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Apply suggestions from code review

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
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John Ericson 2025-02-09 19:42:05 -05:00 committed by GitHub
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@ -161,26 +161,20 @@ There are two types of placeholder, corresponding to the two cases where this pr
>
> - In the output case this is impossible:
>
> We cannot built the output until we have a correct derivation, and we cannot have a correct derivation (without using placeholders) until we have the output path.
> We cannot build the output until we have a correct derivation, and we cannot have a correct derivation (without using placeholders) until we have the output path.
>
> - In the input case this is impractical:
>
> We an always built a dependency, and then refer to its output by store path, but by doing so we loose the ability for a derivation graph to describe an entire build plan consisting of multiple build steps.
> **Note**
>
> The current method of creating hashes which we substitute for string fields should be seen as an artifact of the current "ATerm" serialization format.
> In order to be more explicit, and avoid gotchas analogous to [SQL injection](https://en.wikipedia.org/wiki/SQL_injection),
> we ought to consider switching two a different format where we explicitly use a syntax for the concatenation of plain strings and [deriving paths] written more explicitly.
> If we always build a dependency first, and then refer to its output by store path, we would lose the ability for a derivation graph to describe an entire build plan consisting of multiple build steps.
### System {#system}
The system type on which the [`builder`](#attr-builder) executable is meant to be run.
A necessary condition for Nix to schedule a given derivation on given Nix instance is for the "system" of that derivation to match that instance's [`system` configuration option].
A necessary condition for Nix to schedule a given derivation on some Nix instance is for the "system" of that derivation to match that instance's [`system` configuration option].
By putting the `system` in each derivation, Nix allows *heterogenous* build plans, where not all steps can be run on the same machine or same sort of machine.
A Nix isntance scheduling builds can automatically [build on other platforms](@docroot@/language/derivations.md#attr-builder) by forwarding build requests to other Nix instances.
Nix can schedule builds such that it automatically builds on other platforms by [forwarding build requests](@docroot@/advanced-topics/distributed-builds.md) to other Nix instances.
[`system` configuration option]: @docroot@/command-ref/conf-file.md#conf-system
@ -197,7 +191,7 @@ There are two formats, documented separately:
- The legacy ["ATerm" format](@docroot@/protocols/derivation-aterm.md)
- The experimental [JSON format](@docroot@/protocols/json/derivation.md)
- The experimental, currently under development and changing [JSON format](@docroot@/protocols/json/derivation.md)
Every derivation has a canonical choice of encoding used to serialize it to a store object.
This ensures that there is a canonical [store path] used to refer to the derivation, as described in [Referencing derivations](#derivation-path).
@ -207,27 +201,15 @@ This ensures that there is a canonical [store path] used to refer to the derivat
> Currently, the canonical encoding for every derivation is the "ATerm" format,
> but this is subject to change for types derivations which are not yet stable.
Regardless of the format used, when serializing to store objects, content-addressing is always used.
Regardless of the format used, when serializing a derivation to a store object, that store object will be content-addressed.
In the common case the inputs to store objects are either:
In the common case, the inputs to store objects are either:
- constant deriving paths for content-addressed source objects, which are "initial inputs" rather than the outputs of some other derivation
- [constant deriving paths](#deriving-path-constant) for content-addressed source objects, which are "initial inputs" rather than the outputs of some other derivation
- the outputs of other derivations abiding by this same invariant.
- the outputs of other derivations
This common case makes for the following useful property:
when we serialize such a derivation graph to store objects, the resulting closures are *entirely* content-addressed.
Here is a sketch at the proof of this:
- The inputs which are constant deriving paths become references of the serialized derivations, but they are content-addressed per the above.
- For inputs which are output deriving paths, we cannot directly reference the input because in general it is not built yet.
We instead "peel back" the output deriving path to take its underlying serialized derivation (the `drvPath` field), and reference that.
Since it is a derivation, it must be content-addressed
- There are no other ways a store object would end up in an input closure.
The references of a derivation in store object form always come from solely from the inputs of the derivation.
If those other derivations *also* abide by this common case (and likewise for transitive inputs), then the entire closure of the serialized derivation will be content-addressed.
### Deriving Path {#deriving-path-encoding}
@ -266,9 +248,20 @@ Here is a sketch at the proof of this:
**Experimental feature**: [`dynamic-derivations`](@docroot@/development/experimental-features.md#xp-feature-dynamic-derivations)
We can apply the same extension discussed for the abstract model to the concrete model.
Again, only the data type for Deriving Paths needs to be modified.
Derivations are the same except for using the new extended deriving path data type.
So far, we have used store paths to refer to derivations.
That works because we've implicitly assumed that all derivations are created *statically* --- created by some mechanism out of band, and then manually inserted into the store.
But what if derivations could also be created dynamically within Nix?
In other words, what if derivations could be the outputs of other derivations?
:::{.note}
In the parlance of "Build Systems à la carte", we are generalizing the Nix store layer to be a "Monadic" instead of "Applicative" build system.
:::
How should we refer to such derivations?
A deriving path works, the same as how we refer to other derivation outputs.
But what about a dynamic derivations output?
(i.e. how do we refer to the output of an output of a derivation?)
For that we need to generalize the definition of deriving path, replacing the store path used to refer to the derivation with a nested deriving path:
```idris
type OutputName = String