[lldb/docs] Clarify python/swig version incompatibility

[labath]

Summary

The problematic combo is a debug python>=3.7 && swig<4.0.

[labath]

Jonas, the notion that 3.7 is incompatible comes from ae7389116. Do you happen to remember what was the issue you ran into?

I find it unlikely that 3.7 is incompatible but 3.8 is ok, and I think you probably ran into https://github.com/swig/swig/issues/1321 (which I recently ran into as well).

[tatyana-krasnukha]

I used Swig 3.0.12 + cherry-picked commit that fixes that bug (https://github.com/swig/swig/issues/1321) on Windows. It worked well with both Python 3.7 and 3.8.

The only issue I still have with Python 3.8 and later is importing liblldb.dll from a python script (https://llvm.org/pr46891) while running the test-suite.

[JDevlieghere]

In D88906#2314615, @labath wrote:

Jonas, the notion that 3.7 is incompatible comes from ae7389116. Do you happen to remember what was the issue you ran into?

I find it unlikely that 3.7 is incompatible but 3.8 is ok, and I think you probably ran into https://github.com/swig/swig/issues/1321 (which I recently ran into as well).

That's very possible. I don't remember the details, but when I used 3.7 on Windows all the tests were failing. I asked someone (it might have been @compnerd or @stella.stamenova) who told me it was a known issue.

lldb/docs/resources/build.rst

81

The rest of this page seems to be using LLDB rather than lldb. I have no preference as long as things are consistent.

[amccarth]

Yes, I also (independently) discovered this problem. I probably have notes somewhere with sources for details about the incompatibility. I believe I also brought it up on the lldb-dev list.

[labath]

In D88906#2314777, @amccarth wrote:

Yes, I also (independently) discovered this problem.

Yeah, that seems to be a fairly common problem. I'm gonna try to add some kind of a cmake warning about this.

lldb/docs/resources/build.rst

81

sounds good.

lldb/docs/resources/build.rst

 ::
 
   > yum install libedit-devel libxml2-devel ncurses-devel python-devel swig
   > sudo apt-get install build-essential subversion swig python3-dev libedit-dev libncurses5-dev
   > pkg install swig python
   > pkgin install swig python27 cmake ninja-build
   > brew install swig cmake ninja
 
+Note that there's an `incompatibility
+<https://github.com/swig/swig/issues/1321>` between Python version 3.7 and later
+and swig versions older than 4.0.0 which makes builds of LLDB using debug
+versions of python unusable. This primarily affects Windows, as debug builds of
+LLDB must use debug python as well.
+
 Windows
 *******
 
 * Visual Studio 2017.
 * The latest Windows SDK.
 * The Active Template Library (ATL).
 * `GnuWin32 <http://gnuwin32.sourceforge.net/>`_ for CoreUtils and Make.
-* `Python 3.6 or 3.8 <https://www.python.org/downloads/windows/>`_. Python 3.7
-  is known to be incompatible. Make sure to (1) get the x64 variant if that's
-  what you're targetting and (2) install the debug library if you want to build
-  a debug lldb.
+* `Python 3 <https://www.python.org/downloads/windows/>`_.  Make sure to (1) get
+  the x64 variant if that's what you're targetting and (2) install the debug
+  library if you want to build a debug lldb.
 * `Python Tools for Visual Studio
   <https://github.com/Microsoft/PTVS/releases>`_. If you plan to debug test
   failures or even write new tests at all, PTVS is an indispensable debugging
   extension to VS that enables full editing and debugging support for Python
   (including mixed native/managed debugging).
 
 The steps outlined here describes how to set up your system and install the
 required dependencies such that they can be found when needed during the build