refactor(castore/directory): separate order logic from ClosureValidator r/8282

ClosureValidator was previously only suitable for a very narrow use case:
Validating incoming uploads, which are in leaves-to-root order.
This is because the ordering validation was hard-wired into the add()
function.
This
- Re-name ClosureValidator to DirectoryGraph, which is more suitable
  since it actually stores the Directory structs and is drained in the end.
- Move the ordering-related logic to a separate OrderValidator, which
  can be used independently.
- re-write DirectoryGraph to be a general purpose
  validator which can accept the input in both orders
  and can be drained in both orders as well.

This means the DirectoryGraph and OrderValidator can now serve
multiple new purposes:
- Validating the incoming closure on the client while downloading.
- Validating the incoming closure downloaded in a caching layer from the
  `far` cache, and re-order it for insertion into the `near` cache.

Change-Id: I2b4b226348416912d7a31935bec050e53d911b70
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11708
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: yuka <yuka@yuka.dev>

8 files changed, 640 insertions, 328 deletions

diff --git a/tvix/castore/src/directoryservice/closure_validator.rs b/tvix/castore/src/directoryservice/closure_validator.rs
deleted file mode 100644
index b9746a5a05..0000000000
--- a/tvix/castore/src/directoryservice/closure_validator.rs
+++ /dev/null
@@ -1,309 +0,0 @@
-use std::collections::{HashMap, HashSet};
-
-use bstr::ByteSlice;
-
-use petgraph::{
-    graph::{DiGraph, NodeIndex},
-    visit::{Bfs, Walker},
-};
-use tracing::instrument;
-
-use crate::{
-    proto::{self, Directory},
-    B3Digest, Error,
-};
-
-type DirectoryGraph = DiGraph<Directory, ()>;
-
-/// This can be used to validate a Directory closure (DAG of connected
-/// Directories), and their insertion order.
-///
-/// Directories need to be inserted (via `add`), in an order from the leaves to
-/// the root (DFS Post-Order).
-/// During insertion, We validate as much as we can at that time:
-///
-///  - individual validation of Directory messages
-///  - validation of insertion order (no upload of not-yet-known Directories)
-///  - validation of size fields of referred Directories
-///
-/// Internally it keeps all received Directories in a directed graph,
-/// with node weights being the Directories and edges pointing to child
-/// directories.
-///
-/// Once all Directories have been inserted, a finalize function can be
-/// called to get a (deduplicated and) validated list of directories, in
-/// insertion order.
-/// During finalize, a check for graph connectivity is performed too, to ensure
-/// there's no disconnected components, and only one root.
-#[derive(Default)]
-pub struct ClosureValidator {
-    // A directed graph, using Directory as node weight, without edge weights.
-    // Edges point from parents to children.
-    graph: DirectoryGraph,
-
-    // A lookup table from directory digest to node index.
-    digest_to_node_ix: HashMap<B3Digest, NodeIndex>,
-
-    /// Keeps track of the last-inserted directory graph node index.
-    /// On a correct insert, this will be the root node, from which the DFS post
-    /// order traversal will start from.
-    last_directory_ix: Option<NodeIndex>,
-}
-
-impl ClosureValidator {
-    /// Insert a new Directory into the closure.
-    /// Perform individual Directory validation, validation of insertion order
-    /// and size fields.
-    #[instrument(level = "trace", skip_all, fields(directory.digest=%directory.digest(), directory.size=%directory.size()), err)]
-    pub fn add(&mut self, directory: proto::Directory) -> Result<(), Error> {
-        let digest = directory.digest();
-
-        // If we already saw this node previously, it's already validated and in the graph.
-        if self.digest_to_node_ix.contains_key(&digest) {
-            return Ok(());
-        }
-
-        // Do some general validation
-        directory
-            .validate()
-            .map_err(|e| Error::InvalidRequest(e.to_string()))?;
-
-        // Ensure the directory only refers to directories which we already accepted.
-        // We lookup their node indices and add them to a HashSet.
-        let mut child_ixs = HashSet::new();
-        for dir in &directory.directories {
-            let child_digest = B3Digest::try_from(dir.digest.to_owned()).unwrap(); // validated
-
-            // Ensure the digest has already been seen
-            let child_ix = *self.digest_to_node_ix.get(&child_digest).ok_or_else(|| {
-                Error::InvalidRequest(format!(
-                    "'{}' refers to unseen child dir: {}",
-                    dir.name.as_bstr(),
-                    &child_digest
-                ))
-            })?;
-
-            // Ensure the size specified in the child node matches the directory size itself.
-            let recorded_child_size = self
-                .graph
-                .node_weight(child_ix)
-                .expect("node not found")
-                .size();
-
-            // Ensure the size specified in the child node matches our records.
-            if dir.size != recorded_child_size {
-                return Err(Error::InvalidRequest(format!(
-                    "'{}' has wrong size, specified {}, recorded {}",
-                    dir.name.as_bstr(),
-                    dir.size,
-                    recorded_child_size
-                )));
-            }
-
-            child_ixs.insert(child_ix);
-        }
-
-        // Insert node into the graph, and add edges to all children.
-        let node_ix = self.graph.add_node(directory);
-        for child_ix in child_ixs {
-            self.graph.add_edge(node_ix, child_ix, ());
-        }
-
-        // Record the mapping from digest to node_ix in our lookup table.
-        self.digest_to_node_ix.insert(digest, node_ix);
-
-        // Update last_directory_ix.
-        self.last_directory_ix = Some(node_ix);
-
-        Ok(())
-    }
-
-    /// Ensure that all inserted Directories are connected, then return a
-    /// (deduplicated) and validated list of directories, in from-leaves-to-root
-    /// order.
-    /// In case no elements have been inserted, returns an empty list.
-    #[instrument(level = "trace", skip_all, err)]
-    pub(crate) fn finalize(self) -> Result<Vec<Directory>, Error> {
-        let (graph, _) = match self.finalize_raw()? {
-            None => return Ok(vec![]),
-            Some(v) => v,
-        };
-        // Dissolve the graph, returning the nodes as a Vec.
-        // As the graph was populated in a valid DFS PostOrder, we can return
-        // nodes in that same order.
-        let (nodes, _edges) = graph.into_nodes_edges();
-        Ok(nodes.into_iter().map(|x| x.weight).collect())
-    }
-
-    /// Ensure that all inserted Directories are connected, then return a
-    /// (deduplicated) and validated list of directories, in from-root-to-leaves
-    /// order.
-    /// In case no elements have been inserted, returns an empty list.
-    #[instrument(level = "trace", skip_all, err)]
-    pub(crate) fn finalize_root_to_leaves(self) -> Result<Vec<Directory>, Error> {
-        let (graph, root) = match self.finalize_raw()? {
-            None => return Ok(vec![]),
-            Some(v) => v,
-        };
-
-        // do a BFS traversal of the graph, starting with the root node to get
-        // all nodes reachable from there.
-        let traversal = Bfs::new(&graph, root);
-
-        let order = traversal.iter(&graph).collect::<Vec<_>>();
-
-        let (nodes, _edges) = graph.into_nodes_edges();
-
-        // Convert to option, so that we can take individual nodes out without messing up the
-        // indices
-        let mut nodes = nodes.into_iter().map(Some).collect::<Vec<_>>();
-
-        Ok(order
-            .iter()
-            .map(|i| nodes[i.index()].take().unwrap().weight)
-            .collect())
-    }
-
-    /// Internal implementation of closure validation
-    #[instrument(level = "trace", skip_all, err)]
-    fn finalize_raw(self) -> Result<Option<(DirectoryGraph, NodeIndex)>, Error> {
-        // If no nodes were inserted, an empty list is returned.
-        let last_directory_ix = if let Some(x) = self.last_directory_ix {
-            x
-        } else {
-            return Ok(None);
-        };
-
-        // do a BFS traversal of the graph, starting with the root node to get
-        // (the count of) all nodes reachable from there.
-        let mut traversal = Bfs::new(&self.graph, last_directory_ix);
-
-        let mut visited_directory_count = 0;
-        #[cfg(debug_assertions)]
-        let mut visited_directory_ixs = HashSet::new();
-        #[cfg_attr(not(debug_assertions), allow(unused))]
-        while let Some(directory_ix) = traversal.next(&self.graph) {
-            #[cfg(debug_assertions)]
-            visited_directory_ixs.insert(directory_ix);
-
-            visited_directory_count += 1;
-        }
-
-        // If the number of nodes collected equals the total number of nodes in
-        // the graph, we know all nodes are connected.
-        if visited_directory_count != self.graph.node_count() {
-            // more or less exhaustive error reporting.
-            #[cfg(debug_assertions)]
-            {
-                let all_directory_ixs: HashSet<_> = self.graph.node_indices().collect();
-
-                let unvisited_directories: HashSet<_> = all_directory_ixs
-                    .difference(&visited_directory_ixs)
-                    .map(|ix| self.graph.node_weight(*ix).expect("node not found"))
-                    .collect();
-
-                return Err(Error::InvalidRequest(format!(
-                    "found {} disconnected directories: {:?}",
-                    self.graph.node_count() - visited_directory_ixs.len(),
-                    unvisited_directories
-                )));
-            }
-            #[cfg(not(debug_assertions))]
-            {
-                return Err(Error::InvalidRequest(format!(
-                    "found {} disconnected directories",
-                    self.graph.node_count() - visited_directory_count
-                )));
-            }
-        }
-
-        Ok(Some((self.graph, last_directory_ix)))
-    }
-}
-
-#[cfg(test)]
-mod tests {
-    use crate::{
-        fixtures::{DIRECTORY_A, DIRECTORY_B, DIRECTORY_C},
-        proto::{self, Directory},
-    };
-    use lazy_static::lazy_static;
-    use rstest::rstest;
-
-    lazy_static! {
-        pub static ref BROKEN_DIRECTORY : Directory = Directory {
-            symlinks: vec![proto::SymlinkNode {
-                name: "".into(), // invalid name!
-                target: "doesntmatter".into(),
-            }],
-            ..Default::default()
-        };
-
-        pub static ref BROKEN_PARENT_DIRECTORY: Directory = Directory {
-            directories: vec![proto::DirectoryNode {
-                name: "foo".into(),
-                digest: DIRECTORY_A.digest().into(),
-                size: DIRECTORY_A.size() + 42, // wrong!
-            }],
-            ..Default::default()
-        };
-    }
-
-    use super::ClosureValidator;
-
-    #[rstest]
-    /// Uploading an empty directory should succeed.
-    #[case::empty_directory(&[&*DIRECTORY_A], false, Some(vec![&*DIRECTORY_A]))]
-    /// Uploading A, then B (referring to A) should succeed.
-    #[case::simple_closure(&[&*DIRECTORY_A, &*DIRECTORY_B], false, Some(vec![&*DIRECTORY_A, &*DIRECTORY_B]))]
-    /// Uploading A, then A, then C (referring to A twice) should succeed.
-    /// We pretend to be a dumb client not deduping directories.
-    #[case::same_child(&[&*DIRECTORY_A, &*DIRECTORY_A, &*DIRECTORY_C], false, Some(vec![&*DIRECTORY_A, &*DIRECTORY_C]))]
-    /// Uploading A, then C (referring to A twice) should succeed.
-    #[case::same_child_dedup(&[&*DIRECTORY_A, &*DIRECTORY_C], false, Some(vec![&*DIRECTORY_A, &*DIRECTORY_C]))]
-    /// Uploading A, then C (referring to A twice), then B (itself referring to A) should fail during close,
-    /// as B itself would be left unconnected.
-    #[case::unconnected_node(&[&*DIRECTORY_A, &*DIRECTORY_C, &*DIRECTORY_B], false, None)]
-    /// Uploading B (referring to A) should fail immediately, because A was never uploaded.
-    #[case::dangling_pointer(&[&*DIRECTORY_B], true, None)]
-    /// Uploading a directory failing validation should fail immediately.
-    #[case::failing_validation(&[&*BROKEN_DIRECTORY], true, None)]
-    /// Uploading a directory which refers to another Directory with a wrong size should fail.
-    #[case::wrong_size_in_parent(&[&*DIRECTORY_A, &*BROKEN_PARENT_DIRECTORY], true, None)]
-    fn test_uploads(
-        #[case] directories_to_upload: &[&Directory],
-        #[case] exp_fail_upload_last: bool,
-        #[case] exp_finalize: Option<Vec<&Directory>>, // Some(_) if finalize successful, None if not.
-    ) {
-        let mut dcv = ClosureValidator::default();
-        let len_directories_to_upload = directories_to_upload.len();
-
-        for (i, d) in directories_to_upload.iter().enumerate() {
-            let resp = dcv.add((*d).clone());
-            if i == len_directories_to_upload - 1 && exp_fail_upload_last {
-                assert!(resp.is_err(), "expect last put to fail");
-
-                // We don't really care anymore what finalize() would return, as
-                // the add() failed.
-                return;
-            } else {
-                assert!(resp.is_ok(), "expect put to succeed");
-            }
-        }
-
-        // everything was uploaded successfully. Test finalize().
-        let resp = dcv.finalize();
-
-        match exp_finalize {
-            Some(directories) => {
-                assert_eq!(
-                    Vec::from_iter(directories.iter().map(|e| (*e).to_owned())),
-                    resp.expect("drain should succeed")
-                );
-            }
-            None => {
-                resp.expect_err("drain should fail");
-            }
-        }
-    }
-}
diff --git a/tvix/castore/src/directoryservice/directory_graph.rs b/tvix/castore/src/directoryservice/directory_graph.rs
new file mode 100644
index 0000000000..e6b9b16337
--- /dev/null
+++ b/tvix/castore/src/directoryservice/directory_graph.rs
@@ -0,0 +1,413 @@
+use std::collections::HashMap;
+
+use bstr::ByteSlice;
+
+use petgraph::{
+    graph::{DiGraph, NodeIndex},
+    visit::{Bfs, DfsPostOrder, EdgeRef, IntoNodeIdentifiers, Walker},
+    Direction, Incoming,
+};
+use tracing::instrument;
+
+use super::order_validator::{LeavesToRootValidator, OrderValidator, RootToLeavesValidator};
+use crate::{
+    proto::{self, Directory, DirectoryNode},
+    B3Digest,
+};
+
+#[derive(thiserror::Error, Debug)]
+pub enum Error {
+    #[error("{0}")]
+    ValidationError(String),
+}
+
+/// This can be used to validate and/or re-order a Directory closure (DAG of
+/// connected Directories), and their insertion order.
+///
+/// The DirectoryGraph is parametrized on the insertion order, and can be
+/// constructed using the Default trait, or using `with_order` if the
+/// OrderValidator needs to be customized.
+///
+/// If the user is receiving directories from canonical protobuf encoding in
+/// root-to-leaves order, and parsing them, she can call `digest_allowed`
+/// _before_ parsing the protobuf record and then add it with `add_unchecked`.
+/// All other users insert the directories via `add`, in their specified order.
+/// During insertion, we validate as much as we can at that time:
+///
+///  - individual validation of Directory messages
+///  - validation of insertion order
+///  - validation of size fields of referred Directories
+///
+/// Internally it keeps all received Directories in a directed graph,
+/// with node weights being the Directories and edges pointing to child/parent
+/// directories.
+///
+/// Once all Directories have been inserted, a validate function can be
+/// called to perform a check for graph connectivity and ensure there's no
+/// disconnected components or missing nodes.
+/// Finally, the `drain_leaves_to_root` or `drain_root_to_leaves` can be
+/// _chained_ on validate to get an iterator over the (deduplicated and)
+/// validated list of directories in either order.
+#[derive(Default)]
+pub struct DirectoryGraph<O> {
+    // A directed graph, using Directory as node weight.
+    // Edges point from parents to children.
+    //
+    // Nodes with None weigths might exist when a digest has been referred to but the directory
+    // with this digest has not yet been sent.
+    //
+    // The option in the edge weight tracks the pending validation state of the respective edge, for example if
+    // the child has not been added yet.
+    graph: DiGraph<Option<Directory>, Option<DirectoryNode>>,
+
+    // A lookup table from directory digest to node index.
+    digest_to_node_ix: HashMap<B3Digest, NodeIndex>,
+
+    order_validator: O,
+}
+
+pub struct ValidatedDirectoryGraph {
+    graph: DiGraph<Option<Directory>, Option<DirectoryNode>>,
+
+    root: Option<NodeIndex>,
+}
+
+fn check_edge(dir: &DirectoryNode, child: &Directory) -> Result<(), Error> {
+    // Ensure the size specified in the child node matches our records.
+    if dir.size != child.size() {
+        return Err(Error::ValidationError(format!(
+            "'{}' has wrong size, specified {}, recorded {}",
+            dir.name.as_bstr(),
+            dir.size,
+            child.size(),
+        )));
+    }
+    Ok(())
+}
+
+impl DirectoryGraph<LeavesToRootValidator> {
+    /// Insert a new Directory into the closure
+    #[instrument(level = "trace", skip_all, fields(directory.digest=%directory.digest(), directory.size=%directory.size()), err)]
+    pub fn add(&mut self, directory: proto::Directory) -> Result<(), Error> {
+        if !self.order_validator.add_directory(&directory) {
+            return Err(Error::ValidationError(
+                "unknown directory was referenced".into(),
+            ));
+        }
+        self.add_order_unchecked(directory)
+    }
+}
+
+impl DirectoryGraph<RootToLeavesValidator> {
+    /// If the user is parsing directories from canonical protobuf encoding, she can
+    /// call `digest_allowed` _before_ parsing the protobuf record and then add it
+    /// with `add_unchecked`.
+    pub fn digest_allowed(&self, digest: B3Digest) -> bool {
+        self.order_validator.digest_allowed(&digest)
+    }
+
+    /// Insert a new Directory into the closure
+    #[instrument(level = "trace", skip_all, fields(directory.digest=%directory.digest(), directory.size=%directory.size()), err)]
+    pub fn add(&mut self, directory: proto::Directory) -> Result<(), Error> {
+        let digest = directory.digest();
+        if !self.order_validator.digest_allowed(&digest) {
+            return Err(Error::ValidationError("unexpected digest".into()));
+        }
+        self.order_validator.add_directory_unchecked(&directory);
+        self.add_order_unchecked(directory)
+    }
+}
+
+impl<O: OrderValidator> DirectoryGraph<O> {
+    /// Customize the ordering, i.e. for pre-setting the root of the RootToLeavesValidator
+    pub fn with_order(order_validator: O) -> Self {
+        Self {
+            graph: Default::default(),
+            digest_to_node_ix: Default::default(),
+            order_validator,
+        }
+    }
+
+    /// Adds a directory which has already been confirmed to be in-order to the graph
+    pub fn add_order_unchecked(&mut self, directory: proto::Directory) -> Result<(), Error> {
+        // Do some basic validation
+        directory
+            .validate()
+            .map_err(|e| Error::ValidationError(e.to_string()))?;
+
+        let digest = directory.digest();
+
+        // Teach the graph about the existence of a node with this digest
+        let ix = *self
+            .digest_to_node_ix
+            .entry(digest)
+            .or_insert_with(|| self.graph.add_node(None));
+
+        if self.graph[ix].is_some() {
+            // The node is already in the graph, there is nothing to do here.
+            return Ok(());
+        }
+
+        // set up edges to all child directories
+        for subdir in &directory.directories {
+            let subdir_digest: B3Digest = subdir.digest.clone().try_into().unwrap();
+
+            let child_ix = *self
+                .digest_to_node_ix
+                .entry(subdir_digest)
+                .or_insert_with(|| self.graph.add_node(None));
+
+            let pending_edge_check = match &self.graph[child_ix] {
+                Some(child) => {
+                    // child is already available, validate the edge now
+                    check_edge(subdir, child)?;
+                    None
+                }
+                None => Some(subdir.clone()), // pending validation
+            };
+            self.graph.add_edge(ix, child_ix, pending_edge_check);
+        }
+
+        // validate the edges from parents to this node
+        // this collects edge ids in a Vec because there is no edges_directed_mut :'c
+        for edge_id in self
+            .graph
+            .edges_directed(ix, Direction::Incoming)
+            .map(|edge_ref| edge_ref.id())
+            .collect::<Vec<_>>()
+            .into_iter()
+        {
+            let edge_weight = self
+                .graph
+                .edge_weight_mut(edge_id)
+                .expect("edge not found")
+                .take()
+                .expect("edge is already validated");
+            check_edge(&edge_weight, &directory)?;
+        }
+
+        // finally, store the directory information in the node weight
+        self.graph[ix] = Some(directory);
+
+        Ok(())
+    }
+
+    #[instrument(level = "trace", skip_all, err)]
+    pub fn validate(self) -> Result<ValidatedDirectoryGraph, Error> {
+        // find all initial nodes (nodes without incoming edges)
+        let mut roots = self
+            .graph
+            .node_identifiers()
+            .filter(|&a| self.graph.neighbors_directed(a, Incoming).next().is_none());
+
+        let root = roots.next();
+        if roots.next().is_some() {
+            return Err(Error::ValidationError(
+                "graph has disconnected roots".into(),
+            ));
+        }
+
+        // test that the graph is complete
+        if self.graph.raw_nodes().iter().any(|n| n.weight.is_none()) {
+            return Err(Error::ValidationError("graph is incomplete".into()));
+        }
+
+        Ok(ValidatedDirectoryGraph {
+            graph: self.graph,
+            root,
+        })
+    }
+}
+
+impl ValidatedDirectoryGraph {
+    /// Return the list of directories in from-root-to-leaves order.
+    /// In case no elements have been inserted, returns an empty list.
+    ///
+    /// panics if the specified root is not in the graph
+    #[instrument(level = "trace", skip_all)]
+    pub fn drain_root_to_leaves(self) -> impl Iterator<Item = Directory> {
+        let order = match self.root {
+            Some(root) => {
+                // do a BFS traversal of the graph, starting with the root node
+                Bfs::new(&self.graph, root)
+                    .iter(&self.graph)
+                    .collect::<Vec<_>>()
+            }
+            None => vec![], // No nodes have been inserted, do not traverse
+        };
+
+        let (mut nodes, _edges) = self.graph.into_nodes_edges();
+
+        order
+            .into_iter()
+            .filter_map(move |i| nodes[i.index()].weight.take())
+    }
+
+    /// Return the list of directories in from-leaves-to-root order.
+    /// In case no elements have been inserted, returns an empty list.
+    ///
+    /// panics when the specified root is not in the graph
+    #[instrument(level = "trace", skip_all)]
+    pub fn drain_leaves_to_root(self) -> impl Iterator<Item = Directory> {
+        let order = match self.root {
+            Some(root) => {
+                // do a DFS Post-Order traversal of the graph, starting with the root node
+                DfsPostOrder::new(&self.graph, root)
+                    .iter(&self.graph)
+                    .collect::<Vec<_>>()
+            }
+            None => vec![], // No nodes have been inserted, do not traverse
+        };
+
+        let (mut nodes, _edges) = self.graph.into_nodes_edges();
+
+        order
+            .into_iter()
+            .filter_map(move |i| nodes[i.index()].weight.take())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{
+        fixtures::{DIRECTORY_A, DIRECTORY_B, DIRECTORY_C},
+        proto::{self, Directory},
+    };
+    use lazy_static::lazy_static;
+    use rstest::rstest;
+
+    lazy_static! {
+        pub static ref BROKEN_DIRECTORY : Directory = Directory {
+            symlinks: vec![proto::SymlinkNode {
+                name: "".into(), // invalid name!
+                target: "doesntmatter".into(),
+            }],
+            ..Default::default()
+        };
+
+        pub static ref BROKEN_PARENT_DIRECTORY: Directory = Directory {
+            directories: vec![proto::DirectoryNode {
+                name: "foo".into(),
+                digest: DIRECTORY_A.digest().into(),
+                size: DIRECTORY_A.size() + 42, // wrong!
+            }],
+            ..Default::default()
+        };
+    }
+
+    use super::{DirectoryGraph, LeavesToRootValidator, RootToLeavesValidator};
+
+    #[rstest]
+    /// Uploading an empty directory should succeed.
+    #[case::empty_directory(&[&*DIRECTORY_A], false, Some(vec![&*DIRECTORY_A]))]
+    /// Uploading A, then B (referring to A) should succeed.
+    #[case::simple_closure(&[&*DIRECTORY_A, &*DIRECTORY_B], false, Some(vec![&*DIRECTORY_A, &*DIRECTORY_B]))]
+    /// Uploading A, then A, then C (referring to A twice) should succeed.
+    /// We pretend to be a dumb client not deduping directories.
+    #[case::same_child(&[&*DIRECTORY_A, &*DIRECTORY_A, &*DIRECTORY_C], false, Some(vec![&*DIRECTORY_A, &*DIRECTORY_C]))]
+    /// Uploading A, then C (referring to A twice) should succeed.
+    #[case::same_child_dedup(&[&*DIRECTORY_A, &*DIRECTORY_C], false, Some(vec![&*DIRECTORY_A, &*DIRECTORY_C]))]
+    /// Uploading A, then C (referring to A twice), then B (itself referring to A) should fail during close,
+    /// as B itself would be left unconnected.
+    #[case::unconnected_node(&[&*DIRECTORY_A, &*DIRECTORY_C, &*DIRECTORY_B], false, None)]
+    /// Uploading B (referring to A) should fail immediately, because A was never uploaded.
+    #[case::dangling_pointer(&[&*DIRECTORY_B], true, None)]
+    /// Uploading a directory failing validation should fail immediately.
+    #[case::failing_validation(&[&*BROKEN_DIRECTORY], true, None)]
+    /// Uploading a directory which refers to another Directory with a wrong size should fail.
+    #[case::wrong_size_in_parent(&[&*DIRECTORY_A, &*BROKEN_PARENT_DIRECTORY], true, None)]
+    fn test_uploads(
+        #[case] directories_to_upload: &[&Directory],
+        #[case] exp_fail_upload_last: bool,
+        #[case] exp_finalize: Option<Vec<&Directory>>, // Some(_) if finalize successful, None if not.
+    ) {
+        let mut dcv = DirectoryGraph::<LeavesToRootValidator>::default();
+        let len_directories_to_upload = directories_to_upload.len();
+
+        for (i, d) in directories_to_upload.iter().enumerate() {
+            let resp = dcv.add((*d).clone());
+            if i == len_directories_to_upload - 1 && exp_fail_upload_last {
+                assert!(resp.is_err(), "expect last put to fail");
+
+                // We don't really care anymore what finalize() would return, as
+                // the add() failed.
+                return;
+            } else {
+                assert!(resp.is_ok(), "expect put to succeed");
+            }
+        }
+
+        // everything was uploaded successfully. Test finalize().
+        let resp = dcv
+            .validate()
+            .map(|validated| validated.drain_leaves_to_root().collect::<Vec<_>>());
+
+        match exp_finalize {
+            Some(directories) => {
+                assert_eq!(
+                    Vec::from_iter(directories.iter().map(|e| (*e).to_owned())),
+                    resp.expect("drain should succeed")
+                );
+            }
+            None => {
+                resp.expect_err("drain should fail");
+            }
+        }
+    }
+
+    #[rstest]
+    /// Downloading an empty directory should succeed.
+    #[case::empty_directory(&*DIRECTORY_A, &[&*DIRECTORY_A], false, Some(vec![&*DIRECTORY_A]))]
+    /// Downlading B, then A (referenced by B) should succeed.
+    #[case::simple_closure(&*DIRECTORY_B, &[&*DIRECTORY_B, &*DIRECTORY_A], false, Some(vec![&*DIRECTORY_A, &*DIRECTORY_B]))]
+    /// Downloading C (referring to A twice), then A should succeed.
+    #[case::same_child_dedup(&*DIRECTORY_C, &[&*DIRECTORY_C, &*DIRECTORY_A], false, Some(vec![&*DIRECTORY_A, &*DIRECTORY_C]))]
+    /// Downloading C, then B (both referring to A but not referring to each other) should fail immediately as B has no connection to C (the root)
+    #[case::unconnected_node(&*DIRECTORY_C, &[&*DIRECTORY_C, &*DIRECTORY_B], true, None)]
+    /// Downloading B (specified as the root) but receiving A instead should fail immediately, because A has no connection to B (the root).
+    #[case::dangling_pointer(&*DIRECTORY_B, &[&*DIRECTORY_A], true, None)]
+    /// Downloading a directory failing validation should fail immediately.
+    #[case::failing_validation(&*BROKEN_DIRECTORY, &[&*BROKEN_DIRECTORY], true, None)]
+    /// Downloading a directory which refers to another Directory with a wrong size should fail.
+    #[case::wrong_size_in_parent(&*BROKEN_PARENT_DIRECTORY, &[&*BROKEN_PARENT_DIRECTORY, &*DIRECTORY_A], true, None)]
+    fn test_downloads(
+        #[case] root: &Directory,
+        #[case] directories_to_upload: &[&Directory],
+        #[case] exp_fail_upload_last: bool,
+        #[case] exp_finalize: Option<Vec<&Directory>>, // Some(_) if finalize successful, None if not.
+    ) {
+        let mut dcv =
+            DirectoryGraph::with_order(RootToLeavesValidator::new_with_root_digest(root.digest()));
+        let len_directories_to_upload = directories_to_upload.len();
+
+        for (i, d) in directories_to_upload.iter().enumerate() {
+            let resp = dcv.add((*d).clone());
+            if i == len_directories_to_upload - 1 && exp_fail_upload_last {
+                assert!(resp.is_err(), "expect last put to fail");
+
+                // We don't really care anymore what finalize() would return, as
+                // the add() failed.
+                return;
+            } else {
+                assert!(resp.is_ok(), "expect put to succeed");
+            }
+        }
+
+        // everything was uploaded successfully. Test finalize().
+        let resp = dcv
+            .validate()
+            .map(|validated| validated.drain_leaves_to_root().collect::<Vec<_>>());
+
+        match exp_finalize {
+            Some(directories) => {
+                assert_eq!(
+                    Vec::from_iter(directories.iter().map(|e| (*e).to_owned())),
+                    resp.expect("drain should succeed")
+                );
+            }
+            None => {
+                resp.expect_err("drain should fail");
+            }
+        }
+    }
+}
diff --git a/tvix/castore/src/directoryservice/mod.rs b/tvix/castore/src/directoryservice/mod.rs
index 3f180ef162..0d717a8a3f 100644
--- a/tvix/castore/src/directoryservice/mod.rs
+++ b/tvix/castore/src/directoryservice/mod.rs
@@ -2,11 +2,12 @@ use crate::{proto, B3Digest, Error};
 use futures::stream::BoxStream;
 use tonic::async_trait;
 
-mod closure_validator;
+mod directory_graph;
 mod from_addr;
 mod grpc;
 mod memory;
 mod object_store;
+mod order_validator;
 mod simple_putter;
 mod sled;
 #[cfg(test)]
@@ -14,11 +15,12 @@ pub mod tests;
 mod traverse;
 mod utils;
 
-pub use self::closure_validator::ClosureValidator;
+pub use self::directory_graph::DirectoryGraph;
 pub use self::from_addr::from_addr;
 pub use self::grpc::GRPCDirectoryService;
 pub use self::memory::MemoryDirectoryService;
 pub use self::object_store::ObjectStoreDirectoryService;
+pub use self::order_validator::{LeavesToRootValidator, OrderValidator, RootToLeavesValidator};
 pub use self::simple_putter::SimplePutter;
 pub use self::sled::SledDirectoryService;
 pub use self::traverse::descend_to;
diff --git a/tvix/castore/src/directoryservice/object_store.rs b/tvix/castore/src/directoryservice/object_store.rs
index 64ce335edb..90e53f9286 100644
--- a/tvix/castore/src/directoryservice/object_store.rs
+++ b/tvix/castore/src/directoryservice/object_store.rs
@@ -16,7 +16,7 @@ use tonic::async_trait;
 use tracing::{instrument, trace, warn, Level};
 use url::Url;
 
-use super::{ClosureValidator, DirectoryPutter, DirectoryService};
+use super::{DirectoryGraph, DirectoryPutter, DirectoryService, LeavesToRootValidator};
 use crate::{proto, B3Digest, Error};
 
 /// Stores directory closures in an object store.
@@ -177,7 +177,7 @@ struct ObjectStoreDirectoryPutter {
     object_store: Arc<dyn ObjectStore>,
     base_path: Path,
 
-    directory_validator: Option<ClosureValidator>,
+    directory_validator: Option<DirectoryGraph<LeavesToRootValidator>>,
 }
 
 impl ObjectStoreDirectoryPutter {
@@ -197,7 +197,9 @@ impl DirectoryPutter for ObjectStoreDirectoryPutter {
         match self.directory_validator {
             None => return Err(Error::StorageError("already closed".to_string())),
             Some(ref mut validator) => {
-                validator.add(directory)?;
+                validator
+                    .add(directory)
+                    .map_err(|e| Error::StorageError(e.to_string()))?;
             }
         }
 
@@ -214,7 +216,11 @@ impl DirectoryPutter for ObjectStoreDirectoryPutter {
         // retrieve the validated directories.
         // It is important that they are in topological order (root first),
         // since that's how we want to retrieve them from the object store in the end.
-        let directories = validator.finalize_root_to_leaves()?;
+        let directories = validator
+            .validate()
+            .map_err(|e| Error::StorageError(e.to_string()))?
+            .drain_root_to_leaves()
+            .collect::<Vec<_>>();
 
         // Get the root digest
         let root_digest = directories
diff --git a/tvix/castore/src/directoryservice/order_validator.rs b/tvix/castore/src/directoryservice/order_validator.rs
new file mode 100644
index 0000000000..6045f5d241
--- /dev/null
+++ b/tvix/castore/src/directoryservice/order_validator.rs
@@ -0,0 +1,181 @@
+use std::collections::HashSet;
+use tracing::warn;
+
+use crate::{proto::Directory, B3Digest};
+
+pub trait OrderValidator {
+    /// Update the order validator's state with the directory
+    /// Returns whether the directory was accepted
+    fn add_directory(&mut self, directory: &Directory) -> bool;
+}
+
+#[derive(Default)]
+/// Validates that newly introduced directories are already referenced from
+/// the root via existing directories.
+/// Commonly used when _receiving_ a directory closure _from_ a store.
+pub struct RootToLeavesValidator {
+    /// Only used to remember the root node, not for validation
+    expected_digests: HashSet<B3Digest>,
+}
+
+impl RootToLeavesValidator {
+    /// Use to validate the root digest of the closure upon receiving the first
+    /// directory.
+    pub fn new_with_root_digest(root_digest: B3Digest) -> Self {
+        let mut this = Self::default();
+        this.expected_digests.insert(root_digest);
+        this
+    }
+
+    /// Checks if a directory is in-order based on its digest.
+    ///
+    /// Particularly useful when receiving directories in canonical protobuf
+    /// encoding, so that directories not connected to the root can be rejected
+    /// without parsing.
+    ///
+    /// After parsing, the directory must be passed to `add_directory_unchecked`
+    /// to add its children to the list of expected digests.
+    pub fn digest_allowed(&self, digest: &B3Digest) -> bool {
+        self.expected_digests.is_empty() // we don't know the root node; allow any
+            || self.expected_digests.contains(digest)
+    }
+
+    /// Update the order validator's state with the directory
+    pub fn add_directory_unchecked(&mut self, directory: &Directory) {
+        // No initial root was specified and this is the first directory
+        if self.expected_digests.is_empty() {
+            self.expected_digests.insert(directory.digest());
+        }
+
+        for subdir in &directory.directories {
+            // Allow the children to appear next
+            let subdir_digest = subdir.digest.clone().try_into().unwrap();
+            self.expected_digests.insert(subdir_digest);
+        }
+    }
+}
+
+impl OrderValidator for RootToLeavesValidator {
+    fn add_directory(&mut self, directory: &Directory) -> bool {
+        if !self.digest_allowed(&directory.digest()) {
+            return false;
+        }
+        self.add_directory_unchecked(directory);
+        true
+    }
+}
+
+#[derive(Default)]
+/// Validates that newly uploaded directories only reference directories which
+/// have already been introduced.
+/// Commonly used when _uploading_ a directory closure _to_ a store.
+pub struct LeavesToRootValidator {
+    /// This is empty in the beginning, and gets filled as leaves and intermediates are
+    /// inserted
+    allowed_references: HashSet<B3Digest>,
+}
+
+impl OrderValidator for LeavesToRootValidator {
+    fn add_directory(&mut self, directory: &Directory) -> bool {
+        let digest = directory.digest();
+
+        for subdir in &directory.directories {
+            let subdir_digest = subdir.digest.clone().try_into().unwrap(); // this has been validated in validate_directory()
+            if !self.allowed_references.contains(&subdir_digest) {
+                warn!(
+                    directory.digest = %digest,
+                    subdirectory.digest = %subdir_digest,
+                    "unexpected directory reference"
+                );
+                return false;
+            }
+        }
+
+        self.allowed_references.insert(digest.clone());
+
+        true
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{LeavesToRootValidator, RootToLeavesValidator};
+    use crate::directoryservice::order_validator::OrderValidator;
+    use crate::fixtures::{DIRECTORY_A, DIRECTORY_B, DIRECTORY_C};
+    use crate::proto::Directory;
+    use rstest::rstest;
+
+    #[rstest]
+    /// Uploading an empty directory should succeed.
+    #[case::empty_directory(&[&*DIRECTORY_A], false)]
+    /// Uploading A, then B (referring to A) should succeed.
+    #[case::simple_closure(&[&*DIRECTORY_A, &*DIRECTORY_B], false)]
+    /// Uploading A, then A, then C (referring to A twice) should succeed.
+    /// We pretend to be a dumb client not deduping directories.
+    #[case::same_child(&[&*DIRECTORY_A, &*DIRECTORY_A, &*DIRECTORY_C], false)]
+    /// Uploading A, then C (referring to A twice) should succeed.
+    #[case::same_child_dedup(&[&*DIRECTORY_A, &*DIRECTORY_C], false)]
+    /// Uploading A, then C (referring to A twice), then B (itself referring to A) should fail during close,
+    /// as B itself would be left unconnected.
+    #[case::unconnected_node(&[&*DIRECTORY_A, &*DIRECTORY_C, &*DIRECTORY_B], false)]
+    /// Uploading B (referring to A) should fail immediately, because A was never uploaded.
+    #[case::dangling_pointer(&[&*DIRECTORY_B], true)]
+    fn leaves_to_root(
+        #[case] directories_to_upload: &[&Directory],
+        #[case] exp_fail_upload_last: bool,
+    ) {
+        let mut validator = LeavesToRootValidator::default();
+        let len_directories_to_upload = directories_to_upload.len();
+
+        for (i, d) in directories_to_upload.iter().enumerate() {
+            let resp = validator.add_directory(d);
+            if i == len_directories_to_upload - 1 && exp_fail_upload_last {
+                assert!(!resp, "expect last put to fail");
+
+                // We don't really care anymore what finalize() would return, as
+                // the add() failed.
+                return;
+            } else {
+                assert!(resp, "expect put to succeed");
+            }
+        }
+    }
+
+    #[rstest]
+    /// Downloading an empty directory should succeed.
+    #[case::empty_directory(&*DIRECTORY_A, &[&*DIRECTORY_A], false)]
+    /// Downlading B, then A (referenced by B) should succeed.
+    #[case::simple_closure(&*DIRECTORY_B, &[&*DIRECTORY_B, &*DIRECTORY_A], false)]
+    /// Downloading C (referring to A twice), then A should succeed.
+    #[case::same_child_dedup(&*DIRECTORY_C, &[&*DIRECTORY_C, &*DIRECTORY_A], false)]
+    /// Downloading C, then B (both referring to A but not referring to each other) should fail immediately as B has no connection to C (the root)
+    #[case::unconnected_node(&*DIRECTORY_C, &[&*DIRECTORY_C, &*DIRECTORY_B], true)]
+    /// Downloading B (specified as the root) but receiving A instead should fail immediately, because A has no connection to B (the root).
+    #[case::dangling_pointer(&*DIRECTORY_B, &[&*DIRECTORY_A], true)]
+    fn root_to_leaves(
+        #[case] root: &Directory,
+        #[case] directories_to_upload: &[&Directory],
+        #[case] exp_fail_upload_last: bool,
+    ) {
+        let mut validator = RootToLeavesValidator::new_with_root_digest(root.digest());
+        let len_directories_to_upload = directories_to_upload.len();
+
+        for (i, d) in directories_to_upload.iter().enumerate() {
+            let resp1 = validator.digest_allowed(&d.digest());
+            let resp = validator.add_directory(d);
+            assert_eq!(
+                resp1, resp,
+                "digest_allowed should return the same value as add_directory"
+            );
+            if i == len_directories_to_upload - 1 && exp_fail_upload_last {
+                assert!(!resp, "expect last put to fail");
+
+                // We don't really care anymore what finalize() would return, as
+                // the add() failed.
+                return;
+            } else {
+                assert!(resp, "expect put to succeed");
+            }
+        }
+    }
+}
diff --git a/tvix/castore/src/directoryservice/simple_putter.rs b/tvix/castore/src/directoryservice/simple_putter.rs
index 25617ebcac..dc54e3d11d 100644
--- a/tvix/castore/src/directoryservice/simple_putter.rs
+++ b/tvix/castore/src/directoryservice/simple_putter.rs
@@ -1,6 +1,6 @@
-use super::ClosureValidator;
 use super::DirectoryPutter;
 use super::DirectoryService;
+use super::{DirectoryGraph, LeavesToRootValidator};
 use crate::proto;
 use crate::B3Digest;
 use crate::Error;
@@ -14,7 +14,7 @@ use tracing::warn;
 pub struct SimplePutter<DS: DirectoryService> {
     directory_service: DS,
 
-    directory_validator: Option<ClosureValidator>,
+    directory_validator: Option<DirectoryGraph<LeavesToRootValidator>>,
 }
 
 impl<DS: DirectoryService> SimplePutter<DS> {
@@ -33,7 +33,9 @@ impl<DS: DirectoryService + 'static> DirectoryPutter for SimplePutter<DS> {
         match self.directory_validator {
             None => return Err(Error::StorageError("already closed".to_string())),
             Some(ref mut validator) => {
-                validator.add(directory)?;
+                validator
+                    .add(directory)
+                    .map_err(|e| Error::StorageError(e.to_string()))?;
             }
         }
 
@@ -46,7 +48,11 @@ impl<DS: DirectoryService + 'static> DirectoryPutter for SimplePutter<DS> {
             None => Err(Error::InvalidRequest("already closed".to_string())),
             Some(validator) => {
                 // retrieve the validated directories.
-                let directories = validator.finalize()?;
+                let directories = validator
+                    .validate()
+                    .map_err(|e| Error::StorageError(e.to_string()))?
+                    .drain_leaves_to_root()
+                    .collect::<Vec<_>>();
 
                 // Get the root digest, which is at the end (cf. insertion order)
                 let root_digest = directories
diff --git a/tvix/castore/src/directoryservice/sled.rs b/tvix/castore/src/directoryservice/sled.rs
index 9490a49c00..bd98ed6b1e 100644
--- a/tvix/castore/src/directoryservice/sled.rs
+++ b/tvix/castore/src/directoryservice/sled.rs
@@ -8,7 +8,7 @@ use tonic::async_trait;
 use tracing::{instrument, warn};
 
 use super::utils::traverse_directory;
-use super::{ClosureValidator, DirectoryPutter, DirectoryService};
+use super::{DirectoryGraph, DirectoryPutter, DirectoryService, LeavesToRootValidator};
 
 #[derive(Clone)]
 pub struct SledDirectoryService {
@@ -135,7 +135,7 @@ pub struct SledDirectoryPutter {
 
     /// The directories (inside the directory validator) that we insert later,
     /// or None, if they were already inserted.
-    directory_validator: Option<ClosureValidator>,
+    directory_validator: Option<DirectoryGraph<LeavesToRootValidator>>,
 }
 
 #[async_trait]
@@ -145,7 +145,9 @@ impl DirectoryPutter for SledDirectoryPutter {
         match self.directory_validator {
             None => return Err(Error::StorageError("already closed".to_string())),
             Some(ref mut validator) => {
-                validator.add(directory)?;
+                validator
+                    .add(directory)
+                    .map_err(|e| Error::StorageError(e.to_string()))?;
             }
         }
 
@@ -162,7 +164,11 @@ impl DirectoryPutter for SledDirectoryPutter {
                     let tree = self.tree.clone();
                     move || {
                         // retrieve the validated directories.
-                        let directories = validator.finalize()?;
+                        let directories = validator
+                            .validate()
+                            .map_err(|e| Error::StorageError(e.to_string()))?
+                            .drain_leaves_to_root()
+                            .collect::<Vec<_>>();
 
                         // Get the root digest, which is at the end (cf. insertion order)
                         let root_digest = directories
diff --git a/tvix/castore/src/proto/grpc_directoryservice_wrapper.rs b/tvix/castore/src/proto/grpc_directoryservice_wrapper.rs
index 5c1428690c..ce1d2bcd24 100644
--- a/tvix/castore/src/proto/grpc_directoryservice_wrapper.rs
+++ b/tvix/castore/src/proto/grpc_directoryservice_wrapper.rs
@@ -1,4 +1,5 @@
-use crate::directoryservice::ClosureValidator;
+use crate::directoryservice::DirectoryGraph;
+use crate::directoryservice::LeavesToRootValidator;
 use crate::proto;
 use crate::{directoryservice::DirectoryService, B3Digest};
 use futures::stream::BoxStream;
@@ -78,14 +79,20 @@ where
     ) -> Result<Response<proto::PutDirectoryResponse>, Status> {
         let mut req_inner = request.into_inner();
 
-        // We put all Directory messages we receive into ClosureValidator first.
-        let mut validator = ClosureValidator::default();
+        // We put all Directory messages we receive into DirectoryGraph.
+        let mut validator = DirectoryGraph::<LeavesToRootValidator>::default();
         while let Some(directory) = req_inner.message().await? {
-            validator.add(directory)?;
+            validator
+                .add(directory)
+                .map_err(|e| tonic::Status::new(tonic::Code::Internal, e.to_string()))?;
         }
 
         // drain, which validates connectivity too.
-        let directories = validator.finalize()?;
+        let directories = validator
+            .validate()
+            .map_err(|e| tonic::Status::new(tonic::Code::Internal, e.to_string()))?
+            .drain_leaves_to_root()
+            .collect::<Vec<_>>();
 
         let mut directory_putter = self.directory_service.put_multiple_start();
         for directory in directories {