[n42]

not sure if I completely understand the race you describe, but I don't think there's any race. the paths and links are created at build/install time. nothing is switching around after the expression has been applied.

edit: maybe you're describing what happens if a program is run while a Nix expression is being applied; I believe what happens in that case is that the program that was being run will work fine since its environment is pointing to the previous generation, and applying the new expression creates a new generation