Write to file atomically

What it does

This tiny package provides a single funtion, atomic_write(f, path; backup=nothing, overwrite_backup=false), to atomically write to a file. f is applied to an open IOStream on a temporary file, which is then moved to path. Thus, writing is committed to path only if it completed without error, otherwise the file at path is left untouched.

The backup keyword may be used to specify a backup filename which will contain the original contents of path if f completed successfully.

What it doesn't do

  • There is no support for appending, only overwriting.
  • It's not thread-safe when the backup keyword is used, since the backup is created by copying. It is only intended to be atomic when run from a single thread.


using AtomicFileWrite

filename = "tmp.txt"
open(io->print(io,"original contents"), filename, "w")
    atomic_write(filename) do io
        print(io, "new ")
        print(io, "contents")
catch e
    showerror(stderr, e, catch_backtrace())

# original contents still intact after error
@assert read(filename, String) == "original contents"

backup = filename * ".bak"
atomic_write(io->print(io,"new contents"), filename, backup=backup, overwrite_backup=true)

@assert read(filename, String) == "new contents"
@assert read(backup, String) == "original contents"