feat(tvix/store): add NARCalculationService trait and impl

This adds a NARCalculationService trait, which will take a root node,
and provide a proto::CalculateNarResponse in return.

It also adds a NonCachingNARCalculationService implementation, that will
simply always render the NAR in memory to calculate the size and sha256,
without any caching.

Change-Id: Id1ffb18559212fa6001f70f2634bbc3dfd0aa343
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8096
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/store/src/nar/mod.rs

@@ -1,8 +1,11 @@
+use crate::proto;
 use data_encoding::BASE64;
 use thiserror::Error;
 
+mod non_caching_calculation_service;
 mod renderer;
 
+pub use non_caching_calculation_service::NonCachingNARCalculationService;
 pub use renderer::NARRenderer;
 
 /// Errors that can encounter while rendering NARs.
@@ -23,3 +26,11 @@ pub enum RenderError {
     #[error("failure using the NAR writer: {0}")]
     NARWriterError(std::io::Error),
 }
+
+/// The base trait for something calculating NARs, and returning their size and sha256.
+pub trait NARCalculationService {
+    fn calculate_nar(
+        &self,
+        root_node: proto::node::Node,
+    ) -> Result<proto::CalculateNarResponse, RenderError>;
+}