force_symlink: do nothing if correct symlink already exists (#1002)

src/setup.jl

@@ -26,14 +26,17 @@ gaproot() = @get_scratch!(scratch_key)
 # ensure `link` is a symlink pointing to `target` in a way that is hopefully
 # safe against races with other Julia processes doing the exact same thing
 function force_symlink(target::AbstractString, link::AbstractString)
- # We previously used `rm` followed by `symlink`, but this can cause a
- # race if multiple processes invoke `rm` concurrently (which works if
- # one uses `force=true`), and then try to invoke `symlink`
- # concurrently (which then fails in all but one process).
- #
- # So instead we create the symlink with a temporary name, and then use
+ # Do nothing if the symlink already exists and points at the right
+ # target
+ if Base.islink(link) && Base.readlink(link) == target
+ return nothing
+ end
+
+ # Otherwise we create the symlink with a temporary name, and then use
  # an atomic `rename` to rename it to the `link` name. The latter
  # unfortunately requires invoking an undocumented function.
+ # But all of this together helps avoid a race condition if multiple
+ # Julia instances try to create the symlink concurrently
  tmpfile = tempname(dirname(abspath(link)); cleanup=false)
  symlink(target, tmpfile)
  Base.Filesystem.rename(tmpfile, link)