Python plugin compiled with incomplete dependencies

[jonmz]

Using WeeChat 1.1.1, compiled on CentOS 6 against a manually compiled Python 2.7 (because CentOS 6 only has Python 2.6), I've stumbled upon this problem loading the python.so plugin:

Error: unable to load plugin "/package/host/localhost/weechat-1.1.1/lib/weechat/plugins/python.so": /package/host/localhost/weechat-1.1.1/lib/weechat/plugins/python.so: undefined symbol: forkpty

ldd provides the following linker information on python.so:

$ ldd /package/host/localhost/weechat-1.1.1/lib/weechat/plugins/python.so
    linux-vdso.so.1 =>  (0x00007fff2fd9e000)
    libc.so.6 => /lib64/libc.so.6 (0x00007f58af344000)
    /lib64/ld-linux-x86-64.so.2 (0x00007f58afafa000)

The undefined symbol: forkpty is a result of python.so not being linked to libutil.so. I could quickly verify that starting WeeChat like this ...

$ LD_PRELOAD=/lib64/libutil.so.1 weechat

... effectively fixed the problem. That was amazingly hard to debug because as soon as another plugin that correctly links to libutil.so - like, e.g., the perl.so plugin - is loaded before the python.so plugin, the problem silently disappears. This left me with the weird situation that exactly the same binaries, used on two identically installed CentOS 6 hosts, provided different results, based on the order of plugins in the plugins folder as provided by the underlying filesystem.

I have found out that cmake/FindPython.cmake uses this code to create the list of libraries to link python.so against:

  execute_process(
    COMMAND ${PYTHON_EXECUTABLE} -c "import sys; from distutils.sysconfig import *; sys.stdout.write(get_config_var('LINKFORSHARED'))"
    OUTPUT_VARIABLE PYTHON_LFLAGS
    )

I'm not pretty sure if that's the correct way to do it. The section Compiling and Linking under Unix-like systems of the official Python 2 docs states that one is expected to use the pythonX.Y-config script to find out which linker flags have to be used to compile against Python. These flags correctly include -lutil:

$ python2.7-config --ldflags
-L/package/host/localhost/python-2.7.3/lib/python2.7/config -ldl -lutil -lm -lpython2.7 -Xlinker -export-dynamic

In contrast, the LINKFORSHARED variable from the sysconfig module doesn't:

$ python2.7 -c "import sys; from distutils.sysconfig import *; sys.stdout.write(get_config_var('LINKFORSHARED'))"
-Xlinker -export-dynamic

The Python docs admit, however, that the pythonX.Y-config procedure is "not guaranteed to work for all Unix-like platforms" and also references LINKFORSHARED as an alternative way, but combines it together with the value of LIBS (which does provides -ldl -lutil). So ... for now I'm solving the problem by explicitly adding the LIBS value to the code generation:

--- weechat-1.1.1/cmake/FindPython.cmake.orig   2015-01-25 08:39:58.000000000 +0100
+++ weechat-1.1.1/cmake/FindPython.cmake    2015-05-07 16:19:30.026200107 +0200
@@ -57,7 +57,7 @@
     )

   execute_process(
-    COMMAND ${PYTHON_EXECUTABLE} -c "import sys; from distutils.sysconfig import *; sys.stdout.write(get_config_var('LINKFORSHARED'))"
+    COMMAND ${PYTHON_EXECUTABLE} -c "import sys; from distutils.sysconfig import *; sys.stdout.write(get_config_var('LIBS') + ' ' + get_config_var('LINKFORSHARED'))"
     OUTPUT_VARIABLE PYTHON_LFLAGS
     )

I'd be happy if somebody with better compiler knowledge could review this and possibly fix it in the code base.

[yegle]

I'm using pkgsrc.org on Linux system to build and install weechat. Unfortunately I got the exact same error "undefined symbol forkpty"

[flashcode]

Similar to #1383, which is closed: now that WeeChat uses pkg-config to find and compile with Python, the problem should be solved.
Closing the issue.