[Nix-dev] Improve docs for overrideDerivation - overridable attributes

[Alex Berg]

Oh wow! I never would have guessed this was a behavior of the `rec`
keyword! Nice idea!
I'll play around with it to verify my understanding and then submit a PR to
the NixPkgs manual to note this behavior for other people who might
discover this pitfall. That'll take me several days. :)

Much appreciated, Layus!

On Thu, Jun 30, 2016 at 12:01 PM, Layus <layus.on at gmail.com> wrote:

> On 30/06/16 20:18, Alex Berg wrote:
>
> I created a new "~/.nixpkgs/config.nix" file to customize the
> nix-channel-obtained nixpkgs copy on my system - my goal was to bump the
> version of Vim to a specific version.
>
> My first attempt was to override the derivation and simply set the
> "version" attribute, like this:
>
> {
>   packageOverrides = pkgs: rec {
>     vim = pkgs.vim.overrideDerivation (oldAttrs: {
>       version = "7.4.1941";
>     });
>   };
> }
>
> But this had zero effect on the name of Vim I saw when using "nix-env
> -qaP" to see package details. The Vim package's definition has the "name"
> attribute defined like this:
>
> name = "vim-${version}";
> version = "7.4.1585";
>
> so I expected my overriding the "version" attribute to affect the
> package's name, but it did not.
>
> No, because the definition above relies on the "rec" keyword, but "rec"
> has already been applied *before* overrideDerivation.
> Overriding "version" will not override "name", and in turn will not
> override "src".
> In fact, overriding "version" will have no impact on the output path as
> that parameter is not used by mkDerivation.
> But the attribute is overriden. ` nix-instantiate "<nixpkgs>" --eval -A
> vim.version ` will give you the new version string.
>
> Overriding "name", for example, will have impact as it is used by
> mkDerivation to compute the output path.
> Again, it is not sufficient to build a different version of vim. See below.
>
>
> After asking the #nixos IRC channel, one person suggested the "version"
> attribute isn't overridable because it isn't an attribute the *primitive
> derivation* set. Based on this guess, I changed my ~/.nixpkgs/config.nix
> definition from that to this:
>
> let
>   vim-version = "7.4.1941";
> in
> {
>   packageOverrides = pkgs: rec {
>     vim = pkgs.vim.overrideDerivation (oldAttrs: {
>       name = "vim-${vim-version}";
>       src = pkgs.fetchFromGitHub {
>         owner = "vim";
>         repo = "vim";
>         rev = "v${vim-version}";
>         sha256 = "0apq7sv1fbyfzqh4q0r2z19bn4gbjlb97wyl80kj7qsqmsy7m1lm";
>       };
>     });
>   };
> }
>
> My guess is this works because I'm overriding the "name" attribute.
>
> No, it works because you override the "src" attribute.
> src is used by mkDerivation, so overriding modifies the package to be
> built with the new source tree.
> Overriding the name is not strictly necessary, but is a *very* good
> practice, as otherwise you would get a 7.4.1941 vim with the old version in
> the name... confusing :-)
>
>
> I read the definition of the "mkDerivation" function (pkgs/stdenv/generic/default.nix),
> but it doesn't have a simple list of attributes that are overridable, but
> is rather flexible. Also, that definition doesn't mention a "src"
> attribute, but "src" attribute is overridable, so I wonder why overriding
> the "src" attribute works.
>
> Where can I find explanation for this? If there is a restriction on which
> attributes are overridable, then I'd like to note this in the NixPkgs
> manual, here:
> <https://nixos.org/nixpkgs/manual/index.html#sec-pkg-overrideDerivation>
> https://nixos.org/nixpkgs/manual/index.html#sec-pkg-overrideDerivation
>
> Everything is overridable separately, but you cannot count on the rec
> keyword at override time.
> Every attribute already has a fully defined value.
>
>
>
>
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