[jbboehr]

If you use a content-addressable scheme (fetchurl with sha256, for example), it will retain the source archives until you run garbage collect.

If you use builtins.fetchTarball, I don't think this is the case.

Since this all uses CAS, you can use the nix prefetch scripts to import an arbitrary file:// or other URI into the nix store.

[manveru]

fetchTarball now also supports an optional sha256 argument. It'll then be used indefinitely without checking for changes after the TTL expires.