177 lines
6.9 KiB
Nix
177 lines
6.9 KiB
Nix
{ lib }:
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let
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inherit (lib)
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genAttrs
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isString
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throwIfNot
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;
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showMaybeAttrPosPre = prefix: attrName: v:
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let pos = builtins.unsafeGetAttrPos attrName v;
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in if pos == null then "" else "${prefix}${pos.file}:${toString pos.line}:${toString pos.column}";
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showMaybePackagePosPre = prefix: pkg:
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if pkg?meta.position && isString pkg.meta.position
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then "${prefix}${pkg.meta.position}"
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else "";
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in
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{
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/*
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Restrict a derivation to a predictable set of attribute names, so
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that the returned attrset is not strict in the actual derivation,
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saving a lot of computation when the derivation is non-trivial.
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This is useful in situations where a derivation might only be used for its
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passthru attributes, improving evaluation performance.
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The returned attribute set is lazy in `derivation`. Specifically, this
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means that the derivation will not be evaluated in at least the
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situations below.
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For illustration and/or testing, we define derivation such that its
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evaluation is very noticeable.
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let derivation = throw "This won't be evaluated.";
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In the following expressions, `derivation` will _not_ be evaluated:
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(lazyDerivation { inherit derivation; }).type
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attrNames (lazyDerivation { inherit derivation; })
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(lazyDerivation { inherit derivation; } // { foo = true; }).foo
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(lazyDerivation { inherit derivation; meta.foo = true; }).meta
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In these expressions, `derivation` _will_ be evaluated:
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"${lazyDerivation { inherit derivation }}"
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(lazyDerivation { inherit derivation }).outPath
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(lazyDerivation { inherit derivation }).meta
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And the following expressions are not valid, because the refer to
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implementation details and/or attributes that may not be present on
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some derivations:
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(lazyDerivation { inherit derivation }).buildInputs
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(lazyDerivation { inherit derivation }).passthru
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(lazyDerivation { inherit derivation }).pythonPath
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*/
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lazyDerivation =
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args@{
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# The derivation to be wrapped.
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derivation
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, # Optional meta attribute.
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#
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# While this function is primarily about derivations, it can improve
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# the `meta` package attribute, which is usually specified through
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# `mkDerivation`.
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meta ? null
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, # Optional extra values to add to the returned attrset.
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#
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# This can be used for adding package attributes, such as `tests`.
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passthru ? { }
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, # Optional list of assumed outputs. Default: ["out"]
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#
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# This must match the set of outputs that the returned derivation has.
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# You must use this when the derivation has multiple outputs.
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outputs ? [ "out" ]
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}:
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let
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# These checks are strict in `drv` and some `drv` attributes, but the
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# attrset spine returned by lazyDerivation does not depend on it.
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# Instead, the individual derivation attributes do depend on it.
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checked =
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throwIfNot (derivation.type or null == "derivation")
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"lazyDerivation: input must be a derivation."
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throwIfNot
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# NOTE: Technically we could require our outputs to be a subset of the
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# actual ones, or even leave them unchecked and fail on a lazy basis.
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# However, consider the case where an output is added in the underlying
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# derivation, such as dev. lazyDerivation would remove it and cause it
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# to fail as a buildInputs item, without any indication as to what
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# happened. Hence the more stringent condition. We could consider
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# adding a flag to control this behavior if there's a valid case for it,
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# but the documentation must have a note like this.
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(derivation.outputs == outputs)
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''
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lib.lazyDerivation: The derivation ${derivation.name or "<unknown>"} has outputs that don't match the assumed outputs.
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Assumed outputs passed to lazyDerivation${showMaybeAttrPosPre ",\n at " "outputs" args}:
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${lib.generators.toPretty { multiline = false; } outputs};
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Actual outputs of the derivation${showMaybePackagePosPre ",\n defined at " derivation}:
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${lib.generators.toPretty { multiline = false; } derivation.outputs}
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If the outputs are known ahead of evaluating the derivation,
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then update the lazyDerivation call to match the actual outputs, in the same order.
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If lazyDerivation is passed a literal value, just change it to the actual outputs.
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As a result it will work as before / as intended.
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Otherwise, when the outputs are dynamic and can't be known ahead of time, it won't
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be possible to add laziness, but lib.lazyDerivation may still be useful for trimming
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the attributes.
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If you want to keep trimming the attributes, make sure that the package is in a
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variable (don't evaluate it twice!) and pass the variable and its outputs attribute
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to lib.lazyDerivation. This largely defeats laziness, but keeps the trimming.
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If none of the above works for you, replace the lib.lazyDerivation call by the
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expression in the derivation argument.
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''
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derivation;
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in
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{
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# Hardcoded `type`
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#
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# `lazyDerivation` requires its `derivation` argument to be a derivation,
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# so if it is not, that is a programming error by the caller and not
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# something that `lazyDerivation` consumers should be able to correct
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# for after the fact.
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# So, to improve laziness, we assume correctness here and check it only
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# when actual derivation values are accessed later.
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type = "derivation";
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# A fixed set of derivation values, so that `lazyDerivation` can return
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# its attrset before evaluating `derivation`.
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# This must only list attributes that are available on _all_ derivations.
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inherit (checked) outPath outputName drvPath name system;
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inherit outputs;
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# The meta attribute can either be taken from the derivation, or if the
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# `lazyDerivation` caller knew a shortcut, be taken from there.
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meta = args.meta or checked.meta;
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}
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// genAttrs outputs (outputName: checked.${outputName})
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// passthru;
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/* Conditionally set a derivation attribute.
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Because `mkDerivation` sets `__ignoreNulls = true`, a derivation
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attribute set to `null` will not impact the derivation output hash.
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Thus, this function passes through its `value` argument if the `cond`
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is `true`, but returns `null` if not.
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Type: optionalDrvAttr :: Bool -> a -> a | Null
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Example:
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(stdenv.mkDerivation {
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name = "foo";
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x = optionalDrvAttr true 1;
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y = optionalDrvAttr false 1;
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}).drvPath == (stdenv.mkDerivation {
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name = "foo";
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x = 1;
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}).drvPath
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=> true
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*/
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optionalDrvAttr =
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# Condition
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cond:
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# Attribute value
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value: if cond then value else null;
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}
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