Windows: getAbsolutePathname() to prepend \\?\ for too long pathnames?

[HenrikBengtsson]

On Windows, there's a 260 character limit on the canonical pathname, cf. https://www.aroma-project.org/howtos/UseLongFilenamesOnWindows/ and https://docs.microsoft.com/en-us/windows/win32/fileio/naming-a-file#maximum-path-length-limitation.

However, as https://bugs.r-project.org/bugzilla/show_bug.cgi?id=17631 suggests, it is possible to work around this by using a \\?\ on the absolute path, which then supports 32,767 characters.

Should getAbsolutePathname() automatically prepend \\?\ for long path/pathnames on Windows?

[HenrikBengtsson]

Hmm... although better than 259 characters, there seems to be an upper limit of 298 even when using the \\?\ trick.

Example:

long_pathname <- function(n=259L, path=tempdir(), unc=FALSE) {
  fsep <- if (.Platform$OS.type == "windows") "\\" else "/"
  filename <- paste(rep(letters, length.out=n-1L-nchar(path)), collapse="")
  normalizePath(file.path(path, filename, fsep=fsep), mustWork=FALSE)
}

unc <- function(pathname) {
  paste0("\\\\?\\", pathname)
}

First of all, here is the regular limitation:

pn <- long_pathname(n=259)
cat("hello", file=pn)
readLines(pn, warn=FALSE)
## [1] "hello"

pn <- long_pathname(n=260)
cat("hello", file=pn)
## Error in file(file, ifelse(append, "a", "w")) : 
##   cannot open the connection
## In addition: Warning message:
## In file(file, ifelse(append, "a", "w")) :
##   cannot open file 'C:\Users\hb\AppData\Local\Temp\RtmpszjQBA\abc...j': No such file or directory

This can be somewhat circumvented with \\?\;

pn <- unc(long_pathname(n=259))
cat("hello", file=pn)
readLines(pn, warn=FALSE)
## [1] "hello"

pn <- unc(long_pathname(n=260))
cat("hello", file=pn)
readLines(pn, warn=FALSE)
## [1] "hello"

pn <- unc(long_pathname(n=297))
cat("hello", file=pn)
readLines(pn, warn=FALSE)
## [1] "hello"

but even this will hit an upper limit;

pn <- unc(long_pathname(n=298))
cat("hello", file=pn)
## Error in file(file, ifelse(append, "a", "w")) : 
##   cannot open the connection
## In addition: Warning message:
## In file(file, ifelse(append, "a", "w")) :
##   cannot open file '\\?\C:\Users\hb\AppData\Local\Temp\RtmpszjQBA\abc...v': Invalid argument

BTW, note also that file.exists() doesn't work with UNC pathnames, e.g.

> pn <- unc(tempfile())
> pn
[1] "\\\\?\\C:\\Users\\hb\\AppData\\Local\\Temp\\RtmpszjQBA\\file1da4bd969df"
> cat("hello", file=pn)
> readLines(pn, warn=FALSE)
[1] "hello"
> file.exists(pn)
[1] FALSE

Session info

> sessionInfo()
R version 3.6.1 Patched (2019-09-11 r77179)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 18362)

Matrix products: default

locale:
[1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United States.1252    LC_MONETARY=English_United States.1252 LC_NUMERIC=C                          
[5] LC_TIME=English_United States.1252    

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

loaded via a namespace (and not attached):
[1] compiler_3.6.1 tools_3.6.1