[Rd] NEWS item for bugfix in normalizePath and file.exists?

[Tomas Kalibera]

On 4/28/21 5:22 PM, Martin Maechler wrote:
>>>>>> Toby Hocking
>>>>>>      on Wed, 28 Apr 2021 07:21:05 -0700 writes:
>      > Hi Tomas, thanks for the thoughtful reply. That makes sense about the
>      > problems with C locale on windows. Actually I did not choose to use C
>      > locale, but instead it was invoked automatically during a package check.
>      > To be clear, I do NOT have a file with that name, but I do want file.exists
>      > to return a reasonable value, FALSE (with no error). If that behavior is
>      > unspecified, then should I use something like tryCatch(file.exists(x),
>      > error=function(e)FALSE) instead of assuming that file.exists will always
>      > return a logical vector without error? For my particular application that
>      > work-around should probably be sufficient, but one may imagine a situation
>      > where you want to do
>
>      > x <- "\360\237\247\222\n| \360\237\247\222\360\237\217\273\n|
>      > \360\237\247\222\360\237\217\274\n| \360\237\247\222\360\237\217\275\n|
>      > \360\237\247\222\360\237\217\276\n| \360\237\247\222\360\237\217\277\n"
>      > Encoding(x) <- "unknown"
>      > Sys.setlocale(locale="C")
>      > f <- tempfile()
>      > cat("", file = f)
>      > two <- c(x, f)
>      > file.exists(two)
>
>      > and in that case the correct response from R, in my opinion, would be
>      > c(FALSE, TRUE) -- not an error.
>      > Toby
>
> Indeed, thanks a lot to Tomas!
>
> # A remark
> We *could* -- and according to my taste should -- try to have file.exists()
> return a logical vector in almost all cases, namely, e.g., still give an
> error for file.exists(pi) :
> Notably  if  `c(...)`  {for the  `...`  arguments of file.exists() }
> is a character vector, always return a logical vector of the same
> length, *and* we could notably make use of the fact that R's
> logical type is not binary but ternary, and hence that return
> value could contain values from {TRUE, NA, FALSE}  and interpret NA
> as "don't know" in all cases where the corresponding string in
> the input had an Encoding(.) that was "fishy" in some sense
> given the "context" (OS, locale, OS_version, ICU-presence, ...).
>
> In particular, when the underlying code sees encoding-translation issues
> for a string,  NA  would be returned instead of an error.

Yes, I agree with Toby and you that there is benefit in allowing 
per-element, vectorized use of file.exists(), and well it is the case 
now, we just fall back to FALSE. NA might be be better in case of error 
that prevents the function from deciding whether the file exists or not 
(e.g. an invalid name in form that make is clear such file cannot exist 
might be a different case...).

But, the only way to get a translation error is by passing a string to 
file.exists() which is invalid in its declared encoding (or which is in 
"C" encoding). I would hope that we could get to the point where such 
situation is prevented (we only allow creation of strings that can be 
translated to Unicode). If we get there, the example would fail with 
error (yet, right, before getting to file.exists()).

My point that I would not write tests of this behavior stands. One 
should not use such file names, and after the change Toby reported from 
ERROR to FALSE, Martin's proposal would change to NA, mine eventually to 
ERROR, etc. So it is best for now to leave it unspecified and not 
trigger it, I think.

Tomas

>
> Martin
>
>      > On Wed, Apr 28, 2021 at 3:10 AM Tomas Kalibera <tomas.kalibera using gmail.com>
>      > wrote:
>
>      >> Hi Toby,
>      >>
>      >> a defensive, portable approach would be to use only file names regarded
>      >> portable by POSIX, so characters including ASCII letters, digits,
>      >> underscore, dot, hyphen (but hyphen should not be the first character).
>      >> That would always work on all systems and this is what I would use.
>      >>
>      >> Individual operating systems and file systems and their configurations
>      >> differ in which additional characters they support and how. On some,
>      >> file names are just sequences of bytes, on some, they have to be valid
>      >> strings in certain encoding (and then with certain exceptions).
>      >>
>      >> On Windows, file names are at the lowest level in UTF-16LE encoding (and
>      >> admitting unpaired surrogates for historical reasons). R stores strings
>      >> in other encodings (UTF-8, native, Latin-1), so file names have to be
>      >> translated to/from UTF-16LE, either directly by R or by Windows.
>      >>
>      >> But, there is no way to convert (non-ASCII) strings in "C" encoding to
>      >> UTF16-LE, so the examples cannot be made to work on Windows.
>      >>
>      >> When the translation is left on Windows, it assumes the non-UTF-16LE
>      >> strings are in the Active Code Page encoding (shown as "system encoding"
>      >> in sessionInfo() in R, Latin-1 in your example) instead of the current C
>      >> library encoding ("C" in your example). So, file names coming from
>      >> Windows will be either the bytes of their UTF-16LE representation or the
>      >> bytes of their Latin-1 representation, but which one is subject to the
>      >> implementation details, so the result is really unusable.
>      >>
>      >> I would say using "C" as encoding in R is not a good idea, and
>      >> particularly not on Windows.
>      >>
>      >> I would say that what happens with such file names in "C" encoding is
>      >> unspecified behavior, which is subject to change at any time without
>      >> notice, and that both the R 4.0.5 and R-devel behavior you are observing
>      >> are acceptable. I don't think it should be mentioned in the NEWS.
>      >> Personally, I would prefer some stricter checks of strings validity and
>      >> perhaps disallowing the "C" encoding in R, so yet another behavior where
>      >> it would be clearer that this cannot really work, but that would require
>      >> more thought and effort.
>      >>
>      >> Best
>      >> Tomas
>      >>
>      >>
>      >> On 4/27/21 9:53 PM, Toby Hocking wrote:
>      >>
>      >> > Hi all, Today I noticed bug(s?) in R-4.0.5, which seem to be fixed in
>      >> > R-devel already. I checked on
>      >> > https://developer.r-project.org/blosxom.cgi/R-devel/NEWS and there is no
>      >> > mention of these changes, so I'm wondering if they are intentional? If
>      >> so,
>      >> > could someone please add a mention of the bugfix in the NEWS?
>      >> >
>      >> > The problem involves file.exists, on windows, when a long/strange input
>      >> > file name Encoding is unknown, in C locale. I expected that FALSE should
>      >> be
>      >> > returned (and it is on R-devel), but I got an error in R-4.0.5. Code to
>      >> > reproduce is:
>      >> >
>      >> > x <- "\360\237\247\222\n| \360\237\247\222\360\237\217\273\n|
>      >> > \360\237\247\222\360\237\217\274\n| \360\237\247\222\360\237\217\275\n|
>      >> > \360\237\247\222\360\237\217\276\n| \360\237\247\222\360\237\217\277\n"
>      >> > Encoding(x) <- "unknown"
>      >> > Sys.setlocale(locale="C")
>      >> > sessionInfo()
>      >> > file.exists(x)
>      >> >
>      >> > Output I got from R-4.0.5 was
>      >> >
>      >> >> sessionInfo()
>      >> > R version 4.0.5 (2021-03-31)
>      >> > Platform: x86_64-w64-mingw32/x64 (64-bit)
>      >> > Running under: Windows 10 x64 (build 19042)
>      >> >
>      >> > Matrix products: default
>      >> >
>      >> > locale:
>      >> > [1] C
>      >> > system code page: 1252
>      >> >
>      >> > attached base packages:
>      >> > [1] stats     graphics  grDevices utils     datasets  methods   base
>      >> >
>      >> > loaded via a namespace (and not attached):
>      >> > [1] compiler_4.0.5
>      >> >> file.exists(x)
>      >> > Error in file.exists(x) : file name conversion problem -- name too long?
>      >> > Execution halted
>      >> >
>      >> > Output I got from R-devel was
>      >> >
>      >> >> sessionInfo()
>      >> > R Under development (unstable) (2021-04-26 r80229)
>      >> > Platform: x86_64-w64-mingw32/x64 (64-bit)
>      >> > Running under: Windows 10 x64 (build 19042)
>      >> >
>      >> > Matrix products: default
>      >> >
>      >> > locale:
>      >> > [1] C
>      >> >
>      >> > attached base packages:
>      >> > [1] stats     graphics  grDevices utils     datasets  methods   base
>      >> >
>      >> > loaded via a namespace (and not attached):
>      >> > [1] compiler_4.2.0
>      >> >> file.exists(x)
>      >> > [1] FALSE
>      >> >
>      >> > I also observed similar results when using normalizePath instead of
>      >> > file.exists (error in R-4.0.5, no error in R-devel).
>      >> >
>      >> >> normalizePath(x) #R-4.0.5
>      >> > Error in path.expand(path) : unable to translate 'p'
>      >> > | p'p;
>      >> > | p'p<
>      >> > | p'p=
>      >> > | p'p>
>      >> > | p'p<bf>
>      >> > ' to UTF-8
>      >> > Calls: normalizePath -> path.expand
>      >> > Execution halted
>      >> >
>      >> >> normalizePath(x) #R-devel
>      >> > [1] "C:\\Users\\th798\\R\\\360\237\247\222\n|
>      >> > \360\237\247\222\360\237\217\273\n| \360\237\247\222\360\237\217\274\n|
>      >> > \360\237\247\222\360\237\217\275\n| \360\237\247\222\360\237\217\276\n|
>      >> > \360\237\247\222\360\237\217\277\n"
>      >> > Warning message:
>      >> > In normalizePath(path.expand(path), winslash, mustWork) : path[1]="🧒
>      >> > | 🧒🏻
>      >> > | 🧒🏼
>      >> > | 🧒🏽
>      >> > | 🧒🏾
>      >> > | 🧒🏿
>      >> > ": The filename, directory name, or volume label syntax is incorrect
>