Detect homebrew python and fail

[sophiajt]

Currently, in OS X, if we find homebrew python we'll use it instead of the system python. Unfortunately, this seems to conflict with the lldb python module.

We don't warn the users that this is the case, and instead happily configure homebrew and promptly crash when the user tries to run any of the debuginfo-lldb tests. We should instead warn the user to not use python from homebrew but instead use the system python.

[alexcrichton]

Specifically, the error here (that I'm getting at least) is:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Applications/Xcode.app/Contents/SharedFrameworks/LLDB.framework/Resources/Python/lldb/__init__.py", line 98, in <module>
    import six
ImportError: No module named six
Fatal Python error: PyThreadState_Get: no current thread

(the major part being Fatal Python error: PyThreadState_Get: no current thread)

I personally have no idea what that error means, but I suspect it's because we're using a compiled plugin for lldb which only works against one version of python (presumably) and that version of python is likely the OSX system python.

We should detect this in either or both of the ./configure script and src/bootstrap/build/sanity.rs. Basically on OSX we should only use /usr/bin/python, not anything from /usr/local. An alternative is to run a smoke test to see if the found python works at all and then just error out if it fails.

cc @michaelwoerister, lldb weirdness!

[sophiajt]

I also tried installing the 'six' module manually, and still was unable to get past the crash with homebrew python.

[michaelwoerister]

We could also adapt compiletest to always use /usr/bin/python for the LLDB debuginfo tests? I'll look into it. Maybe LLDB even comes with its own Python version that we can fall back on.

[alexcrichton]

Yeah that may not be a bad idea to just hardcode that for OSX. #32994 worries me though and indicates that even that may be wrong, although it could also just be a problem with our script. If LLDB has its own Python version then we should probably definitely be using that!

[michaelwoerister]

Changing the version of Python from the one shipping with OSX is unsupported

from http://stackoverflow.com/a/19642204

[sanmai-NL]

LLDB does not appear to come with a Python distribution, and explicitly depends on ‘Python 3.5’. I assume they simply depend on the latest minor version of CPython 3.x. Given the proliferation of Python code in the Rust code base, perhaps shipping a minimal Python distribution with a known version that works well across all Python code may cause the least headache.

You can set PYTHON_HOME when building LLDB, see the previous link.

[sophiajt]

I'm pretty sure the trouble I was hitting with homebrew was with the 2.x python. It was the one installed as 'python' in /usr/local/bin. Likewise, system Python is 2.7.10 for OS X 10.11.4. I haven't see it use 3.x, yet, though maybe I'm looking in the wrong place?

[alexcrichton]

I believe we since fixed this by hardcoding that the system python is used for LLDB tests, so I'm gonna close.