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#pragma once
///@file
#include "hash.hh"
#include "path.hh"
namespace nix {
/**
* Downstream Placeholders are opaque and almost certainly unique values
* used to allow derivations to refer to store objects which are yet to
* be built and for we do not yet have store paths for.
*
* They correspond to `DerivedPaths` that are not `DerivedPath::Opaque`,
* except for the cases involving input addressing or fixed outputs
* where we do know a store path for the derivation output in advance.
*
* Unlike `DerivationPath`, however, `DownstreamPlaceholder` is
* purposefully opaque and obfuscated. This is so they are hard to
* create by accident, and so substituting them (once we know what the
* path to store object is) is unlikely to capture other stuff it
* shouldn't.
*
* We use them with `Derivation`: the `render()` method is called to
* render an opaque string which can be used in the derivation, and the
* resolving logic can substitute those strings for store paths when
* resolving `Derivation.inputDrvs` to `BasicDerivation.inputSrcs`.
*/
class DownstreamPlaceholder
{
/**
* `DownstreamPlaceholder` is just a newtype of `Hash`.
* This its only field.
*/
Hash hash;
/**
* Newtype constructor
*/
DownstreamPlaceholder(Hash hash) : hash(hash) { }
public:
/**
* This creates an opaque and almost certainly unique string
* deterministically from the placeholder.
*/
std::string render() const;
/**
* Create a placeholder for an unknown output of a content-addressed
* derivation.
*
* The derivation itself is known (we have a store path for it), but
* the output doesn't yet have a known store path.
*/
static DownstreamPlaceholder unknownCaOutput(
const StorePath & drvPath,
std::string_view outputName);
/**
* Create a placehold for the output of an unknown derivation.
*
* The derivation is not yet known because it is a dynamic
* derivaiton --- it is itself an output of another derivation ---
* and we just have (another) placeholder for it.
*
* @param xpSettings Stop-gap to avoid globals during unit tests.
*/
static DownstreamPlaceholder unknownDerivation(
const DownstreamPlaceholder & drvPlaceholder,
std::string_view outputName,
const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
};
}
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