[Rd] Bug in file.access on Windows when using network shares

[Nick Kennedy]

Dear R-Devel,

I've run into an issue with a package (vcfR) that uses file.access to check
a file is readable before opening it. The issue is actually in base R
though. I've looked into the package code, and it calls file.access(path,
mode = 4). I've created a minimal working example of the code in winAccessW
from src/gnuwin32/extra.c, and the problem arises when GetFileSecurityW is
called on shared files under certain circumstances.

One situation I've seen it in are when a file is shared from a non-Windows
host (e.g. Linux), which is similar to the situation documented at
https://social.msdn.microsoft.com/Forums/sqlserver/en-US/f57928d3-d89b-426d-a174-d06d97355afc/how-to-check-if-a-filefolder-is-writable-or-not?forum=windowssdk
.

The other situation arises when a file is cached offline by Windows Offline
files feature. The call to GetFileSecurityW works fine when the network is
up (and so the file is being accessed from the share), but fails when the
network is down and the file is being accessed from the offline files cache.

Is there any reason that there is a custom function here? Windows supports
the use of access (as is used on other OSes), although the ISO C++ _waccess
would be preferred. This seems to work well even in situations where the
current code does not.

BW

Nick

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