How do I use GDB to edit a python 2.7 global variable?

Question

I've got a script that's been running for days and is about halfway done. It's got a stupid bug in it which is going to cause it to crash before it finishes if a certain counter gets too high (the counter is otherwise unused; I don't mind mangling it to keep the script going). I did not have the foresight to import a module or write a backdoor to let me change state in any way; I've been told I can still get at the counter using gdb.

For simplicity, imagine instead I'm talking about this script below, which I have run from the command line as "python foo.py"

from time import sleep
i = 0
while(True):
    i += 1
    if (i > 100):
        raise Exception("Explosion")
    sleep(10)

What exactly would I need to type in gdb from time to time to change the value of the variable i to 0, and avoid the explosion? (If it matters, my python binary is 2.7.3).

jdowdell · Accepted Answer · 2013-09-06 01:17:06Z

Went ahead and powered through on my own. This worked for me (not shown - 'file' to load the python binary, 'attach'; 'detach' from process):

"bt" - see the stack trace
"frame foo" - where foo is the first frame mentioning "globals" as arguments to a Py* function
"print globals" (or, print *(PyDictObject*)globals) - confirm that your variable is there
"down" - move just below the frame you're interested in.
"finish" - (maybe unnecessary?) - let python complete whatever it was doing and release any locks or etc. that might interfere with interfacing with the globals object.
"call PyDict_SetItemString(globals, "i", PyLong_FromLong(0))" - hope it doesn't segfault. Change "i" to whatever your variable is.
"continue" - if you're lucky, the counter should now have been reset to 0.

However, for this to succeed, I think python2.7-gdb has to be installed/setup BEFORE you begin running the python process - can't do it after the fact, or gdb sees a garbled stack.

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