ModuleNotFoundError when running Python

[dynamite-bud]

Describe the bug

The problem is that the default current directory is /. Python gets its standard library from the files embedded in the python/python package, which are initially mounted in their default spot. When mounting the host's current directory to the guest's current directory, we end up clobbering the existing filesystem and core modules like encodings will no longer be available.

Steps to reproduce

$ wasmer run python/python --mapdir=.:. test.py
Could not find platform dependent libraries <exec_prefix>
Consider setting $PYTHONHOME to <prefix>[:<exec_prefix>]
Fatal Python error: Py_Initialize: Unable to get the locale encoding
ModuleNotFoundError: No module named 'encodings'

Current thread 0x00000000 (most recent call first):
error: RuntimeError: unreachable
    at <unnamed> (<module>[4661]:0x224edf)
    at <unnamed> (<module>[2033]:0xce8da)
    at <unnamed> (<module>[2030]:0xcde7a)
    at <unnamed> (<module>[2028]:0xcd9d0)
    at <unnamed> (<module>[2037]:0xcef38)
    at <unnamed> (<module>[43]:0x4743)
    at <unnamed> (<module>[42]:0x3fec)
    at <unnamed> (<module>[4709]:0x22985e)
    at <unnamed> (<module>[30]:0x3e58)

Additional context

The issue isn't specific to the python wapm package

[dynamite-bud]

The fix (from @Michael-F-Bryan ) is to mount the current directory on the host to a different directory in the guest (i.e. run wasmer run python/python --mapdir=/app:. /app/test.py).

[dynamite-bud]

We can mitigate the user experience impact. some ideas from @waynr :

1. Wasmer could warn users when they have overlapping directory mappings
2. Wasmer could refuse to overlap directory mappings (especially the environment's root directory) without some kind of --force-overlapped-mappings flag
3. Reconsider for a future breaking change using a different default working directory