Expand manual on derivation outputs #12442
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Ericson2314
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fricklerhandwerk and
roberth
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This pull request has been mentioned on NixOS Discourse. There might be relevant details there: https://discourse.nixos.org/t/description-of-the-attribute-inputdrvs-in-the-nix-manual/60190/13 |
Ericson2314
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Feb 13, 2025
Comment on lines
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| ```nix | ||
| fetchurl { | ||
| url = "http://ftp.gnu.org/pub/gnu/hello/hello-2.1.1.tar.gz"; | ||
| sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465"; | ||
| } | ||
| ``` | ||
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| It sometimes happens that the URL of the file changes, e.g., because | ||
| servers are reorganised or no longer available. We then must update | ||
| the call to `fetchurl`, e.g., | ||
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| ```nix | ||
| fetchurl { | ||
| url = "ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz"; | ||
| sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465"; | ||
| } | ||
| ``` | ||
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| If a `fetchurl` derivation was treated like a normal derivation, the | ||
| output paths of the derivation and *all derivations depending on it* | ||
| would change. For instance, if we were to change the URL of the | ||
| Glibc source distribution in Nixpkgs (a package on which almost all | ||
| other packages depend) massive rebuilds would be needed. This is | ||
| unfortunate for a change which we know cannot have a real effect as | ||
| it propagates upwards through the dependency graph. | ||
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| For fixed-output derivations, on the other hand, the name of the | ||
| output path only depends on the `outputHash*` and `name` attributes, | ||
| while all other attributes are ignored for the purpose of computing | ||
| the output path. (The `name` attribute is included because it is | ||
| part of the path.) | ||
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| As an example, here is the (simplified) Nix expression for | ||
| `fetchurl`: | ||
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| ```nix | ||
| { stdenv, curl }: # The curl program is used for downloading. | ||
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| { url, sha256 }: | ||
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| stdenv.mkDerivation { | ||
| name = baseNameOf (toString url); | ||
| builder = ./builder.sh; | ||
| buildInputs = [ curl ]; | ||
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| # This is a fixed-output derivation; the output must be a regular | ||
| # file with SHA256 hash sha256. | ||
| outputHashMode = "flat"; | ||
| outputHashAlgo = "sha256"; | ||
| outputHash = sha256; | ||
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| inherit url; | ||
| } | ||
| ``` |
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This motivation is taken from (going back far enough) Eelco's dissertation; it should be reworded to not use the language, since in this new context, we are describing the store without reference to the language.
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Motivation
Describe derivation outputs, and topics about them like input addressing in detail.
Context
Very nice that there is more pre-exsting material in the language manual we can move out, and give a better home, with this.
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