Dear Nick,
thank you for your report. In general one cannot know reliably in
advance whether reading a file will work, if nothing else, there are
possible race conditions with other applications and/or the system (e.g.
a service may lock the file, move it away temporarily, etc). The only
correct way to handle errors is try the read and catch errors. Please
note there is a disclaimer in ?file.access to this effect.
If you still want to report a bug in file.access, please provide an
example that does not require a package, and provide more details (what
do you think would be the correct behavior, etc). From looking at the
code, when winAccessW/GetFileSecurityW fails to retrieve information
about the file, file.access would signal an error, which seems to be a
fine behavior for me. It is not surprising to me that in some cases
(like working on a local copy of a file in a distributed file-system),
the OS would not know whether a file is readable/writeable before trying
out on/syncing the distributed version.
Best
Tomas
On 07/03/2018 01:11 AM, Nick Kennedy wrote:
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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