Creates a file atomically

Creates a file atomically by first creating and writing to a temporary file which is then renamed.

# S3 method for default
createFileAtomically(filename, path=NULL, FUN, ..., skip=FALSE, overwrite=FALSE,
  backup=TRUE, verbose=FALSE)

Arguments

filename

The filename of the file to create.

path

The path to the file.

FUN

A function that creates and writes to the pathname that is passed as the first argument. This pathname is guaranteed to be a non-existing temporary pathname.

...

Additional arguments passed to pushTemporaryFile() and popTemporaryFile().

skip

If TRUE and a file with the same pathname already exists, nothing is done/written.

overwrite

If TRUE and a file with the same pathname already exists, the existing file is overwritten. This is also done atomically such that if the new file was not successfully created, the already original file is restored. If restoration also failed, the original file remains as the pathname with suffix ".bak" appended.

backup

If TRUE and a file with the same pathname already exists, then it is backed up while creating the new file. If the new file was not successfully created, the original file is restored from the backup copy.

verbose

A logical or Verbose.

Value

Returns (invisibly) the pathname.

Author

Henrik Bengtsson

See also

Internally, pushTemporaryFile() and popTemporaryFile() are used for working toward a temporary file, and pushBackupFile() and popBackupFile() are used for backing up and restoring already existing file.

Examples

# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
# Create a file atomically
# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
n <- 10
pathname <- createFileAtomically("foobar.txt", path=tempdir(), FUN=function(pathname) {
  cat(file=pathname, "This file was created atomically.\n")
  cat(file=pathname, "Timestamp: ", as.character(Sys.time()), "\n", sep="")
  for (kk in 1:n) {
    cat(file=pathname, kk, "\n", append=TRUE)
    # Emulate a slow process
    if (interactive()) Sys.sleep(0.1)
  }
  cat(file=pathname, "END OF FILE\n", append=TRUE)
}, overwrite=TRUE)

bfr <- readLines(pathname)
cat(bfr, sep="\n")
#> Timestamp: 2023-11-17 17:03:36.452271
#> 1 
#> 2 
#> 3 
#> 4 
#> 5 
#> 6 
#> 7 
#> 8 
#> 9 
#> 10 
#> END OF FILE


# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
# Overwrite the file atomically (emulate write failure)
# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
tryCatch({
  pathname <- createFileAtomically("foobar.txt", path=tempdir(), FUN=function(pathname) {
    cat(file=pathname, "Trying to create a new file.\n")
    cat(file=pathname, "Writing a bit, but then an error...\n", append=TRUE)
    # Emulate write error
    stop("An error occured while writing to the new file.")
    cat(file=pathname, "END OF FILE\n", append=TRUE)
  }, overwrite=TRUE)
}, error = function(ex) {
  print(ex$message)
})

# The original file was never overwritten
bfr2 <- readLines(pathname)
cat(bfr2, sep="\n")
#> Timestamp: 2023-11-17 17:03:36.452271
#> 1 
#> 2 
#> 3 
#> 4 
#> 5 
#> 6 
#> 7 
#> 8 
#> 9 
#> 10 
#> END OF FILE
stopifnot(identical(bfr2, bfr))

# The partially temporary file remains
pathnameT <- sprintf("%s.tmp", pathname)
stopifnot(isFile(pathnameT))
bfr3 <- readLines(pathnameT)
cat(bfr3, sep="\n")
#> Trying to create a new file.
#> Writing a bit, but then an error...

file.remove(pathnameT)
#> [1] TRUE
file.remove(pathname)
#> [1] TRUE