Features/DebugPythonStacks

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Debug Python stacks

Summary

Fedora now ships debug versions of Python 2 and Python 3 in addition to the traditional optimized builds. This will be of use to advanced Python users, such as developers of extension modules.

Owner

• Name: Dave Malcolm

• Email: <dmalcolm@redhat.com>

Current status

• Targeted release: Fedora 40
• Last updated: 2010-05-20
• Percentage of completion: 10%

Initial notes on this: DaveMalcolm/PythonIdeas

See email

See the upstream notes on what all the flags do

Package	Latest build	Debug flags
python	python-2.6.5-9.fc14	--with-py-debug (implies Py_DEBUG , which implies LLTRACE, Py_REF_DEBUG, Py_TRACE_REFS, and PYMALLOC_DEBUG)
python3	python3-3.1.2-5.fc14	No debug build yet

• Other flags to investigate:
  • COUNT_ALLOCS
  • LLTRACE
  • CALL_PROFILE
  • WITH_TSC
• Figure out sane RPM conventions for packaging debug builds of extension modules
• Package debug builds of important extension modules

Detailed Description

In previous releases we have configured our build of Python for the typical use-case: as much optimization as reasonable.

However, upstream Python supports a number of useful debug options which use more RAM and CPU cycles, but make it easier to track down bugs [2]

Typically these are of use to people working on Python C extensions, for example, for tracking down awkward reference-counting mistakes.

In Fedora 14 we now supply a python-debug package containing a debug build of Python with these settings turned on.

It is intended for use by advanced Python users, and is installable on top of the normal (optimized) build. The builds share the same .py and .pyc files, but have their own compiled libraries and extension modules.

Technical notes

The Fedora 14 python.src.rpm now configures and builds, and installs the python sources twice, once with the regular optimized settings, and again with debug settings. (in most cases the files are identical between the two installs, and for the files that are different, they get separate paths)

The builds are set up so that they can share the same .py and .pyc files - they have the same bytecode format.

However, they are incompatible at the machine-code level: the extra debug-checking options change the layout of Python objects in memory, so the configurations have different shared library ABIs. A compiled C extension built for one will not work with the other.

The key to keeping the different module ABIs separate is that module "foo.so" for the standard optimized build will instead be "foo_d.so i.e. gaining a "_d" suffix to the filename, and this is what the "import" routine will look for. This convention ultimately comes from the way the Windows build is set up in the upstream build process, via a similar patch that Debian apply.

Similarly, the optimized libpython2.6.so.1.0 now has a libpython2.6_d.so.1.0 cousin for the debug build: all of the extension modules are linked against the appropriate libpython, and there's a /usr/include/python2.6-debug directory, parallel with the /usr/include/python2.6 directory. There's a new "sys.pydebug" boolean to distinguish the two configurations, and the distutils module uses this to supply the appropriate header paths ,and linker flags when building C extension modules.

Finally, the debug build's python binary is /usr/bin/python2.6-debug, hardlinked as /usr/bin/python-debug (as opposed to /usr/bin/python2.6 and /usr/bin/python)

User Experience

Fedora 14 now has a python-debug package containing debug versions of all of the content of the regular subpackages emitted by the python build (as opposed to the python-debuginfo package, which contains data for use by gdb (and thus is of use by the optimized stack).

The optimized build should be unaffected by the presence (or availability) of the debug build: all of the paths and the ELF metadata for the standard build should be unchanged compared to how they were before adding the debug configuration.

Installing the debug package gives you a /usr/bin/python-debug, analogous to the regular /usr/bin/python

The interactive mode of this version tells you the total reference count of all live Python objects after each command:

[david@fedora14 devel]$ python-debug
Python 2.6.5 (r265:79063, May 19 2010, 18:20:14) 
[GCC 4.4.3 20100422 (Red Hat 4.4.3-18)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> print "hello world"
hello world
[28748 refs]
>>> 
[28748 refs]
[15041 refs]

The debug build shares most of the files with the regular build (.py/.pyc/.pyo files; directories; support data; documentation); the only differences are the ELF files (binaries/shared libraries), and infrastructure relating to configuration (Include files, Makefile, python-config => python-debug-config, etc) that are different.

Comments and Discussion

• See Talk:Features/YourFeatureName