Windows path length limitations - gracefully handle ~260 char limit when present,

[jsirois]

Although the Python windows installer has a final page that allows you to click to enable long path support, you have to actually click. Worse, this assumes you install Python on the target machine running a PEX. You may not; most sharply, in a PEX scie case.

[jsirois]

Ok, so even on a system with long paths enabled, you get this:

>>> import os
>>> os.path.isdir(r"C:\Users\jsirois\AppData\Local\pex-tool.org\pex\Cache\pip\1\20.3.4-patched\pip.pex\cd68b6ca220083d5773ff357ed1c7e4cd1da0b40\no-extra-requirements\.deps\pip-20.3.4-py2.py3-none-any.whl")
>>> os.path.exists(r"C:\Users\jsirois\AppData\Local\pex-tool.org\pex\Cache\pip\1\20.3.4-patched\pip.pex\cd68b6ca220083d5773ff357ed1c7e4cd1da0b40\no-extra-requirements\.deps\pip-20.3.4-py2.py3-none-any.whl")
False
>>> os.path.islink(r"C:\Users\jsirois\AppData\Local\pex-tool.org\pex\Cache\pip\1\20.3.4-patched\pip.pex\cd68b6ca220083d5773ff357ed1c7e4cd1da0b40\no-extra-requireme
nts\.deps\pip-20.3.4-py2.py3-none-any.whl")
True
>>> os.readlink(r"C:\Users\jsirois\AppData\Local\pex-tool.org\pex\Cache\pip\1\20.3.4-patched\pip.pex\cd68b6ca220083d5773ff357ed1c7e4cd1da0b40\no-extra-requirements
\.deps\pip-20.3.4-py2.py3-none-any.whl")
'..\\..\\..\\..\\..\\..\\..\\installed_wheels\\0\\09de3454168e9999cc459c28a36ca0572466559839baa7cd17ddf7c830d5213b\\pip-20.3.4-py2.py3-none-any.whl'
>>> len(r"C:\Users\jsirois\AppData\Local\pex-tool.org\pex\Cache\pip\1\20.3.4-patched\pip.pex\cd68b6ca220083d5773ff357ed1c7e4cd1da0b40\no-extra-requirements\.deps\p
ip-20.3.4-py2.py3-none-any.whl")
183
>>> len('..\\..\\..\\..\\..\\..\\..\\installed_wheels\\0\\09de3454168e9999cc459c28a36ca0572466559839baa7cd17ddf7c830d5213b\\pip-20.3.4-py2.py3-none-any.whl')
136
>>> len(r"C:\Users\jsirois\AppData\Local\pex-tool.org\pex\Cache\pip\1\20.3.4-patched\pip.pex\cd68b6ca220083d5773ff357ed1c7e4cd1da0b40\no-extra-requirements\.deps\..\..\..\..\..\..\..\installed_wheels\0\09de3454168e9999cc459c28a36ca0572466559839baa7cd17ddf7c830d5213b\pip-20.3.4-py2.py3-none-any.whl")
288
>>> os.path.exists(r"C:\Users\jsirois\AppData\Local\pex-tool.org\pex\Cache\pip\1\20.3.4-patched\pip.pex\cd68b6ca220083d5773ff357ed1c7e4cd1da0b40\no-extra-requirements\.deps\..\..\..\..\..\..\..\installed_wheels\0\09de3454168e9999cc459c28a36ca0572466559839baa7cd17ddf7c830d5213b\pip-20.3.4-py2.py3-none-any.whl")
True
>>> os.path.isdir(r"C:\Users\jsirois\AppData\Local\pex-tool.org\pex\Cache\pip\1\20.3.4-patched\pip.pex\cd68b6ca220083d5773ff357ed1c7e4cd1da0b40\no-extra-requirements\.deps\..\..\..\..\..\..\..\installed_wheels\0\09de3454168e9999cc459c28a36ca0572466559839baa7cd17ddf7c830d5213b\pip-20.3.4-py2.py3-none-any.whl")
True
>>> os.path.isdir(os.path.normpath(r"C:\Users\jsirois\AppData\Local\pex-tool.org\pex\Cache\pip\1\20.3.4-patched\pip.pex\cd68b6ca220083d5773ff357ed1c7e4cd1da0b40\no-extra-requirements\.deps\pip-20.3.4-py2.py3-none-any.whl"))
False
>>> os.path.isdir(os.path.abspath(r"C:\Users\jsirois\AppData\Local\pex-tool.org\pex\Cache\pip\1\20.3.4-patched\pip.pex\cd68b6ca220083d5773ff357ed1c7e4cd1da0b40\no-extra-requirements\.deps\pip-20.3.4-py2.py3-none-any.whl"))
False
>>> os.path.isdir(os.path.realpath(r"C:\Users\jsirois\AppData\Local\pex-tool.org\pex\Cache\pip\1\20.3.4-patched\pip.pex\cd68b6ca220083d5773ff357ed1c7e4cd1da0b40\no-extra-requirements\.deps\pip-20.3.4-py2.py3-none-any.whl"))
True

So ... relative wheel symlinks in the PEX_ROOT will tend to run afoul of path length limitations in ~silent incorrect results for os.path.isXXX checks unless those paths are 1st resolved through symlinks with an os.path.realpath.