[Rd] file.access() on network (mounted) drive on Windows Vista?

[Henrik Bengtsson]

Hi,

I have a writable and readable file on a small network file system
(Cisco NSLU2 Unslung; non-NTFS) that I access via a mounted drive on
Windows Vista.  My problem could be due to a "funny" file
system/server, but here it goes:

> pathname <- "Q:/foo.txt"
> cat(file=pathname, "Hello world!\n")
> readLines(pathname)
[1] "Hello world!"
> file.info(pathname)
           size isdir mode               mtime               ctime
Q:/foo.txt   14 FALSE  666 2008-11-26 11:45:53 2008-11-26 11:45:53
                         atime exe
Q:/foo.txt 2008-11-26 11:45:57  no

The mode == 666 reported by file.info() indicates that it is readable
& writable by all users.  This is also what Windows Vista file
properties reports.  So far so good.  However, when I use
file.access() to test for file permissions, I get:

> file.access(pathname, 0)  # 0 test for existence.
Q:/foo.txt
         0
> file.access(pathname, 1) # 1 test for execute permission.
Q:/foo.txt
        -1
> file.access(pathname, 2) # 2 test for write permission.
Q:/foo.txt
        -1
> file.access(pathname, 4) # 4 test for read permission.
Q:/foo.txt
        -1

I obviously can write to and read from the file, and this is what
file.info()$mode says too.  However, file.access() tells a different
story.  More troubleshooting: When I log into the file server and do:

# chmod ugo-w foo.txt
# ls -l foo.txt
-r-xr-----    1 admin    everyone       14 Nov 26 11:48 foo.txt

The changes in permission are seen by file.info():

> file.info(pathname)
           size isdir mode               mtime               ctime
Q:/foo.txt   14 FALSE  444 2008-11-26 11:48:50 2008-11-26 11:48:50
                         atime exe
Q:/foo.txt 2008-11-26 11:56:40  no

The output from file.access() remains the same though.

>From help(file.info) I read:

     File modes are probably only useful on NTFS file systems, and it
     seems all three digits refer to the file's owner. The
     execute/search bits are set for directories, and for files based
     on their extensions (e.g., '.exe', '.com', '.cmd' and '.bat'
     files).  'file.access' will give a more reliable view of
     read/write access availability to the R process.

>From what I conclude, file.access() is not reliable in this case.  Is
this a feature or a bug?

I need a cross-platform test for file permissions, and I am looking
for safer workaround.  For instance, could it be that a zero result
from file.access() can be trusted, but a -1 could occur either from a
true lack of permission as well as a failure to test for the
permission?  If that would be case, I could try other measures (e.g.
try to open the file) whenever I receive a -1 before throwing an
exception.

Any feedback or suggestions would be great.

Thanks

/Henrik

> sessionInfo()
R version 2.8.0 Patched (2008-10-21 r46766)
i386-pc-mingw32

locale:
LC_COLLATE=English_United States.1252;LC_CTYPE=English_United States.1252;LC_MON
ETARY=English_United States.1252;LC_NUMERIC=C;LC_TIME=English_United States.1252


attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base