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Create (experimental) outputOf primop. #8813

Merged
merged 3 commits into from
Aug 14, 2023
Merged

Create (experimental) outputOf primop. #8813

merged 3 commits into from
Aug 14, 2023

Commits on Aug 14, 2023

  1. Rename optOutputPath to optStaticOutputPath 

    This choice of variable name makes it more clear what is going on.

Co-authored-by: Robert Hensing <[email protected]>
    @Ericson2314 @roberth
    Ericson2314 and roberth committed Aug 14, 2023
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  2. Rework evaluator SingleDerivedPath infra 

    `EvalState::mkSingleDerivedPathString` previously contained its own
inverse (printing, rather than parsing) in order to validate what was
parsed. Now that is pulled out into its own separate function:
`EvalState::coerceToSingleDerivedPath`.

In additional that pulled out logic is deduplicated with
`EvalState::mkOutputString` via `EvalState::mkOutputStringRaw`, which is
itself deduplicated (and generalized) with
`DownstreamPlaceholder::mkOutputStringRaw`.

All these changes make the unit tests simpler.

(We would ideally write more unit tests for `mkSingleDerivedPathString`
`coerceToSingleDerivedPath` directly, but we cannot yet do that because
the IO in reading the store path won't work when the dummy store cannot
hold anything. Someday we'll have a proper in-memory store which will
work for this.)

Co-authored-by: Robert Hensing <[email protected]>
    @Ericson2314 @roberth
    Ericson2314 and roberth committed Aug 14, 2023
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  3. Create outputOf primop. 

    In the Nix language, given a drv path, we should be able to construct
another string referencing to one of its output. We can do this today
with `(import drvPath).output`, but this only works for derivations we
already have.

With dynamic derivations, however, that doesn't work well because the
`drvPath` isn't yet built: importing it like would need to trigger IFD,
when the whole point of this feature is to do "dynamic build graph"
without IFD!

Instead, what we want to do is create a placeholder value with the right
string context to refer to the output of the as-yet unbuilt derivation.
A new primop in the language, analogous to `builtins.placeholder` can be
used to create one. This will achieve all the right properties. The
placeholder machinery also will match out the `outPath` attribute for CA
derivations works.

In 60b7121 we added that type of
placeholder, and the derived path and string holder changes necessary to
support it. Then in the previous commit we cleaned up the code
(inspiration finally hit me!) to deduplicate the code and expose exactly
what we need. Now, we can wire up the primop trivally!

Part of RFC 92: dynamic derivations (tracking issue NixOS#6316)

Co-authored-by: Robert Hensing <[email protected]>
    @Ericson2314 @roberth
    Ericson2314 and roberth committed Aug 14, 2023
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