Adventures in Python Core Dumping
After watching Bryan Cantrill's presentation on Running Aground: Debugging Docker in Production I got all excited (and strangely nostalgic) about the possibility of core-dumping server-side Python apps whenever they go awry. This would theoretically allow me to fully inspect the state of the program at the point it exploded, rather than relying solely on the information of a stack trace.
I decided to try exploring a core dump on my own by writing a simple Python script that generated one.
Doing this required a bit of setup on my Ubuntu 14.04 server.
First, I had to
apt-get install python2.7-dbg to install a version
of Python with debug symbols, so that
gdb could actually
make sense of the core dump. It seems Ubuntu comes pre-configured with
a Python debugging extension for
gdb built-in, so I didn't have to
do any extra configuration here, which was great.
I also had to add the following line to
actually enable core dump files to be created:
#<domain> <type> <item> <value> * soft core 100000
After that, I created a file called
explode.py in my home directory:
import os def my_exploding_func(): my_local_var = 'hi' os.abort() my_exploding_func()
Then I ran the script:
$ python2.7-dbg explode.py Aborted (core dumped)
This created a
core file in my home directory.
Exploring The Stack
I opened the core dump in
$ gdb /usr/bin/python2.7-dbg core GNU gdb (Ubuntu 7.7.1-0ubuntu5~14.04.2) 7.7.1 Copyright (C) 2014 Free Software Foundation, Inc. ... warning: core file may not match specified executable file. [New LWP 10020] [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1". Core was generated by `/usr/bin/python2.7-dbg ./explode.py'. Program terminated with signal SIGABRT, Aborted. #0 0x00007f996aff7cc9 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56 56 ../nptl/sysdeps/unix/sysv/linux/raise.c: No such file or directory. (gdb)
Now I could use all of
Python debugging extension commands. For example, running
py-bt gave me:
(gdb) py-bt #4 Frame 0x7f996bf28240, for file ./explode.py, line 7, in my_exploding_func (my_local_var='hi') os.abort() #7 Frame 0x7f996bf28060, for file ./explode.py, line 9, in <module> () my_exploding_func()
I could also use
py-locals to show me the values of local variables
in the current stack frame, and
py-down to traverse the
This was all pretty awesome, and will be very useful if my Python programs actually segfault. But it'd be cool if I could actually get all this rich information any time one of my servers returned a 500. That's a bit of a different situation since Python servers don't usually segfault when they return a 500--instead, they catch exceptions, return an error code, and continue running.
For now I'm going to ignore the "continue running" part; there are ways to core dump without killing a process, but right now I'm more interested in figuring out how to get information about handled exceptions.
Obtaining Information About Handled Exceptions
Let's assume we have a script called
import os def my_exploding_func(): a = 1 call_nonexistent_func() try: my_exploding_func() except Exception, e: os.abort()
The thing about the core dump generated from this script is that running
py-bt only gives us the stack trace from the point that we called
os.abort(), which is pretty useless:
(gdb) py-bt #4 Frame 0x7f3767430450, for file ./explode3.py, line 12, in <module> () os.abort()
What we really want is a way to introspect the exception that was currently
being handled at the time that
os.abort() was called.
There isn't a particularly easy way to do this with the Python debugging
gdb, but one nice thing about
gdb is that its extensions
are written in Python. This means we can write our own extension
that gives us easy access to the information we need.
Doing this took some research. It looks like the latest version of the
Python debugging extension for
gdb is in a file in the CPython codebase
libpython.py, but this is actually a much newer
version than the one that ships with Ubuntu 14.04. I had to use
to find the actual version on my system, which was at
After poring through the code and consulting the CPython source code and
documentation on extending gdb using Python, I wrote my first
gdb extension, which is in the attached
py_exc_print.py file. It adds a
command that gives us what we need:
(gdb) source py_exc_print.py (gdb) py-exc-print Traceback (most recent call last): Frame 0x7f3767430450, for file ./explode2.py, line 12, in <module> () Frame 0x7f37673f3060, for file ./explode2.py, line 7, in my_exploding_func (a=1) exceptions.NameError("global name 'call_nonexistent_func' is not defined",)
Note that it's more useful than a standard stack trace, as the values of local variables are included in the printout. But more work on the extension needs to be done in order to make those locals easily introspectable.
Thus concludes my first foray into Python core dumping.
Some open questions:
I'm not sure how feasible core dumping on every uncaught exception actually is. For instance, how big do core files become in production environments?
In order for the core dump to be useful, a debug build of the Python interpreter needs to be used. How is performance impacted by this? As the aforementioned Bryan Cantrill talk mentions, we should be able to inspect core dumps from production environments: yet is it feasible to run a debug build of Python in a production environment?