #668.

Fault

• EXC_BAD_ACCESS (SIGSEGV), KERN_INVALID_ADDRESS at 0x0000000000000008 (a dereference through a near-null pointer).
• At crash time, 6 of 29 threads were executing inside libmsodbcsql.18.dylib and 2 were in a fetch call. That points at a race in the driver's read path under concurrency, not at bad input.

Native stack (crashing thread)

The interpreter side is a plain SQLAlchemy result iteration (_manyrow_getter, i.e. fetchmany) calling into the driver:

libmsodbcsql.18.dylib              SQLFetchScroll          <-- faulting frame
ddbc_bindings.cp312-universal2.so  FetchBatchData(...)
ddbc_bindings.cp312-universal2.so  FetchMany_wrap(...)
ddbc_bindings.cp312-universal2.so  pybind11 dispatcher
libpython3.12.dylib                _PyEval_EvalFrameDefault
_result_cy.cpython-312-darwin.so   sqlalchemy BaseResultInternal _manyrow_getter
libpython3.12.dylib                builtin_next

Full frames from the .ips crash report:

 0  libmsodbcsql.18.dylib + 1127984
 1  libmsodbcsql.18.dylib + 1171428
 2  libmsodbcsql.18.dylib + 1199848
 3  libmsodbcsql.18.dylib + 1140492
 4  libmsodbcsql.18.dylib + 943376
 5  libmsodbcsql.18.dylib + 859712
 6  libmsodbcsql.18.dylib + 867548
 7  libmsodbcsql.18.dylib + 905332
 8  libmsodbcsql.18.dylib + 164368
 9  libmsodbcsql.18.dylib + 450000
10  libmsodbcsql.18.dylib + 462896
11  libmsodbcsql.18.dylib + 463820  SQLFetchScroll
12  ddbc_bindings.cp312-universal2.so + 111852  FetchBatchData(...)
13  ddbc_bindings.cp312-universal2.so + 123228  FetchMany_wrap(...)
14  ddbc_bindings.cp312-universal2.so + 373200  pybind11 argument_loader::call
15  ddbc_bindings.cp312-universal2.so + 186332  pybind11::cpp_function::dispatcher
16  libpython3.12.dylib + 677576  _PyEval_EvalFrameDefault
17  _result_cy.cpython-312-darwin.so + 92288  sqlalchemy BaseResultInternal _manyrow_getter
18  libpython3.12.dylib + 2721008  gen_iternext
19  libpython3.12.dylib + 2918080  builtin_next
20  libpython3.12.dylib + 1906720  thread_run
21  libsystem_pthread.dylib + 27736  _pthread_start
22  libsystem_pthread.dylib + 7196  thread_start

Likely relation to #668

#668 reports concurrent threaded reads on macOS arm64 with Encrypt=yes corrupting the TDS/TLS stream: single-character substitutions in returned strings, Communication link failure, Protocol error in TDS stream, and timeouts. It reproduces with pyodbc plus msodbcsql18 too, so it lives in the ODBC/TLS layer. A commenter there suspects the read path is corrupting adjacent heap memory. A near-null dereference in SQLFetchScroll fits that reading: the same corruption, landing on a pointer rather than a data byte. Same trigger (macOS arm64, mssql-python 1.10.0, Encrypt=yes, concurrent reads), heavier outcome.