1 files changed, 186 insertions, 0 deletions
diff --git a/tvix/glue/src/known_paths.rs b/tvix/glue/src/known_paths.rs
new file mode 100644
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+++ b/tvix/glue/src/known_paths.rs
@@ -0,0 +1,186 @@
+//! This module implements logic required for persisting known paths
+//! during an evaluation.
+//!
+//! Tvix needs to be able to keep track of each Nix store path that it
+//! knows about during the scope of a single evaluation and its
+//! related builds.
+//!
+//! This data is required to scan derivation inputs for the build
+//! references (the "build closure") that they make use of.
+//!
+//! Please see //tvix/eval/docs/build-references.md for more
+//! information.
+
+use crate::refscan::{ReferenceScanner, STORE_PATH_LEN};
+use nix_compat::nixhash::NixHash;
+use std::{
+    collections::{hash_map, BTreeSet, HashMap},
+    ops::Index,
+};
+
+#[derive(Debug, PartialEq)]
+pub enum PathKind {
+    /// A literal derivation (`.drv`-file), and the *names* of its outputs.
+    Derivation { output_names: BTreeSet<String> },
+
+    /// An output of a derivation, its name, and the path of its derivation.
+    Output { name: String, derivation: String },
+
+    /// A plain store path (e.g. source files copied to the store).
+    Plain,
+}
+
+#[derive(Debug, PartialEq)]
+pub struct KnownPath {
+    pub path: String,
+    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)]
+#[derive(Clone, Debug, Default, PartialEq, PartialOrd, Ord, Eq, Hash)]
+pub struct PathName(String);
+
+impl From<&str> for PathName {
+    fn from(s: &str) -> Self {
+        PathName(s[..STORE_PATH_LEN].to_string())
+    }
+}
+
+/// This instance is required to pass PathName instances as needles to
+/// the reference scanner.
+impl AsRef<[u8]> for PathName {
+    fn as_ref(&self) -> &[u8] {
+        self.0.as_ref()
+    }
+}
+
+#[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) => (),
+                    (PathKind::Output { .. }, PathKind::Output { .. }) => (),
+
+                    (
+                        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
+        // been calculated if we get this far.
+        self.derivation_or_fod_hashes[drv_path].clone()
+    }
+
+    pub fn add_hash_derivation_modulo<D: ToString>(
+        &mut self,
+        drv: D,
+        hash_derivation_modulo: &NixHash,
+    ) {
+        #[allow(unused_variables)] // assertions on this only compiled in debug builds
+        let old = self
+            .derivation_or_fod_hashes
+            .insert(drv.to_string(), hash_derivation_modulo.to_owned());
+
+        #[cfg(debug_assertions)]
+        {
+            if let Some(old) = old {
+                debug_assert!(
+                    old == *hash_derivation_modulo,
+                    "hash derivation modulo for a given derivation should always be calculated the same"
+                );
+            }
+        }
+    }
+}