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

[Layus]

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
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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