[Nix-dev] NIX_PATH and tarballs

[Daniel Hlynskyi]

Hi. The manual says on tarballs in NIX_PATH this:

> If a path in the Nix search path starts with http:// or https://, it is
interpreted
> as the URL of a tarball that will be downloaded and unpacked to a
temporary
> location. The tarball must consist of a single top-level directory.

But when I run `nixos-rebuild` twice, I notice that tarball is not
downloaded second time.

root# NIX_PATH=nixos-config=/etc/nixos/configuration.nix:nixpkgs=
https://github.com/NixOS/nixpkgs-channels/archive/nixos-unstable.tar.gz \
root#   nixos-rebuild dry-build
building the system configuration...
# ----->  tarball is not downloaded second time <-----------
these derivations will be built:
  /nix/store/h2sp5p9x3gzryiq57znqdzc3jrw3dlpa-nixos-help.drv
  /nix/store/9gzsm78klcassgs9yvpmnd38naczndc1-system-path.drv
  /nix/store/rflgcrmas917yzdihxd9kqbp8n27w2qj-dbus-conf.drv

What kind of caching is this and what happens when upstream tarball changes?

Question 2: In his talk Eelco mentioned the possible deprecation of
nix-channel in favor of tarball links. Is anybody aware of what the new
interface for channels will look like?
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