Find fixed output derivations

find-fixed-outputs.nix

# This expression returns a list of all fixed output derivations used by ‘expr’.
# eg.
# $ nix-instantiate find-fixed-outputs.nix --eval --strict --json --arg expr '(import <nixpkgs> {}).hello'

with import <nixpkgs> { };
with lib;

{ expr }:

let

  root = expr;

  uniqueSrcs = map (x: x.file) (genericClosure {
    startSet = map (file: { key = file.hash; inherit file; }) srcs;
    operator = const [ ];
  });

  srcs = map (drv: { drv = drv.drvPath; hash = drv.outputHash; type = drv.outputHashAlgo; mode = drv.outputHashMode; name = drv.name; }) drvDependencies;

  drvDependencies =
    filter
      (drv: drv.outputHash or "" != "")
      dependencies;

  dependencies = map (x: x.value) (genericClosure {
    startSet = map keyDrv (derivationsIn' root);
    operator = { key, value }: map keyDrv (immediateDependenciesOf value);
  });

  derivationsIn' = x:
    if !canEval x then []
    else if isDerivation x then optional (canEval x.drvPath) x
    else if isList x then concatLists (map derivationsIn' x)
    else if isAttrs x then concatLists (mapAttrsToList (n: v: addErrorContext "while finding fixed outputs in '${n}':" (derivationsIn' v)) x)
    else [ ];

  keyDrv = drv: if canEval drv.drvPath then { key = drv.drvPath; value = drv; } else { };

  immediateDependenciesOf = drv:
    concatLists (mapAttrsToList (n: v: derivationsIn v) (removeAttrs drv ["meta" "passthru"]));

  derivationsIn = x:
    if !canEval x then []
    else if isDerivation x then optional (canEval x.drvPath) x
    else if isList x then concatLists (map derivationsIn x)
    else [ ];

  canEval = val: (builtins.tryEval val).success;

in uniqueSrcs