Common pattern of .${stdenv.hostPlatform.system} or (throw "Unsupported platform: ${stdenv.hostPlatform.system}") should be encapsulated in library function

[pluiedev]

Currently there's a lot of code in Nixpkgs that selectively use a different src/hash/rev/etc. depending on the target system. This is especially common in proprietary software that have different download links for pre-built binaries, for instance, in UT1999, the url and hash are different for each given platform:

let
  # ...
  srcs = {
    x86_64-linux = fetchurl {
      url = "https://github.com/OldUnreal/UnrealTournamentPatches/releases/download/v${version}/OldUnreal-UTPatch${version}-Linux-amd64.tar.bz2";
      hash = "sha256-aoGzWuakwN/OL4+xUq8WEpd2c1rrNN/DkffI2vDVGjs=";
    };
    aarch64-linux = fetchurl {
      url = "https://github.com/OldUnreal/UnrealTournamentPatches/releases/download/v${version}/OldUnreal-UTPatch${version}-Linux-arm64.tar.bz2";
      hash = "sha256-2e9lHB12jLTR8UYofLWL7gg0qb2IqFk6eND3T5VqAx0=";
    };
    i686-linux = fetchurl {
      url = "https://github.com/OldUnreal/UnrealTournamentPatches/releases/download/v${version}/OldUnreal-UTPatch${version}-Linux-x86.tar.bz2";
      hash = "sha256-1JsFKuAAj/LtYvOUPFu0Hn+zvY3riW0YlJbLd4UnaKU=";
    };
    x86_64-darwin = fetchurl {
      url = "https://github.com/OldUnreal/UnrealTournamentPatches/releases/download/v${version}/OldUnreal-UTPatch${version}-macOS-Sonoma.dmg";
      hash = "sha256-TbhJbOH4E5WOb6XR9dmqLkXziK3/CzhNjd1ypBkkmvw=";
    };
  };
in
stdenv.mkDerivation {
  src = srcs.${stdenv.hostPlatform.system} or (throw "Unsupported system: ${stdenv.hostPlatform.system}");
  # ...
}

Searching through the entire repository, there are 69 occurrences of this pattern, given by running rg -l '.\$\{stdenv.hostPlatform.system\} or \(throw' | wc -l.
Without the or (throw part, it's more like 205 occurrences.
In my opinion, this already makes it common enough to justify having a library function that simplifies this.

List of files that feature this pattern

pkgs/applications/audio/touchosc/default.nix
pkgs/applications/blockchains/trezor-suite/default.nix
pkgs/applications/gis/udig/default.nix
pkgs/applications/graphics/kodelife/default.nix
pkgs/applications/misc/keeweb/default.nix
pkgs/applications/networking/browsers/mullvad-browser/default.nix
pkgs/applications/networking/browsers/tor-browser/default.nix
pkgs/applications/networking/browsers/vivaldi/default.nix
pkgs/applications/networking/instant-messengers/armcord/default.nix
pkgs/applications/networking/instant-messengers/qq/default.nix
pkgs/applications/networking/mullvad-vpn/default.nix
pkgs/applications/networking/p2p/frostwire/default.nix
pkgs/applications/networking/remote/teamviewer/default.nix
pkgs/applications/networking/resilio-sync/default.nix
pkgs/applications/science/math/scilab-bin/default.nix
pkgs/applications/version-management/p4v/default.nix
pkgs/applications/video/streamlink-twitch-gui/bin.nix
pkgs/by-name/cu/cups-kyocera-3500-4500/package.nix
pkgs/by-name/go/goofcord/package.nix
pkgs/by-name/ou/outfox/package.nix
pkgs/by-name/ph/photonvision/package.nix
pkgs/by-name/pu/pulsar/package.nix
pkgs/by-name/st/stereotool/package.nix
pkgs/by-name/ut/ut1999/package.nix
pkgs/by-name/wi/windmill/package.nix
pkgs/by-name/ye/yesplaymusic/package.nix
pkgs/development/compilers/ccl/default.nix
pkgs/development/compilers/gcc-arm-embedded/10/default.nix
pkgs/development/compilers/gcc-arm-embedded/11/default.nix
pkgs/development/compilers/gcc-arm-embedded/12/default.nix
pkgs/development/compilers/gcc-arm-embedded/13/default.nix
pkgs/development/compilers/gcc-arm-embedded/6/default.nix
pkgs/development/compilers/gcc-arm-embedded/7/default.nix
pkgs/development/compilers/gcc-arm-embedded/8/default.nix
pkgs/development/compilers/gcc-arm-embedded/9/default.nix
pkgs/development/compilers/jetbrains-jdk/default.nix
pkgs/development/compilers/jetbrains-jdk/jcef.nix
pkgs/development/compilers/julia/1.6-bin.nix
pkgs/development/compilers/julia/generic-bin.nix
pkgs/development/compilers/mozart/binary.nix
pkgs/development/compilers/oraclejdk/jdk-linux-base.nix
pkgs/development/libraries/libcef/default.nix
pkgs/development/python-modules/debugpy/default.nix
pkgs/development/python-modules/tensorflow/default.nix
pkgs/development/tools/azure-static-sites-client/default.nix
pkgs/development/tools/confluent-cli/default.nix
pkgs/development/tools/misc/blackfire/default.nix
pkgs/development/tools/misc/segger-ozone/default.nix
pkgs/development/web/postman/linux.nix
pkgs/games/katawa-shoujo/default.nix
pkgs/misc/cups/drivers/kyodialog/default.nix
pkgs/misc/drivers/pantum-driver/default.nix
pkgs/misc/urbit/default.nix
pkgs/os-specific/linux/broadcom-sta/default.nix
pkgs/servers/keycloak/keycloak-metrics-spi/default.nix
pkgs/servers/monitoring/grafana/default.nix
pkgs/servers/search/elasticsearch/7.x.nix
pkgs/servers/sql/cockroachdb/cockroachdb-bin.nix
pkgs/servers/ums/default.nix
pkgs/tools/admin/ibmcloud-cli/default.nix
pkgs/tools/archivers/rar/default.nix
pkgs/tools/bootloaders/refind/default.nix
pkgs/tools/inputmethods/droidmote/default.nix
pkgs/tools/misc/archi/default.nix
pkgs/tools/misc/logstash/7.x.nix
pkgs/tools/networking/cloudflare-warp/default.nix
pkgs/tools/networking/ookla-speedtest/default.nix
pkgs/tools/security/enpass/default.nix
pkgs/tools/security/sonar-scanner-cli/default.nix
   

[Scrumplex]

Perhaps this could be something that is handled by mkDerivation? As it is right now, this pattern makes overriding sources much harder, as they will be different depending on the platform.

[Aleksanaa]

Perhaps this could be something that is handled by mkDerivation?

You are right, maybe we should have a unified pattern like srcForSystem.

[Aleksanaa]

Related: #293838

[eclairevoyant]

Perhaps this could be something that is handled by mkDerivation?

You are right, maybe we should have a unified pattern like srcForSystem.

This is not as important for src, but for version it definitely is needed if versions differ by platform. See #313741. Hence srcForSystem is too specific to src.

[eclairevoyant]

Also after considering this further, this seems like unnecessary boilerplate. If a platform isn't listed in meta.platforms then we shouldn't have to eval src or version in the first place.

I'd rather just remove the need for such a throw at all.