Add docs for troubleshooting the libxml2 mismatch issue (#424)

Author: mxamin

README.md

@@ -87,6 +87,58 @@ apk add build-base openssl libffi-dev openssl-dev libxslt-dev libxml2-dev xmlsec
 
 ## Troubleshooting
 
+### `lxml & xmlsec libxml2 library version mismatch`
+
+`xmlsec` passes `lxml` XML nodes to the underlying `xmlsec1` library. Both
+libraries use `libxml2`, so they must use compatible `libxml2` versions at
+runtime. If `lxml` is installed from a wheel that bundles one `libxml2`
+version while `xmlsec` is built against another system `libxml2`, importing
+or using `xmlsec` can fail with:
+
+``` text
+xmlsec.InternalError: (-1, 'lxml & xmlsec libxml2 library version mismatch')
+```
+
+The most reliable fixes are:
+
+- Use prebuilt wheels for both `lxml` and `xmlsec` when wheels are available
+  for your platform:
+
+  ``` bash
+  pip install --only-binary=lxml,xmlsec lxml xmlsec
+  ```
+
+- If you need to build from source, build both packages against the same
+  locally installed `libxml2`:
+
+  ``` bash
+  pip install --no-binary=lxml,xmlsec lxml xmlsec
+  ```
+
+Do not mix a wheel-built `lxml` with a locally built `xmlsec`, or the other
+way around, unless you know they use the same `libxml2` version.
+
+An `lxml` release upgrade does not by itself mean `xmlsec` is incompatible;
+this error is about the `libxml2` libraries loaded in that environment.
+
+If the error appears only under uWSGI, uWSGI may have loaded the system
+`libxml2` before Python imports `lxml` or `xmlsec`. In that case, make sure
+uWSGI and the Python packages resolve to the same `libxml2`, or rebuild the
+Python packages from source in that environment. If you use `uv`, clear any
+cached mixed builds before reinstalling. For example, to reinstall from
+wheels:
+
+``` bash
+uv pip uninstall lxml xmlsec
+uv cache clean
+uv pip install --only-binary lxml --only-binary xmlsec lxml xmlsec
+```
+
+For background, see
+[issue #356](https://github.com/xmlsec/python-xmlsec/issues/356) and the
+uWSGI edge case in
+[issue #415](https://github.com/xmlsec/python-xmlsec/issues/415).
+
 ### Mac
 
 If you get any fatal errors about missing `.h` files, update your

doc/source/install.rst

@@ -65,6 +65,60 @@ Alpine
 Troubleshooting
 ***************
 
+``lxml & xmlsec libxml2 library version mismatch``
+--------------------------------------------------
+
+``xmlsec`` passes ``lxml`` XML nodes to the underlying ``xmlsec1``
+library. Both libraries use ``libxml2``, so they must use compatible
+``libxml2`` versions at runtime. If ``lxml`` is installed from a wheel
+that bundles one ``libxml2`` version while ``xmlsec`` is built against
+another system ``libxml2``, importing or using ``xmlsec`` can fail with:
+
+.. code-block:: text
+
+   xmlsec.InternalError: (-1, 'lxml & xmlsec libxml2 library version mismatch')
+
+The most reliable fixes are:
+
+* Use prebuilt wheels for both ``lxml`` and ``xmlsec`` when wheels are
+  available for your platform:
+
+  .. code-block:: bash
+
+     pip install --only-binary=lxml,xmlsec lxml xmlsec
+
+* If you need to build from source, build both packages against the same
+  locally installed ``libxml2``:
+
+  .. code-block:: bash
+
+     pip install --no-binary=lxml,xmlsec lxml xmlsec
+
+Do not mix a wheel-built ``lxml`` with a locally built ``xmlsec``, or the
+other way around, unless you know they use the same ``libxml2`` version.
+
+An ``lxml`` release upgrade does not by itself mean ``xmlsec`` is
+incompatible; this error is about the ``libxml2`` libraries loaded in
+that environment.
+
+If the error appears only under uWSGI, uWSGI may have loaded the system
+``libxml2`` before Python imports ``lxml`` or ``xmlsec``. In that case,
+make sure uWSGI and the Python packages resolve to the same ``libxml2``,
+or rebuild the Python packages from source in that environment. If you
+use ``uv``, clear any cached mixed builds before reinstalling. For
+example, to reinstall from wheels:
+
+.. code-block:: bash
+
+   uv pip uninstall lxml xmlsec
+   uv cache clean
+   uv pip install --only-binary lxml --only-binary xmlsec lxml xmlsec
+
+For background, see `issue #356 <https://github.com/xmlsec/python-xmlsec/issues/356>`_
+and the uWSGI edge case in
+`issue #415 <https://github.com/xmlsec/python-xmlsec/issues/415>`_.
+
+
 Mac
 ---