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Document the derived path types.

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This commit is contained in:
John Ericson 2021-04-05 10:56:48 -04:00
parent d8fa7517fa
commit 125a824228
1 changed files with 49 additions and 1 deletions
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50
src/libstore/derived-path.hh
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@ -2,7 +2,6 @@
#include "util.hh"
#include "path.hh"
#include "path.hh"
#include <optional>
@ -12,6 +11,13 @@ namespace nix {
class Store;
/**
* An opaque derived path.
*
* Opaque derived paths are just store paths, and fully evaluated. They
* cannot be simplified further. Since they are opaque, they cannot be
* built, but they can fetched.
*/
struct DerivedPathOpaque {
StorePath path;
@ -20,6 +26,18 @@ struct DerivedPathOpaque {
static DerivedPathOpaque parse(const Store & store, std::string_view);
};
/**
* A derived path that is built from a derivation
*
* Built derived paths are pair of a derivation and some output names.
* They are evaluated by building the derivation, and then replacing the
* output names with the resulting outputs.
*
* Note that does mean a derived store paths evaluates to multiple
* opaque paths, which is sort of icky as expressions are supposed to
* evaluate to single values. Perhaps this should have just a single
* output name.
*/
struct DerivedPathBuilt {
StorePath drvPath;
std::set<std::string> outputs;
@ -33,6 +51,16 @@ using _DerivedPathRaw = std::variant<
DerivedPathBuilt
>;
/**
* A "derived path" is a very simple sort of expression that evaluates
* to (concrete) store path. It is either:
*
* - opaque, in which case it is just a concrete store path with
* possibly no known derivation
*
* - built, in which case it is a pair of a derivation path and an
* output name.
*/
struct DerivedPath : _DerivedPathRaw {
using Raw = _DerivedPathRaw;
using Raw::Raw;
@ -48,6 +76,11 @@ struct DerivedPath : _DerivedPathRaw {
static DerivedPath parse(const Store & store, std::string_view);
};
/**
* A built derived path with hints in the form of optional concrete output paths.
*
* See 'DerivedPathWithHints' for more an explanation.
*/
struct DerivedPathWithHintsBuilt {
StorePath drvPath;
std::map<std::string, std::optional<StorePath>> outputs;
@ -61,6 +94,21 @@ using _DerivedPathWithHintsRaw = std::variant<
DerivedPathWithHintsBuilt
>;
/**
* A derived path with hints in the form of optional concrete output paths in the built case.
*
* This type is currently just used by the CLI. The paths are filled in
* during evaluation for derivations that know what paths they will
* produce in advanced, i.e. input-addressed or fixed-output content
* addressed derivations.
*
* That isn't very good, because it puts floating content-addressed
* derivations "at a disadvantage". It would be better to never rely on
* the output path of unbuilt derivations, and exclusively use the
* realizations types to work with built derivations' concrete output
* paths.
*/
// FIXME Stop using and delete this, or if that is not possible move out of libstore to libcmd.
struct DerivedPathWithHints : _DerivedPathWithHintsRaw {
using Raw = _DerivedPathWithHintsRaw;
using Raw::Raw;