Explanation of how output hashes are derived

[roberth]

          > What kind of changes to the lowlevel implementations in nixpkgs could change the hash of a static text file

None. The contents of the output of writeTextFile is very very stable, which is why you want a content-addressed output instead of an input addressed output. By making it a fixed-output derivation that's exactly what you get. It shields you from the many obscure changes that happen to the derivation closure.

How output path hashes are derived would be a good topic for an explanation, but what we have now is reference docs, https://nixos.org/manual/nix/stable/language/advanced-attributes.html?highlight=outputHash#adv-attr-outputHash

Originally posted by @roberth in NixOS/nixpkgs#246115 (comment)

[roberth]

ie more prosaic explanation of how hashing works instead of having to click through a ton of reference docs to piece it together.