boost_python suffix issues

[mikepurvis]

Somewhat related to #21, but Boost 1.71 was just released into the repo for Ubuntu Focal LTS, and the story for boost_python is not great. From the perspective of an LTS user, over the years there have been five ways of linking to boost_python from CMake for a given Python X.Y:

1. find_package(Boost COMPONENTS python)
2. find_package(Boost COMPONENTS python-pyXY)
3. find_package(Boost COMPONENTS pythonX)
4. find_package(Boost COMPONENTS pythonX-pyXY)
5. find_package(Boost COMPONENTS pythonXY)

Xenial's Boost (1.58) supported 1 and 2. Bionic's Boost (1.65.1) supported 1, 2, 3, and 4. Focal's Boost (currently 1.71) supports only 1 and 5. But using 1 on Focal gets you Python 3 whereas previously 1 got you Python 2. There is no one invocation that consistently gets you the Python 3 bindings.

My appeal is for Boost to continue supporting the python3 and numpy3 component names, at least long enough for any software that wants to support multiple LTS releases from a single branch. Particularly when you consider that Boost_VERSION has now been changed so that it is no longer usable for checks against systems old enough to not yet have Boost_VERSION_MACRO (for example, Ubuntu Bionic, where FindBoost is supplied by CMake 1.10).

Launchpad ticket for the same issue: https://bugs.launchpad.net/ubuntu/+source/boost1.71/+bug/1865209

[pdimov]

I can't do anything about Boost 1.71, it has already been released (as was 1.72). I can make changes for 1.73, if the 1.72 behavior is problematic.

[mikepurvis]

If you're willing to change 1.73 to support the 3 suffix for python components, it may be possible to have the Ubuntu Boost maintainer patch that change on to the version of 1.71 in the release.

[pdimov]

What are the actual XY versions on Focal?

[mikepurvis]

Python 3.8 is the default: https://packages.ubuntu.com/source/focal/python3-defaults

Python 2.7.17 is available in universe (unsupported): https://packages.ubuntu.com/source/focal/python-defaults

But for the purposes of the Boost change, wouldn't you just be aliasing python3 to do whatever python would have done, since python is now Python 3 anyway? Or would you want an additional check to ensure that someone who built Boost from source with Py2 support enabled couldn't accidentally link themselves to libboost_python27 when specifying python3?

[pdimov]

It's a bit more involved. The python component, if more than one Python version variant is present, picks up one at random, unless Boost_PYTHON_VERSION is set. The format of Boost_PYTHON_VERSION is f.ex. 2.7 or 3.8.

For compatibility with FindBoost, components such as pythonXY are recognized and treated as if the component python is requested with Boost_PYTHON_VERSION set to X.Y.

The logical behavior for the component python3 (and python2 for symmetry) would be for it to pick up one of the 3.x versions (but not the 2.x ones.) But this doesn't fit well within the current logic.

I'll try to think of something.

[pdimov]

I've implemented support for the python3 and python2 components in 7d2b3bf. This change will go into the 1.73 release. I'm not 100% sure whether the patch will apply cleanly to 1.71; there were a lot of CMake config fixes in 1.72, but I think the Python part didn't change much, so it might work.

[mikepurvis]

Terrific, thanks! I'll look into the backporting story.

[PPeltzer]

Has this actually been backported to 1.71? I'm running into some issues similar to that on Ubuntu 20.04 with the shipped Boost 1.71 (see above)

[mclow]

Has this actually been backported to 1.71?

In general, we do not make updated versions of old releases.
Use 1.73 (or later), or apply the patch to your local copy of 1.71.

[mikepurvis]

@PPeltzer I wasn't able to get anything done on the Ubuntu side unfortunately— as far as calling in favours, there were more pressing issues with gnuarm and eigen3 that I really needed addressed, so we basically just patched our CMakeList files to deal with it.