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Document the glibc static-TLS ImportError in the FAQ #801

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Document the glibc static-TLS ImportError in the FAQ
1fanwang Sep 12, 2026
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Clarify static TLS loading and the preload workaround
1fanwang Sep 12, 2026
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28 changes: 27 additions & 1 deletion doc/FAQ.rst
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Expand Up @@ -89,6 +89,33 @@ every major release of GCC changes the ABI in some why, so linking
code compiled with GCC-3.3 and GCC-4.0, for example, can be
problematic.

ImportError: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: cannot allocate memory in static TLS block

Previously loaded native extensions can consume glibc's static-TLS
surplus. The dynamic loader can then fail to load ``MySQLdb._mysql`` or
one of its dependencies when it needs more static TLS, particularly
with the initial-exec TLS model. The static allocation cannot grow
after process startup, so this can occur even with a correct
mysqlclient build. Examples include `Apache Airflow #17546
<https://github.com/apache/airflow/issues/17546>`_ and `#40503
<https://github.com/apache/airflow/issues/40503>`_.

Preload the library named in the error before Python starts, as in
`Apache Airflow #19010 <https://github.com/apache/airflow/pull/19010>`_.
An exported ``LD_PRELOAD`` is inherited by child processes, including
programs that would not otherwise load that library. Scope it to the
affected service or container; startup and memory costs depend on the
program and platform.

On Debian-based systems with ``dpkg-dev`` installed, use the platform's
multiarch name rather than ``uname -m``:

.. code-block:: sh

export LD_PRELOAD="/usr/lib/$(dpkg-architecture -qDEB_HOST_MULTIARCH)/libstdc++.so.6"

On other distributions, use the path from the error. For example,
RHEL/CentOS commonly uses ``/lib64/libstdc++.so.6`` instead.

My data disappeared! (or won't go away!)
----------------------------------------
Expand Down Expand Up @@ -133,4 +160,3 @@ Other Resources
* Read `PEP-249`_

.. _`PEP-249`: https://www.python.org/dev/peps/pep-0249/