[ITK] [ITK-dev] ITK+static+wrapping [ref: 21857 "Enable singleton program global timestamp"]

[Matt McCormick]

Hi Isaiah,

Here is some more information and background.


> What is the rationale behind creating so many separate, statically-linked
> Python modules? (78 `_ITK*Python.so` files on my machine). Creating a single
> bundle instead should resolve the original issue, and might also reduce the
> size of the final library [3].

For background, the GlobalTimeStamp, is a very important variable in
ITK (and VTK). This is an atomic integer that gets incremented when
any itk::Object (most classes in ITK) is "Modified", i.e. changed to
note that it should be updated in a processing pipeline. So, almost
all ITK-based code needs it, and it needs to be global across the
entire program.

The GlobalTimeStamp lives in and is used my itk::TimeStamp.cxx, and it
is incremented on every call to ::Modified(). The itk::TimeStamp class
is linked into libITKCommon.

When ITK is built with shared libraries, libITKCommon is shared, and
everything shares the GlobalTimeStamp. When ITK is built with static
libraries, every binary that links in libITKCommon will get its own
GlobalTimeStamp.

Unfortunately, a shared libITKCommon is not always possible. For the
official Python packaging format, wheels, native libraries are not yet
officially supported [1].

The advantages of not linking everything into one, giant, monolithic
`_ITK*Python.so` are that 1) we can lazily load only the parts of the
library that we need, and 2) the brilliant ITK community members can
extend ITK to create their own ITK Python module. For example, see the
awesome ITKMinimalPathExtraction [2] or ITKIsotropicWavelets [3]
modules.

While I also investigated, for example, using CMake object libraries
to make itk::TimeStamp an exported symbol in _ITKCommonPython.so and
only linked there [4], this is a more complex build configuration.
Additionally, according to the Python documentation, exporting extra
symbols in a CPython extension module is officially not supported [5].
In practice, I have found that extra symbols can segfault the process
on module import with Python 3.5 on Windows. So, the sharing is done
the officially supported way, via PyCapsule.


> Additionally, I am wondering if cross-module `dynamic_cast` works correctly
> in the current situation -- because the typeinfos in each static module .so
> have different addresses [4]. (I'm not sure how to create a situation to
> test this from Python)

Yes, since the Python wrapping covers the entire toolkit, it is an
excellent stress test for the dynamic_cast issue. When we developed
the dynamic_cast patch, we used Python wrapping with static libraries
and experimentally enabled hidden symbols by default [6]. Without the
dynamic_cast fix, the Python tests failed, and with the fix, they
pass. It turns out that we missed some mark-up on a few classes as
indicated by warnings in the Python wrapping and non-wrapping test
failures, but we are working on resolving those now.

Thanks,
Matt


[1] https://mail.python.org/pipermail/wheel-builders/2016-April/000090.html

[2] https://github.com/InsightSoftwareConsortium/ITKMinimalPathExtraction

[3] https://github.com/phcerdan/ITKIsotropicWavelets

[4] https://github.com/thewtex/ITK/commits/thewtex-PythonModifiedTime55

[5] https://docs.python.org/3.5/extending/extending.html#providing-a-c-api-for-an-extension-module

[6] http://review.source.kitware.com/#/c/21766/
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