[PATCH] Fix shared library loading in Python bindings.

Thu May 30 09:06:42 PDT 2013

On Thu, May 30 2013, Justus Winter <4winter at informatik.uni-hamburg.de> wrote:

> Hi Julian :)
>
> Quoting Julian Berman (2013-05-26 05:53:38)
>> Specifically, fixes loading on OS X, where libnotmuch will be
>> a dylib.:
>> ---
>>  bindings/python/notmuch/globals.py | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>> 
>> diff --git a/bindings/python/notmuch/globals.py b/bindings/python/notmuch/globals.py
>> index c7632c3..5e08e73 100644
>> --- a/bindings/python/notmuch/globals.py
>> +++ b/bindings/python/notmuch/globals.py
>> @@ -18,11 +18,12 @@ Copyright 2010 Sebastian Spaeth <Sebastian at SSpaeth.de>
>>  """
>>  
>>  from ctypes import CDLL, Structure, POINTER
>> +from ctypes.util import find_library
>>  
>>  #-----------------------------------------------------------------------------
>>  #package-global instance of the notmuch library
>>  try:
>> -    nmlib = CDLL("libnotmuch.so.3")
>> +    nmlib = CDLL(find_library("libnotmuch"))
>
> Does this work for you on Darwin? On my box (Debian/Linux) I have to
> use "notmuch" instead of "libnotmuch" to get anything from
> find_library:
>
> In [5]: print ctypes.util.find_library("notmuch")
> libnotmuch.so.3
>
> In [6]: print ctypes.util.find_library("libnotmuch")
> None
>
> Then again, find_library is different for every architecture under the
> sun...
>
>>  except:
>>      raise ImportError("Could not find shared 'notmuch' library.")
>
> I'm leaning towards declining the patch in it's current form. If the
> bindings do not work on OS X, we need to find another solution. There
> are two reasons for this:
>
> 1. find_library was once used, but was removed since it is (has?) been
> problematic wrt to LD_LIBRARY_PATH usage:
>
> % git show f378f458
> commit f378f45893bb4263402008b2abd8aab522901fb6
> Author: Cedric Cabessa <ced at ryick.net>
> Date:   Mon Apr 5 03:03:51 2010 +0200
>
>     find_library does not read LD_LIBRARY_PATH, but CDLL does.
>
> diff --git a/cnotmuch/globals.py b/cnotmuch/globals.py
> index ef2686f..fa20ae8 100644
> --- a/cnotmuch/globals.py
> +++ b/cnotmuch/globals.py
> @@ -3,17 +3,17 @@ from ctypes.util import find_library
>  
>  #-----------------------------------------------------------------------------
>  #package-global instance of the notmuch library
> -#TODO: lazy load this on first access?
> -so = find_library('notmuch')
> -if so is None:
> -  raise ImportError("Could not find shared 'notmuch' library.")
> -nmlib = CDLL(so)
> +try:
> +    nmlib = CDLL("libnotmuch.so.1")
> +except:
> +    raise ImportError("Could not find shared 'notmuch' library.")
> [...]
>
> As a heavy LD_LIBRARY_PATH user I don't want to break this. So I tried
> to test whether find_library now respects LD_LIBRARY_PATH or not:
>
> teythoon at thinkbox ~ % echo $LD_LIBRARY_PATH
> /home/teythoon/.local/lib
> teythoon at thinkbox ~ % strace -e trace=open python -c 'import ctypes; ctypes.CDLL("libnotmuch.so.3")' 2>&1 | grep notmuch
> open("/home/teythoon/.local/lib/libnotmuch.so.3", O_RDONLY) = 3
>
> That's how it's done now and indeed it finds my libnotmuch.
>
> teythoon at thinkbox ~ % strace -f -e trace=stat,open python -c 'import ctypes.util; ctypes.util.find_library("notmuch")' 2>&1 | grep notmuch
>
> Nothing. Funny. Let's see:
>
> teythoon at thinkbox ~ % strace -f -e trace=fork,execve,clone python -c 'import ctypes.util; print ctypes.util.find_library("notmuch")'
> execve("/usr/bin/python", ["python", "-c", "import ctypes.util; print ctypes"...], [/* 63 vars */]) = 0
> clone(Process 12000 attached
> child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f0734a649d0) = 12000
> [pid 12000] execve("/bin/sh", ["sh", "-c", "/sbin/ldconfig -p 2>/dev/null"], [/* 63 vars */]) = 0
> [pid 12000] clone(Process 12001 attached
> child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f119d5479d0) = 12001
> [pid 12001] execve("/sbin/ldconfig", ["/sbin/ldconfig", "-p"], [/* 63 vars */]) = 0
> Process 12001 detached
> [pid 12000] --- SIGCHLD (Child exited) @ 0 (0) ---
> Process 12000 detached
> --- SIGCHLD (Child exited) @ 0 (0) ---
> libnotmuch.so.3
>
> So it also prints libnotmuch.so.3, but only because the version
> installed from the Debian archive is also libnotmuch.so.3:
>
> teythoon at thinkbox ~ % /sbin/ldconfig -p | grep notmuch
>         libnotmuch.so.3 (libc6,x86-64) => /usr/lib/libnotmuch.so.3
>
> So I guess *if* I had a libnotmuch.so.2 in my ldconfig(8) cache (-p
> prints the cache), find_library would have returned libnotmuch.so.2
> and not my libnotmuch.so.3. So your patch would most likely break this
> kind of setup.
>
> 2. Uh. I actually looked at /usr/lib/python2.7/ctypes/util.py and I
> almost got eye cancer from just looking briefly at it... The
> implementation of find_library differs for various operating systems
> and most (all?) of them use fork/exec some program (even gcc in some
> cases) to look up the libraries. I'd like to avoid this if possible,
> maybe even if this means handling OS X as a special case in our
> bindings. For reference, our current solution does not require any
> fork/exec calls:
>
> teythoon at thinkbox ~ % strace -f -e trace=execve,fork,clone python -c 'import ctypes; ctypes.CDLL("libnotmuch.so.3")' 
> execve("/usr/bin/python", ["python", "-c", "import ctypes; ctypes.CDLL(\"libn"...], [/* 63 vars */]) = 0
>
> Thoughts anyone?

Tried this:

>>> from ctypes import CDLL
>>> x = CDLL("foobnar.so")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.6/ctypes/__init__.py", line 353, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: foobnar.so: cannot open shared object file: No such file or
directory

Looked into /usr/lib/python2.6/ctypes/__init__.py -- _dlopen() uses
dlopen() from _ctypes module, which is in 
/usr/lib/python2.6/lib-dynload/_ctypes.so

I'd guess this does pretty much the same as dlopen(3) does in Linux
(or Mac -- whatever it does there)

Could something like

try:
    nmlib = CDLL("libnotmuch.so.3")
except OSError:
    try:
        nmlib = CDLL("something_macosx_understands")
    except:
        ImportError("Could not find shared 'notmuch' library.")

be used ?

Just before sending this email I came up with this page:

https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man3/dlopen.3.html

The 'SEARCHING' section has quite a few words how the file is found.

> Cheers,
> Justus

Tomi