FatFs' VFS unlink implementation does not adhere to POSIX (IDFGH-14405)

[Aric5301]

Answers checklist.

• I have read the documentation ESP-IDF Programming Guide and the issue is not addressed there.
• I have updated my IDF branch (master or release) to the latest version and checked that the issue is present there.
• I have searched the issue tracker for a similar issue and not found a similar issue.

General issue report

Hi.
I've discovered that VFS' implementation of the FatFs file system uses FatFs' implementation of the unlink function as the unlink function implementation.
The problem is that FatFs' unlink function does not support removing a currently open file.
This is contrary to the definition of the POSIX unlink function, which allows it (this is the reason why it's called unlink).

I wasn't able to find any documentation for this discrepancy in ESP-IDF Programming Guide.
I think that it should be either properly documented (like the documentation added for explaining how the link function actually copies a file's content instead of just adding a second link), or it should be resolved in code by writing some special implementation of the unlink function.

Thanks!