feat(tvix/glue): contextful derivation

We calculate the input context by performing the union of context
over all input of the derivation.

Then, we just pass the rest of it to the remaining machinery.

Finally, we re-emit an `outPath` and a `drvPath` containing the expected
contexts.

Change-Id: I74905fb258b5bee8b08d1208c9eb87f51b92a890
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10436
Autosubmit: raitobezarius <tvl@lahfa.xyz>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/glue/src/known_paths.rs

@@ -11,12 +11,9 @@
 //! Please see //tvix/eval/docs/build-references.md for more
 //! information.
 
-use crate::refscan::{ReferenceScanner, STORE_PATH_LEN};
+use crate::refscan::STORE_PATH_LEN;
 use nix_compat::nixhash::NixHash;
-use std::{
-    collections::{hash_map, BTreeSet, HashMap},
-    ops::Index,
-};
+use std::collections::{BTreeSet, HashMap};
 
 #[derive(Debug, PartialEq)]
 pub enum PathKind {
@@ -36,12 +33,6 @@ pub struct KnownPath {
     pub kind: PathKind,
 }
 
-impl KnownPath {
-    fn new(path: String, kind: PathKind) -> Self {
-        KnownPath { path, kind }
-    }
-}
-
 /// Internal struct to prevent accidental leaks of the truncated path
 /// names.
 #[repr(transparent)]
@@ -54,6 +45,12 @@ impl From<&str> for PathName {
     }
 }
 
+impl From<String> for PathName {
+    fn from(s: String) -> Self {
+        s.as_str().into()
+    }
+}
+
 /// This instance is required to pass PathName instances as needles to
 /// the reference scanner.
 impl AsRef<[u8]> for PathName {
@@ -64,125 +61,13 @@ impl AsRef<[u8]> for PathName {
 
 #[derive(Debug, Default)]
 pub struct KnownPaths {
-    /// All known paths, keyed by a truncated version of their store
-    /// path used for reference scanning.
-    paths: HashMap<PathName, KnownPath>,
-
     /// All known derivation or FOD hashes.
     ///
     /// Keys are derivation paths, values is the NixHash.
     derivation_or_fod_hashes: HashMap<String, NixHash>,
 }
 
-impl Index<&PathName> for KnownPaths {
-    type Output = KnownPath;
-
-    fn index(&self, index: &PathName) -> &Self::Output {
-        &self.paths[index]
-    }
-}
-
 impl KnownPaths {
-    fn insert_path(&mut self, path: String, path_kind: PathKind) {
-        match self.paths.entry(path.as_str().into()) {
-            hash_map::Entry::Vacant(entry) => {
-                entry.insert(KnownPath::new(path, path_kind));
-            }
-
-            hash_map::Entry::Occupied(mut entry) => {
-                match (path_kind, &mut entry.get_mut().kind) {
-                    // These variant combinations require no "merging action".
-                    (PathKind::Plain, PathKind::Plain) => (),
-
-                    #[allow(unused_variables)]
-                    (
-                        PathKind::Output {
-                            name: name1,
-                            derivation: drv1,
-                        },
-                        PathKind::Output {
-                            name: ref name2,
-                            derivation: ref drv2,
-                        },
-                    ) => {
-                        #[cfg(debug_assertions)]
-                        {
-                            if &name1 != name2 {
-                                panic!(
-                                    "inserted path {} with two different names: {} and {}",
-                                    path, name1, name2
-                                );
-                            }
-                            if &drv1 != drv2 {
-                                panic!(
-                                    "inserted path {} with two different derivations: {} and {}",
-                                    path, drv1, drv2
-                                );
-                            }
-                        }
-                    }
-
-                    (
-                        PathKind::Derivation { output_names: new },
-                        PathKind::Derivation {
-                            output_names: ref mut old,
-                        },
-                    ) => {
-                        old.extend(new);
-                    }
-
-                    _ => panic!(
-                        "path '{}' inserted twice with different types",
-                        entry.key().0
-                    ),
-                };
-            }
-        };
-    }
-
-    /// Mark a plain path as known.
-    pub fn plain<S: ToString>(&mut self, path: S) {
-        self.insert_path(path.to_string(), PathKind::Plain);
-    }
-
-    /// Mark a derivation as known.
-    pub fn drv<P: ToString, O: ToString>(&mut self, path: P, outputs: &[O]) {
-        self.insert_path(
-            path.to_string(),
-            PathKind::Derivation {
-                output_names: outputs.iter().map(ToString::to_string).collect(),
-            },
-        );
-    }
-
-    /// Mark a derivation output path as known.
-    pub fn output<P: ToString, N: ToString, D: ToString>(
-        &mut self,
-        output_path: P,
-        name: N,
-        drv_path: D,
-    ) {
-        self.insert_path(
-            output_path.to_string(),
-            PathKind::Output {
-                name: name.to_string(),
-                derivation: drv_path.to_string(),
-            },
-        );
-    }
-
-    /// Checks whether there are any known paths. If not, a reference
-    /// scanner can not be created.
-    pub fn is_empty(&self) -> bool {
-        self.paths.is_empty()
-    }
-
-    /// Create a reference scanner from the current set of known paths.
-    pub fn reference_scanner(&self) -> ReferenceScanner<PathName> {
-        let candidates = self.paths.keys().map(Clone::clone).collect();
-        ReferenceScanner::new(candidates)
-    }
-
     /// Fetch the opaque "hash derivation modulo" for a given derivation path.
     pub fn get_hash_derivation_modulo(&self, drv_path: &str) -> NixHash {
         // TODO: we rely on an invariant that things *should* have