[Python-Dev] Integrating Expat

[Martin v. Loewis]

> I think bundling the sources is fine, but it should *ONLY* be a fallback
> if you do not find the Expat library installed on the system. *ALWAYS*
> link against a system-installed library first.

I'm not so sure about that. There are many problems doing that:

- if there is no shared libexpat, a static one is only usable if it is
  compiled either as PIC, or if pyexpat is statically linked into
  the Python interpreter. These conditions are impossible to test.

- if the installed version of expat is older than the one we ship,
  I think Python should still use the included one.

I think the problem you had with Apache does not apply to Python:

> Apache 1.3 would get loaded and export the Expat symbols to the rest
> of the process.

In Python, symbols of one extension module are not exported to
anything else. We would not build a libexpat.so (not sure whether
Apache did), and nobody would link with pyexpat.so. Furthermore,
dynloading pyexpat.so will use RTLD_LOCAL, unless that is overridden
by the application (and you really have to know what you're doing when
changing the dlopen flags).

> Now, some *other* module is loaded and dynamically links against
> /usr/lib/libexpat.so. Now you have *two* sets of Expat symbols and
> crashes are going to start happening.

No, that can't happen. pyexpat.so does not export its symbols (due to
RTLD_LOCAL), so the other module will still use libexpat.so.

Regards,
Martin