feat(tvix/glue): Implement builtins.storePath

This one's relatively simple - we just check if the store path exists,
and if it does we make a new contextful string containing the store path
as its only context element.

Automatic testing seems tricky for this (I think?) so I tested it
manually:

tvix-repl> builtins.storePath /nix/store/yn46i4xx5alh7gs6fpkxk430i34rp2q9-hello-2.12.1
=> "/nix/store/yn46i4xx5alh7gs6fpkxk430i34rp2q9-hello-2.12.1" :: string

Change-Id: I8a0d9726e4102ab872c53c2419679c2c855a5a18
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11696
Tested-by: BuildkiteCI
Autosubmit: aspen <root@gws.fyi>
Reviewed-by: flokli <flokli@flokli.de>

tvix/glue/src/builtins/errors.rs

@@ -4,7 +4,7 @@ use nix_compat::{
     store_path::BuildStorePathError,
 };
 use reqwest::Url;
-use std::rc::Rc;
+use std::{path::PathBuf, rc::Rc};
 use thiserror::Error;
 use tvix_castore::import;
 
@@ -65,8 +65,12 @@ pub enum FetcherError {
 pub enum ImportError {
     #[error("non-file '{0}' cannot be imported in 'flat' mode")]
     FlatImportOfNonFile(String),
+
     #[error("hash mismatch at ingestion of '{0}', expected: '{1}', got: '{2}'")]
     HashMismatch(String, NixHash, NixHash),
+
+    #[error("path '{}' is not in the Nix store", .0.display())]
+    PathNotInStore(PathBuf),
 }
 
 impl From<ImportError> for tvix_eval::ErrorKind {

tvix/glue/src/builtins/import.rs

@@ -104,11 +104,13 @@ async fn filtered_ingest(
 
 #[builtins(state = "Rc<TvixStoreIO>")]
 mod import_builtins {
+    use std::os::unix::ffi::OsStrExt;
     use std::rc::Rc;
 
     use super::*;
 
     use nix_compat::nixhash::{CAHash, NixHash};
+    use nix_compat::store_path::StorePath;
     use tvix_eval::generators::Gen;
     use tvix_eval::{generators::GenCo, ErrorKind, Value};
     use tvix_eval::{NixContextElement, NixString};
@@ -280,6 +282,44 @@ mod import_builtins {
                 .into(),
         )
     }
+
+    #[builtin("storePath")]
+    async fn builtin_store_path(
+        state: Rc<TvixStoreIO>,
+        co: GenCo,
+        path: Value,
+    ) -> Result<Value, ErrorKind> {
+        let p = std::str::from_utf8(match &path {
+            Value::String(s) => s.as_bytes(),
+            Value::Path(p) => p.as_os_str().as_bytes(),
+            _ => {
+                return Err(ErrorKind::TypeError {
+                    expected: "string or path",
+                    actual: path.type_of(),
+                })
+            }
+        })?;
+
+        let path_exists = if let Ok((store_path, sub_path)) = StorePath::from_absolute_path_full(p)
+        {
+            if !sub_path.as_os_str().is_empty() {
+                false
+            } else {
+                state.store_path_exists(store_path.as_ref()).await?
+            }
+        } else {
+            false
+        };
+
+        if !path_exists {
+            return Err(ImportError::PathNotInStore(p.into()).into());
+        }
+
+        Ok(Value::String(NixString::new_context_from(
+            [NixContextElement::Plain(p.into())].into(),
+            p,
+        )))
+    }
 }
 
 pub use import_builtins::builtins as import_builtins;