Trac 0.11 and Apache trouble

[Christian Boos]

This ticket is mostly for documenting a problem that can happen when upgrading an existing Trac 0.10 installation running under Apache to Trac 0.11.

You may get the following segfault:

#0  0x0000002a96408140 in strlen () from /lib64/tls/libc.so.6
#1  0x0000002a96eef8bb in PyString_FromString (str=0x0) at stringobject.c:106
#2  0x0000002a96f3de3c in PyModule_AddStringConstant (m=0x2a984d4210,
    name=0x2a988dd6d3 "XML_ERROR_UNDECLARING_PREFIX",
    value=<value optimized out>) at modsupport.c:567
#3  0x0000002a988b9e98 in initpyexpat ()
    at /usr/local/src/Python-2.4.2/Modules/pyexpat.c:1972
#4  0x0000002a96f3a52b in _PyImport_LoadDynamicModule (
    name=0x7fbfff46cc "pyexpat",
    pathname=0x7fbfff4200 "/opt/python-2.4.2/lib/python2.4/lib-dynload/pyexpat.so", fp=<value optimized out>) at importdl.c:53
#5  0x0000002a96f38f54 in import_submodule (mod=0x2a970a7d90,
    subname=0x7fbfff46cc "pyexpat", fullname=0x7fbfff46cc "pyexpat")
    at import.c:2262

This happens due to a binary incompatibility related to the expat library.

Apache uses one version:

# ldd $(which httpd) | grep expat
        libexpat.so.0 => /usr/lib64/libexpat.so.0 (0x0000002a95a8a000)

# strings /usr/lib64/libexpat.so.0  |grep expat_
expat_1.95.7

Python expects another:

# strings /opt/python-2.4.2/lib/python2.4/lib-dynload/pyexpat.so  | grep expat_
expat_1.95.8

At runtime, Python will get to use some symbols found in the already loaded 1.95.7 one.

At this point, there's not much to do, you need to upgrade either one so that they use the same library (who said DLL hell was Windows specific?).

I believe that Python 2.5 "namespaces" the expat library, so it might be worth to give it a try.

[Matthew Good]

It looks good on my system (Ubuntu Edgy):

$ ldd /usr/sbin/apache2 | grep expat
        libexpat.so.1 => /usr/lib/libexpat.so.1 (0x00002b9241cb9000)
$ strings /usr/lib/libexpat.so.1 | grep expat_
expat_1.95.8
$ strings /usr/lib/python2.4/lib-dynload/pyexpat.so | grep expat_
expat_1.95.8

How did you install Python? One of the features of packaging systems on Linux is to make sure that your programs are consistent in the library versions they're compiled against to prevent these sorts of linking errors.

Besides possibly documenting this in the Wiki I don't see anything here that we can take action on here. So, I think this either needs converted to a task for documenting this, or simply closed.

[Christian Boos]

See also ​http://www.dscpl.com.au/wiki/ModPython/Articles/ExpatCausingApacheCrash which describes this issue at length and suggests some workarounds.

(worksforme as there are documented workarounds)

[anonymous]

set to 0.11 as expat is used since then extensively

[Matthew Good]

Replying to anonymous:

set to 0.11 as expat is used since then extensively

The milestone does not apply since this is closed as worksforme.