Improve termcap detection in setup.py

[pooryorick]

BPO	23284
Nosy	@bitdancer, @skrah, @koobs, @mjeveritt
Files	34d54cc5ecfd.diff

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2015-01-20.22:51:23.645>
labels = ['build']
title = 'Improve termcap detection in setup.py'
updated_at = <Date 2019-09-30.01:35:11.922>
user = 'https://bugs.python.org/pooryorick'

bugs.python.org fields:

activity = <Date 2019-09-30.01:35:11.922>
actor = 'veremitz'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Build']
creation = <Date 2015-01-20.22:51:23.645>
creator = 'pooryorick'
dependencies = []
files = ['37796']
hgrepos = ['292']
issue_num = 23284
keywords = ['patch']
message_count = 12.0
messages = ['234399', '234699', '234981', '234982', '234983', '234987', '234988', '234995', '235005', '235007', '235035', '235036']
nosy_count = 5.0
nosy_names = ['pooryorick', 'r.david.murray', 'skrah', 'koobs', 'veremitz']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue23284'
versions = ['Python 2.7']

[pooryorick]

Building Python-2.7.9 using --prefix, with an ncurses that's linked to libtinfo
and a readline that isn't linked to any termcap library, I ran into the trouble
that the curses module wan't buing build with the needed -L and -l flags for
the libtinfo shared object. I dug into setup.py, and ended up overhauling the
readline, curses, and also dbm handling (more on that in another ticket). I
also made changes to allow setup.py to pick up options like -isystem from
$CPPFLAGS, replacing in the process the "optparse" module with the "argparse"
module. Since argparse requires collections.OrderedDict, which isn't yet built
at this stage, I added Lib_boot/argparse_boot.py, which uses the pure-Python
OrderedDict from

https://pypi.python.org/pypi/ordereddict

and had setup.py use "argparse_boot" module instead.

The build also ran into trouble with system directories that setup.py
explicitly adds to inc_dirs and lib_dirs ahead of those of the alternate
prefix.

The attached files fixed all these issues in my scenario, allowing a succesful
build and install of Python-2.7.9.

[bitdancer]

At a quick glance this does not seem like a reasonable patch...introducing copies of two stdlib modules is basically a non-starter. Nor do I understand from the description what the problem is that it is trying to solve.

[pooryorick]

An attempt to build python-2.7.9 resulted in this error:

renaming "readline" since importing it failed ... undefined symbol: UP

This happened because setup.py chose "-lncursesw" as the readline terminal
library. Although libncursesw.so itself links to libtinfow.so, in order to
succeed with the "--as-needed" linker flag, setup.py would have to directly
choose libtinfow.so. The part of the patch that deals with that does not rely
on the other changes that were made to accomodate -isystem in CPPFLAGS.

[skrah]

What system are you using? The patch is quite large, and the existing libtinfo logic should work on most systems. Is the main problem that ldd is not found?

[pooryorick]

ldd was found. That patch does abstract and generalize ldd usage into a
function for reuse. I'm building Python as part of a rather large software
collection that lives in an alternate prefix. The main issue with the termcap
library is that with the --as-needed linker flag, which more modern build
systems prefer, linking to libncurses.so isn't sufficient, even when
libncurses.so itself links to libtinfo.so, but that's what setup.py decides to
do.

[pooryorick]

The wholesale inclusion of two additional files makes the patch look larger
than it is. The modifications to setup.py address three different issues. The
modifications for each issue don't overlap much, so they could be cherry-picked
fairly easily. For the readline issue, all that's needed is the new function,
"ldd_find_library_file" and then the changes from line 704 to about line 773
that deal directly with readline and ncurses.

[skrah]

I think even the curses changes alone are rather large, especially
for 2.7. Perhaps you could give us exact instructions how to
reproduce the issue (OS, version, build command line).

[pooryorick]

Larger changes represent more work on the part of the submitter ;)
The following should reproduce the readline problem:

Have the current setup.py detect as the readline termcap library a
libncurses.so that itself links against libtinfo.so. Without the --as-needed
flag, this is sufficient, since libtinfo ends up being indirectly linked to
readline.so, through libncurses.so. Not that it was ever the right approach,
but absent --as-needed, it happens to work.

Then, make sure the --as-needed flag is passed to the link command for the
readline.so module. In that case, even though there is a "-lncurses" argument,
the linke will not link libncurses.so to readline.so because readline.so
doesn't directly need any symbols provided by libncurses.so. The readline
module will then fail to load because terminal library symbols like UP will be
missing.

Without --as-needed, superfluous links between shared objects proliferate
through a software collection. --as-needed is becoming more common, so it
would be good if setup.py didn't break because of it.

[skrah]

I still don't understand why libncurses is linked against libtinfo but
libreadline is not. Again, what is your OS?

Example Ubuntu (a modern system):

$ ldd /lib/x86_64-linux-gnu/libncurses.so.5
 linux-vdso.so.1 => (0x00007fff5bfed000)
 libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007fcb8b43a000)
 libtinfo.so.5 => /lib/x86_64-linux-gnu/libtinfo.so.5 (0x00007fcb8b211000)
 libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fcb8ae4a000)
 /lib64/ld-linux-x86-64.so.2 (0x00007fcb8b87d000)

$ ldd /lib/x86_64-linux-gnu/libreadline.so.6
 linux-vdso.so.1 => (0x00007fff8c5fe000)
 libtinfo.so.5 => /lib/x86_64-linux-gnu/libtinfo.so.5 (0x00007f43055a0000)
 libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f43051da000)
 /lib64/ld-linux-x86-64.so.2 (0x00007f4305a2b000)

[pooryorick]

Ths OS is RHEL 6, but that's not so important because the entire collection,
including readline, is built from source and installed into an alternate
location. With the exception of a few essential shared objects from the
system, everything else is contained in the alternate location. Here's what
the readline INSTALL document says:

Readline uses the termcap functions, but does not link with the termcap or
curses library itself, allowing applications which link with readline 
to choose an appropriate library.

So if you don't monkey with the readline distribution, but build and install it
as-is, you get a libreadline.so that's not linked to a termcap library. That's
the situation for our software collection.

[skrah]

Ok thanks, I understand the issue now. I'm focusing this issue
on the termcap detection. For the other parts of the patch
you'd have to open separate issues.

As an aside, the ncurses maintainer seems to endorse a distro
enforced choice for libreadline's termcap:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=602720

[skrah]

FWIW, even http://www.linuxfromscratch.org/lfs/view/stable/chapter06/readline.html
enforces -ncurses linking of readline.

[They should be compiling ncurses with tinfo split out though and
use -tinfo instead.]