Rewrite force_symlink to avoid a race condition

src/setup.jl

@@ -23,9 +23,21 @@ gaproot() = @get_scratch!(scratch_key)
 #
 #############################################################################
 
-function force_symlink(p::AbstractString, np::AbstractString)
-    rm(np; force = true)
-    symlink(p, np)
+# ensure `source` 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(source::AbstractString, target::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
+    # an atomic `rename` to rename it to the `target` name. The latter
+    # unfortunately requires invoking an undocumented function.
+    tmpfile = tempname(dirname(abspath(target)); cleanup=false)
+    symlink(source, tmpfile)
+    Base.Filesystem.rename(tmpfile, target)
+    return nothing
 end
 
 function read_sysinfo_gap(dir::String)