[Patches] [ python-Patches-1454844 ] Use dlopen() to load extensions on Darwin, where possible

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Patches item #1454844, was opened at 2006-03-20 19:41
Message generated for change (Comment added) made by zpincus
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Category: Build
Group: Python 2.5
Status: Open
Resolution: None
Priority: 5
Submitted By: Zach Pincus (zpincus)
Assigned to: Nobody/Anonymous (nobody)
Summary: Use dlopen() to load extensions on Darwin, where possible

Initial Comment:
Here is a patch to make Python on OS X 10.3 and above use dlopen() 
(via dynload_shlib.c) to load extension modules, and to make the dl 
module avaliable. 

The patch was generated against the SVN head as of yesterday, but can 
be applied (and I have done so with success) to Python 2.4.2.

RATIONALE
---------
On most unix-like operating systems, Python uses the dlopen() call to 
load extension modules. In addition, the way that these modules are 
opened can be modified via 'sys.setdlopenflags(...)'.

Modifications of how extensions are loaded are useful for several 
reasons (enough so that the standard Python docs (e.g. http://
docs.python.org/lib/module-sys.html ). In particular, if multiple 
modules need to share symbols, a call to sys.setdlopenflags is 
necessary.

Symbol sharing is especially important for interoperability of modules 
which wrap C++ classes, because GCC creates classes that resolve 
their run-time type identification by symbol identity. Thus, symbols 
must be shared globally for many C++ features to work properly for 
objects passed between modules.

On OS X / Darwin, Python uses some NeXT-derived APIs to load 
modules. Though these APIs provide analogues to the dlopenflags 
used to control how dlopen() loads modules, this interface is *not* 
exposed to the Python interpreter. Worse, sys.setdlopenflags remains 
available on Darwin, though calls to it are never heeded.

Fortunately, on OS X 10.3 and above, Apple has included dlopen as a 
standard function. In 10.3, this call is provided by a compatibility API; 
in 10.4, the dlopen() call is written to interface directly with the library 
loading mechanism and is now the recommended method for libraries 
to be opened for non Carbon/Cocoa tools.

IMPLEMENTATION
--------------
This (trivial) patch instructs the Python build process to use 
dynload_shlib.c (which uses dlopen) instead of dynload_next.c (which 
uses the NeXT-derived APIs). It also allows for the dl module to be built 
in order to provide access to the proper values for the various dlopen 
flags.

TESTING
-------
This patch can be configured and built into executables that build and 
test correctly on 10.3 and 10.4.

Because Python 2.5 and 2.4 do not currently compile properly on OS X 
10.2, I have not built or tested this patch on that OS version. However, 
the configure and compile process does select the appropriate 
dynload_next.c file to use, and compiles that correctly before breaking 
elsewhere. Thus, if the other errors are fixed for 10.2, these patches 
will work fine. (This is because they only change Python's behavior for 
10.3 and up.)

PATCHES
-------
There are three main components to the attached patch.

The first is a patch the 'configure.in' file to use dynload_shlib.c when it 
can, and a patch to the 'configure' file to sync it up with 'configure.in'. 

The second is a minor change to 'setup.py' and the dl module test to 
allow the dl module to be built and tested on OS X systems where dlfcn 
is available. (10.3 and above.)

The last part of the patchfile should be considered optional. This patch 
applies to 'Lib/test/regrtest.py', and it tells the testing suite that the dl 
test is not expected to be skipped anymore.

This is optional because if Python is ever built on 10.2, the test script 
will expect dl to work, when it only works on 10.3 and above. However, 
if Python on 10.2 is officially not supported, then this change should 
be made to properly test the dl functionality on all supported OS X 
platforms.



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>Comment By: Zach Pincus (zpincus)
Date: 2006-03-27 18:40

Message:
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Ronald -
I am unfortunately not an expert on windows DLL loading. I do know that in 
non-GCC C++ implementations, it becomes far less critical to share symbols 
globally in many circumstances, because non-GCC C++ implementations 
tend to use string comparisons to resolve C++ object identity, and not use 
exact address comparisons on the typeinfo objects. Thus, these objects do 
not need to be shared between windows DLLs.

Beyond that, I do not know whether symbols from DLLs are loaded with 
global visibility in windows or not. Certainly the windows dlopen() equivalent 
*does not* support mode options. I presume anything tagged with _declspec
(dllexport) is loaded globally when the DLL is opened, but I am not sure.

However, that issue is largely tangential to the question of whether Python on 
OS X should work like Python on every other Unix-like system...

As to your other question -- have I checked if this will break existing python 
code -- the answer is of course yes. The python regression suite test passes 
on both 10.3 and 10.4 with this modification (as I mentioned), so that's a 
good indication that run-of-the-mill code will not be affected.

In fact, on 10.3, the dlopen() call is implemented under the hood with the 
exact same calls that Python's dynload_next.c file uses. In 10.4 this was 
changed so that both APIs talk to the same underlying system. Thus, nothing 
is really changing: both are functionally identical ways of getting the same OS 
X runtime loader (dyld) to load a bundle from disk. The only difference is that 
by using dlopen(), there are hooks in Python for manipulating the mode that 
is used to load that bundle from disk.

So, I cannot imagine a situation where this change would break code on the 
mac. While argument from failure of imagination is typically weak, this 
combined with the fact that the python tests pass without difficulty (including 
the dl module test), I am fairy confident in this patch. Moreover, I have been 
using this patch on my own computer for some time and have not run into 
any difficulties, even when loading fairly complex custom extensions.

Finally, this patch *only* modifies the codepath on Darwin/OS X, so there's 
zero possibility that it would affect anything on other platforms.

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Comment By: Ronald Oussoren (ronaldoussoren)
Date: 2006-03-27 17:25

Message:
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I have a number of questions. Firstly, how does symbol sharing between 
extensions work on windows? And more importantly, have you checked if this 
will break existing python code?



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Comment By: Zach Pincus (zpincus)
Date: 2006-03-22 08:34

Message:
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I just uploaded a new patch which addresses this issue, and fixes this same 
problem in a few other locations in the configure script.

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Comment By: Andrew Barnert (barnert)
Date: 2006-03-22 02:31

Message:
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One minor issue: Your pattern will match Darwin 10.0, which will come out in 
a few years as part of OS X 10.7, by which time nobody will remember why 
this script is written this way. Since it's easy to handle now, we might as well 
do so. On a side note, there are no 2.x-4.x versions (1.5 was renamed 5.1), 
so anyone who matches Darwin/2.* will be a false match.

Here's what I'd suggest:

+       Darwin/[0156]\..*) DYNLOADFILE="dynload_next.o";;

Meanwhile, your patch seems to work for me on an Intel iMac, but I haven't 
tried anything too sophisticated.


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