Magic Folder on Windows: ignore files with hidden or system attributes

[daira]

The ​Magic Folder remote-to-local-sync specification says:

"Hidden" files, and files with names matching the patterns used for backup, temporary, and conflicted files, will be ignored, i.e. not synchronized in either direction. A file is hidden if it has a filename beginning with "." (on any platform), or has the hidden or system attribute on Windows.

Checking for the hidden or system attribute is not currently implemented.

[daira]

This is not a blocker for merging Magic Folder to trunk.

[daira]

​https://bugs.python.org/issue21719 says that there is no Python 2.7 API for this. Therefore, a ctypes call to either ​GetFileAttributesW or ​GetFileAttributesEx is needed. Note that we already lstat the file in get_pathinfo before deciding whether to upload it, but I guess that determining whether it is hidden/system does not need to be atomic with that, therefore the simpler GetFileAttributesW API would be sufficient. The ctypes call should be implemented in fileutil.py, either as part of get_pathinfo or separately.

[daira]

I suggest implementing a fileutil.is_hidden_or_system function that always returns False on non-Windows platforms.

[exarkun]

magic-folder has been split out into a separate project - ​https://github.com/leastauthority/magic-folder