I am sometimes using shutil.copytree to copy a directory to a destination that does not support setting metadata (like MTP mounts of Android devices).

Using the copy_function parameter allows passing shutil.copy or a custom function to ignore file metadata, however currently shutil.copytree always tries to call copystat on directories, which can fail with OSError (errno set to ENOTSUPP).

The code assumes copystat can fail on Windows, but propagates the error on other OS, even though the tree copy actually succeeds.
See https://github.com/python/cpython/blob/9bb6fe52742340f6c92f0dda18599a4577a94e18/Lib/shutil.py#L352-L357

Ping

What function raises ENOTSUPP exactly (traceback would be welcome)?

Traceback is not very useful in that case:

mkdir /tmp/a
touch /tmp/a/b 

python
Python 3.6.6 (default, Jun 27 2018, 13:11:40) 
[GCC 8.1.1 20180531] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import os, shutil
>>> os.getcwd()
'/run/user/1000/gvfs/mtp:host=%5Busb%3A001%2C006%5D/sdcard1'
>>> shutil.copytree("/tmp/a", os.path.join(os.getcwd(), "test"))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/desbma/py-venvs/main/lib/python3.6/shutil.py", line 359, in copytree
    raise Error(errors)
shutil.Error: [('/tmp/a/b', '/run/user/1000/gvfs/mtp:host=%5Busb%3A001%2C006%5D/sdcard1/test/b', '[Errno 95] Operation not supported'), ('/tmp/a', '/run/user/1000/gvfs/mtp:host=%5Busb%3A001%2C006%5D/sdcard1/test', '[Errno 95] Operation not supported')]
>>> shutil.copytree("/tmp/a", os.path.join(os.getcwd(), "test2"), copy_function=shutil.copy)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/desbma/py-venvs/main/lib/python3.6/shutil.py", line 359, in copytree
    raise Error(errors)
shutil.Error: [('/tmp/a/b', '/run/user/1000/gvfs/mtp:host=%5Busb%3A001%2C006%5D/sdcard1/test2/b', "[Errno 95] Operation not supported: '/run/user/1000/gvfs/mtp:host=%5Busb%3A001%2C006%5D/sdcard1/test2/b'"), ('/tmp/a', '/run/user/1000/gvfs/mtp:host=%5Busb%3A001%2C006%5D/sdcard1/test2', '[Errno 95] Operation not supported')]

The exception is thrown from here https://github.com/python/cpython/blob/9bb6fe52742340f6c92f0dda18599a4577a94e18/Lib/shutil.py#L359

Note that in the examples above both copytree calls actually succeed (only metadata copy failed).

The user can disable file metadata copy by passing 'copy_function=shutil.copy', but there is no way to do the same for directories and the directory copystat call https://github.com/python/cpython/blob/9bb6fe52742340f6c92f0dda18599a4577a94e18/Lib/shutil.py#L353 fails.

It's not clear where the exception originates from. Try to use copy2() instead of copytree().

copy2 always raises "OSError: [Errno 95] Operation not supported" here https://github.com/python/cpython/blob/9bb6fe52742340f6c92f0dda18599a4577a94e18/Lib/shutil.py#L258 but I can work around that by passing copy_function=shutil.copy to copytree as I did above.

The problem is that there is currently no way to avoid the copystat call made on the directory itself.