nixos-and-flakes-book/docs/nixos-with-flakes/downgrade-or-upgrade-packages.md

Downgrading or Upgrading Packages

After using Flakes, most people are currently using the nixos-unstable branch of nixpkgs. However, sometimes you may encounter bugs, such as the chrome/vscode crash problem.

To resolve these issues, you may need to downgrade or upgrade some packages. In Flakes, all package versions and hash values correspond one-to-one to the git commit of their flake input. Therefore, to downgrade or upgrade a package, you need to lock the git commit of its flake input.

For example, you can add multiple nixpkgs, each using a different git commit or branch:

{
  description = "NixOS configuration of Ryan Yin"

  inputs = {
    # default to nixos-unstable branch
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";

    # the latest stable branch of nixpkgs, used to rollback the version of some packages
    # the current latest version is 22.11
    nixpkgs-stable.url = "github:nixos/nixpkgs/nixos-22.11";

    # we can also use git commit hash to lock the version
    nixpkgs-fd40cef8d.url = "github:nixos/nixpkgs/fd40cef8d797670e203a27a91e4b8e6decf0b90c";
  outputs = inputs@{
    self,
    nixpkgs,
    nixpkgs-stable,
    nixpkgs-fd40cef8d,
    ...
  }: {
    nixosConfigurations = {
      nixos-test = nixpkgs.lib.nixosSystem rec {
        system = "x86_64-linux";

        # The core parameter, which passes the non-default nixpkgs instances to other nix modules
        specialArgs = {
          # To use packages from nixpkgs-stable, we need to configure some parameters for it first
          pkgs-stable = import nixpkgs-stable {
            system = system;  # refer the `system` parameter form outer scope recursively
            # To use chrome, we need to allow the installation of non-free software
            config.allowUnfree = true;
          };
          pkgs-fd40cef8d = import nixpkgs-fd40cef8d {
            system = system;
            config.allowUnfree = true;
          };
        };
        modules = [
          ./hosts/nixos-test

          # omit other configuration...
        ];
      };
    };
  };
}

And then refer to the packages from pkgs-stable or pkgs-fd40cef8d in your sub module, a home manager's sub module as an example:

{
  pkgs,
  config,
  # nix will search and jnject this parameter from specialArgs in flake.nix
  pkgs-stable,
  # pkgs-fd40cef8d,
  ...
}:

{
  # refer packages from pkgs-stable instead of pkgs
  home.packages = with pkgs-stable; [
    firefox-wayland

    # chrome wayland support was broken on nixos-unstable branch, so fallback to stable branch for now
    # https://github.com/swaywm/sway/issues/7562
    google-chrome
  ];

  programs.vscode = {
    enable = true;
    package = pkgs-stable.vscode;  # refer vscode from pkgs-stable instead of pkgs
  };
}

After adjusting the configuration, deploy it with sudo nixos-rebuild switch. This will downgrade your Firefox/Chrome/VSCode to the version corresponding to nixpkgs-stable or nixpkgs-fd40cef8d.

According to 1000 instances of nixpkgs, it's not a good practice to use import in submodules to customize nixpkgs. Each import creates a new instance of nixpkgs, which increases build time and memory usage as the configuration grows. To avoid this problem, we create all nixpkgs instances in flake.nix.

according to 1000 instances of nixpkgs, it's not a good practice to use import in sub modules to customize nixpkgs, because each import will create a new instance of nixpkgs, which will increase the build time and memory usage as the configuration grows. So here we create all nixpkgs instances in flake.nix to avoid this problem.