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Re: Python enabled gdb on Windows and relocation


2011/5/15 Ruben Van Boxem <vanboxem.ruben@gmail.com>:
> 2011/5/14 Doug Evans <dje@google.com>:
>> On Sat, May 14, 2011 at 2:09 AM, Ruben Van Boxem
>> <vanboxem.ruben@gmail.com> wrote:
>>> 2011/5/14 Doug Evans <dje@google.com>:
>>>> On Thu, May 12, 2011 at 9:19 AM, Ruben Van Boxem
>>>> <vanboxem.ruben@gmail.com> wrote:
>>>>> (now in plain-text as required by gdb mailing list)
>>>>>
>>>>> Hi,
>>>>>
>>>>> I am currently trying to integrate Python support into my toolchain
>>>>> build (including GDB of course). It is a sysrooted
>>>>> binutils+GCC+GDB+mingw-w64 toolchain.
>>>>>
>>>>> I currently have the basic setup working: I can link gdb with my
>>>>> manually generated import lib to the python dll from the official
>>>>> Windows install. If there is anything I am missing or a very easy
>>>>> solution to the problems decsribed below, please just say so. I am
>>>>> only suggesting what I would like to happen.
>>>>>
>>>>> Now on to the problems I'd like to discuss:
>>>>>
>>>>> 1. gdb.exe won't start without me having set PYTHONPATH manually.
>>>>
>>>> In a properly configured/built gdb on linux this isn't necessary, even
>>>> if python is installed in some random place.
>>>> I'm not sure about windows though.
>>>> Did you specify --with-python when you configured gdb, and if so did
>>>> you specify a value?
>>>> e.g., --with-python=SOME_VALUE
>>>
>>> I was cross-compiling a mingw toolchain+gdb from Linux, so I used
>>> --with-python without a value (because gdb configure tries to find the
>>> Python executabe), and I added -I"/path/to/python/includes" to CFLAGS
>>> and -L"/path/to/pythondll/importlib" to LDFLAGS, which built as it
>>> should. This is hacky though, and gdb configure should provide
>>> --with-python-libs and --with-python-include to make it more
>>> streamlined with any other build prerequisite (like
>>> gmp/mpfr/mpc/cloog/ppl in GCC for example).
>>
>> Ah.
>> Cross-compiling gdb with python is in need of improvement.
>> Alas python hasn't been designed with cross-compilation in mind (e.g.
>> build on linux, run on windows).
>> AIUI, the way to get the parameters required for compiling with
>> libpython is to get them from python's "distutils": kinda hard to do
>> in a cross-compile. ÂDone correctly there's no need to run python.
>>
>> I haven't done anything more to support python in gdb's configure.ac
>> because it's not clear to me what the right thing to do is: distutils
>> provides more than just --libs and --includes (btw, we don't use
>> --libs though, we use --ldflags which includes all of: the directory
>> in which to find libpython, the -l for libpython, and the -l's for all
>> the other libraries python needs). [Which isn't to say that someone
>> else isn't free to tackle this.]
>>
>> In the meantime, what I've been doing is a hack: write a script that
>> responds to:
>> --includes
>> --ldflags
>> --exec-prefix
>> and pass that as --with-python.
>>
>> E.g.
>> bash$ cat $HOME/my-python-for-config
>> #! /bin/sh
>>
>> if [ $# -ne 2 ]
>> then
>> Â Â Â Âecho "Bad # args. ÂBlech!" >&2
>> Â Â Â Âexit 1
>> fi
>>
>> # The first argument is the path to python-config.py, ignore it.
>>
>> case "$2" in
>> --includes) echo "-I/usr/include/python2.6 -I/usr/include/python2.6" ;;
>> --ldflags) echo "-L/usr/lib/python2.6/config -lpthread -ldl -lutil -lm
>> -lpython2.6" ;;
>> --exec-prefix) echo "/usr" ;;
>> *) echo "Bad arg $2. ÂBlech!" >&2 ; exit 1 ;;
>> esac
>>
>> exit 0
>> bash$ ./configure --with-python=$HOME/my-python-for-config [...]
>> [...]
>>
>>
>> Note that --exec-prefix is the runtime location of python.
>> GCC uses this to tell libpython where to find its support files.
>> [grep for Py_SetProgramName in gdb/python/python.c]
>
> OK, I tried your script in a couple of variations. It gets rid of the
> traceback I had before, but still doesn't help the PYTHONPATH problem.
> My directory structure is as follows (this is not in root, "/" is just
> my main build directory, there are several levels below it):
>
> /gdb <-- gdb build dir
> /gdb/gdb <-- where the python configuration is done
> /python <-- temporary install dir for python files for build,
> extracted from the official Windows installer, also location of
> libpython2.7.a import library
> /python/include/python27 <-- python headers, found by gdb in both cases
> /mingw64 <-- toolchain sysroot prefix
> /mingw64/bin <-- install location of gdb and python27.dll
> /mingw64/lib/python27 <-- install location of all python scripts
>
> First variant:
> --includes) echo "-I../../python/include" ;;
> --ldflags) echo "-L../../python -lpython2.7" ;;
> --exec-prefix) echo "../../mingw64/lib/python27" ;;
>
> Here exec-prefix would be the relative path from where "configure"
> does its magic to the final location of the scripts on the build
> system.
>
> Second variant:
> --includes) echo "-I../../python/include" ;;
> --ldflags) echo "-L../../python -lpython2.7" ;;
> --exec-prefix) echo "../lib/python27" ;;
>
> I thought the second points gdb to the installed location of the
> python scripts, but it still needed PYTHONPATH for that. I used
> relative paths in an attempt to "do the right thing", hackwise...
> Remember that the whole "/mingw64" directory gets zipped and moved,
> then it gets extracted to a random location in the Windows filesystem.
>
> Thanks for the help so far, I think we might just get this worked out together.
>
> Ruben

Wow, I think I have a partial solution. Delving into the Python docs,
for example here:
http://docs.python.org/using/windows.html#finding-modules, you can see
that PYTHONPATH is used first, then the Windows registry, then
PYTHONHOME, then some default relative paths. I placed the python
scripts all in the directory structure like so:

/bin/gdb
/bin/Lib/<python scripts>
/bin/python27.dll

This works, even without any manual PYTHONPATH intervention. Problem
is though, that as soon as someone has a PYTHONPATH environment
variable from a (incompatible) Python installation (think different
bitness or version 3.x instead of 2.7.1), I cannot predict what will
go wrong. This problem originates in Python's way of filling in the
search path (sys.path). A true solution in the GDB case to prevent
this collision of an incompatible PYTHONPATH would be that GDB sets an
internal PYTHONPATH as directed by configure, uses that to load its
Python internals, and allows the GDB child processes (apps being
debugged) to use the environment PYTHONPATH. For now, I have a
functional installation, but it will break as soon as someone installs
Python on their system.

The part your script plays in this (if I haven't misunderstood the
intention/effects of the "--exec-prefix" part) is allowing a uniform
way of directing the configure script to the right directories for
includes/libs.

Ruben

>
>>
>>>>> I understand the need for this, but as gdb requires Python 2, and users
>>>>> of my toolchain may have installed Python 3 or a 32-bit version python
>>>>> they want to use from the same environment (without changing their own
>>>>> PYTHONPATH), there is no way to run python-enabled gdb.
>>>>> [...]
>>>>
>>>> Yeah.
>>>> There is a proposal to add GDB_PYTHONPATH (or some such IIRC) and have
>>>> gdb use that instead of PYTHONPATH if it exists, but there's been
>>>> resistance to it.
>>>> I think(!) what would happen is that gdb would set $PYTHONPATH to the
>>>> value of $GDB_PYTHONPATH.
>>>> [Inferiors started by gdb should still get the original value of
>>>> PYTHONPATH though.]
>>>
>>> That way would be almost ideal, but a hardcoded *relative* path to the
>>> python scripts (that is standardized within gdb) wouldn't hurt.
>>
>> See above re: --exec-prefix.
>>
>>> An
>>> extra environment variable would require a lot of explaining for
>>> Windows, and is not "plug-and-play", like the rest of a sysrooted
>>> toolchain is supposed to be like. I think this should work on all
>>> setups:
>>>
>>> 1. Check hardcoded path; my suggestion would be "<gdb
>>> executable>/../lib/python27"
>>> 2. If this fails to find the necessary files/scripts, find it like you
>>> described above in Linux, without PYTHONPATH set.
>>> 3. Check PYTHONPATH.
>>
>> The problem being solved by the proposed GDB_PYTHONPATH is "What if
>> the user has PYTHONPATH set and it points to an incompatible version
>> of python?".
>> Leaving such a value for PYTHONPATH set while gdb's python is running
>> feels wrong (and IIRC has caused some problems).
>>
>> The problem of telling python where to find itself is already solved
>> (or at least is intended to be solved) with gdb's calling
>> Py_SetProgramName with a value derived from the python-provided
>> --exec-prefix.
>>
>>> I would think only number one would change, and perhaps be only
>>> enabled with a special configure option. Nothing else would have to
>>> change, and Windows users would rejoice :)
>>> Again, this is only my suggestion, if there are problems with it in
>>> way I haven't thought of, please say so, and we can come up with
>>> another solution.
>>
>


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