[lttng-dev] [PATCH lttng-ust] Fix: Corrected python version assumptions in lttng-gen-tp

[Gerlando Falauto]

Hi,

On 05/13/2014 08:55 PM, Jérémie Galarneau wrote:
> On Tue, May 13, 2014 at 4:38 AM, Gerlando Falauto
> <gerlando.falauto at keymile.com> wrote:
>> Hi Jesper,
>>
>> [...]
>>
>>> I *thought* I was running on python2.7 but as it turns out I was not.
>>>
>>> When resolving "python" I get python2.7 but when I check the explicit
>>> version laying at /usr/bin/python its actually a 2.4 version so no wonder it
>>> was failing.
>>>
>>> Checking the PEP for when "except XX as yy" was introduced points to that
>>> it was introduced in python 2.6 so entirely my mistake.
>>> Reran it now with the a 2.7 version and it works as intended.
>>>
>>> However, I still think you should run it through env as proposed in the
>>> patch and not use the explicit path to python.
>>
>>
>> I still have some issues, even with this change.
>> As a matter of fact, my default python installation would be a python2.6:
>>
>> $ python
>>
>> python
>> Python 2.6.6 (r266:84292, Jan 22 2014, 05:06:49)
>>
>> I'm however cross-compiling, so I specify a sysroot and I explicitly set the
>> python version in my configure script:
>>
>> $ . /opt/eldk/eldk_init ppc_6xx # set the env for cross-compiling
>>
>> $ python2.7
>> Python 2.7.3 (default, Aug  2 2013, 20:37:10)
>> [GCC 4.7.2] on linux2
>>
>> $ which python2.7
>> /opt/eldk/powerpc/sysroots/i686-eldk-linux/usr/bin/python2.7
>>
>> $ ac_cv_header_linux_perf_event_h=no PYTHON=python2.7 ./configure
>> --target=powerpc-linux --host=powerpc-linux --build=i686-linux
>> --with-libtool-sysroot=/opt/eldk/powerpc/sysroots/powerpc-linux
>> --prefix=/usr
>>
>> $ MAKEFLAGS= && DESTDIR=/opt/eldk/powerpc/sysroots/powerpc-linux make all
>> install install-strip pkgconfigdir=/tmp pkgconfig_DATA=
>>
>> This doesn't however help, as the script will call "python" from the current
>> environment (thereby ignoring the PYTHON= value used by the configure script
>> when checking for python version).
>>
>> So perhaps a solution would be to change the configure script (or whatever
>> it comes from by means of autoconfig/automake) so to create a "python"
>> symlink in the working directory (to the version selected/detected) and then
>> add it to the path (*).
>>
>> I guess a quick-and-dirty alternative could be to change the makefile so to
>> use the "PYTHON" environment variable to explictly set what interpreter to
>> use -- but that would defeat the whole purpose of the shebang so I believe
>> it's a very bad idea:
>>
>> --- a/doc/examples/gen-tp/Makefile
>> +++ b/doc/examples/gen-tp/Makefile
>> @@ -46,7 +46,7 @@ sample.o: sample.c sample_tracepoint.h
>>          CFLAGS="$(CFLAGS) $(AM_CFLAGS)" \
>>          LDFLAGS="$(LDFLAGS) $(AM_LDFLAGS)" \
>>          CC="$(CC)" \
>> -       $(LTTNG_GEN_TP_PATH)lttng-gen-tp -o $@ $<
>> +       $(PYTHON) $(LTTNG_GEN_TP_PATH)lttng-gen-tp -o $@ $<
>>
>>   # The following rule can be used to generate all files instead of having
>> one
>>   # for each file type. Note that the sample.o has a dependency on the
>> @@ -55,10 +55,10 @@ sample.o: sample.c sample_tracepoint.h
>>   #      lttng-gen-tp $<
>>
>>   %.h: %.tp
>> -       $(LTTNG_GEN_TP_PATH)lttng-gen-tp -o $@ $<
>> +       $(PYTHON) $(LTTNG_GEN_TP_PATH)lttng-gen-tp -o $@ $<
>>
>>   %.c: %.tp
>> -       $(LTTNG_GEN_TP_PATH)lttng-gen-tp -o $@ $<
>> +       $(PYTHON) $(LTTNG_GEN_TP_PATH)lttng-gen-tp -o $@ $<
>>
>>   .PHONY: clean
>>   clean:
>>
>>
>> What do you think?
>>
>
> I agree that this is not the proper fix...
>
> Note that this script is not required to build lttng-ust. It will not
> be used during the build process if your host doesn't have a suitable
> Python version installed;

Uhm, that's not entirely correct. Unfortunately, the configure script 
will indeed try detect a suitable python version; however, that doesn't 
mean it will use THAT version (in my case, the one reachable through 
"python2.7") when needed (and that's the whole point of my email!).
I also tried enforcing detection of an older version (so to disable 
building of the examples) by using PYTHON=python before ./configure but 
I get:

checking whether python version is >= 2.7... no
configure: error: Python interpreter is too old

Some other attempts with am_* and ac_* variables led to similar results.
In the end I settled for

am_cv_pathless_PYTHON=none ./configure

which at least lets me continue and disable BUILD_GEN_TP_EXAMPLES 
without touching the code.

 > it is only used to build one of the
> examples.

Indeed.

>
> Regards,
> Jérémie

Thank you!
Gerlando