Bob Ippolito (@etrepum) on Haskell, Python, Erlang, JavaScript, etc.

Advanced Debugging Techniques: ktrace


I had spent the past few days on and off trying to help a py2app user with a very hairy problem: when wxPython was bundled, the main menu didn't work. Running the application as an alias bundle or with pythonw worked just fine, but when built as standalone or semi-standalone, the menu no longer showed up.

Right off the bat I (correctly) assumed it was a problem with wxPython or his sample code, because py2app doesn't do anything that would cause this sort of behavior. It does link to Cocoa, but it doesn't call into any AppKit functionality unless it needs to display an error message. Somewhat trivially converting his source to work with wxPython 2.5 solved this issue, so I was rather stumped.

I had a hunch that perhaps there was something that wxPython wants that didn't end up in the bundle, so I broke out ktrace. Using ktrace is rather simple:

% ktrace ./dist/

This will create a file in the current directory, ktrace.out, with a log of just about everything that happened. For efficiency, this file is in a binary format that you must process with kdump. The output of kdump is quite lengthy, but it looks like this:

15121 ktrace   CALL  execve(0xbffffd1b,0xbffffc84,0xbffffc8c)
15121 ktrace   NAMI  "dist/"
15121 ktrace   NAMI  "/usr/lib/dyld"
15121 test     RET   execve 0

CALL is the actual system call, NAMI is a name to inode translation that the system call used, and RET is the value returned to the application. If there was an error during the system call, kdump will gladly tell you everything you wanted to know:

15121 test     CALL  open(0xbfffe7f0,0,0x1b6)
15121 test     NAMI  "/Library/Preferences/org.pythonmac.unspecified.test.plist"
15121 test     RET   open -1 errno 2 No such file or directory

Since I suspected that wxPython was missing a file, I wanted to narrow down the output, so I naturally used grep on the kdump output to find errors:

% kdump | grep -B 2 errno | grep wx
15121 test     NAMI  "/Users/bob/Desktop/simple/dist/"
15121 test     NAMI  "/Users/bob/Desktop/simple/dist/"
15121 test     NAMI  "/Users/bob/Desktop/simple/dist/"
15121 test     NAMI  "/Users/bob/Desktop/simple/dist/"

Unfortunately, what we're looking at here (and for the next 150 or so lines) is primarily just Python doing its module import search. Since I knew that all of sys.path pointed to locations under Resources, I can just filter that out. It is extremely unlikely that wxPython wants anything from there:

% kdump | grep -B 2 errno | grep wx | grep -v Resources
15121 test     NAMI  "/Users/bob/Desktop/simple/dist/"
15121 test     NAMI  "/libwx_mac-2.4.0.rsrc"

There it is! So now I just need to make the look like the following:

from distutils.core import setup
import py2app
    app = [''],
    data_files = [
        ('../Frameworks', ['/usr/local/lib/libwx_mac-2.4.0.rsrc']),

Now the application works as expected. py2app 0.1.9 will include a recipe for wxPython to make sure that this file ends up in the bundle automagically, among other new features.