Prune unreachable packages from lockfile

crates/uv-resolver/src/lock/mod.rs

@@ -11,6 +11,7 @@ use itertools::Itertools;
 use petgraph::visit::EdgeRef;
 use rustc_hash::{FxHashMap, FxHashSet};
 use toml_edit::{value, Array, ArrayOfTables, InlineTable, Item, Table, Value};
+use tracing::debug;
 use url::Url;
 
 use cache_key::RepositoryUrl;
@@ -94,6 +95,10 @@ impl Lock {
             if !dist.is_base() {
                 continue;
             }
+            if graph.reachability[&node_index].is_false() {
+                debug!("Removing unreachable package: `{}`", dist.package_id());
+                continue;
+            }
             let fork_markers = graph
                 .fork_markers(dist.name(), &dist.version, dist.dist.version_or_url().url())
                 .cloned()
@@ -108,6 +113,10 @@ impl Lock {
                 else {
                     continue;
                 };
+                // Prune edges leading to unreachable nodes.
+                if graph.reachability[&edge.target()].is_false() {
+                    continue;
+                }
                 let marker = edge.weight().clone();
                 package.add_dependency(&requires_python, dependency_dist, marker, root)?;
             }
@@ -126,6 +135,9 @@ impl Lock {
             let ResolutionGraphNode::Dist(dist) = &graph.petgraph[node_index] else {
                 continue;
             };
+            if graph.reachability[&node_index].is_false() {
+                continue;
+            }
             if let Some(extra) = dist.extra.as_ref() {
                 let id = PackageId::from_annotated_dist(dist, root)?;
                 let Some(package) = packages.get_mut(&id) else {
@@ -140,6 +152,10 @@ impl Lock {
                     else {
                         continue;
                     };
+                    // Prune edges leading to unreachable nodes.
+                    if graph.reachability[&edge.target()].is_false() {
+                        continue;
+                    }
                     let marker = edge.weight().clone();
                     package.add_optional_dependency(
                         &requires_python,
@@ -164,6 +180,10 @@ impl Lock {
                     else {
                         continue;
                     };
+                    // Prune edges leading to unreachable nodes.
+                    if graph.reachability[&edge.target()].is_false() {
+                        continue;
+                    }
                     let marker = edge.weight().clone();
                     package.add_dev_dependency(
                         &requires_python,

crates/uv-resolver/src/resolution/graph.rs

@@ -1,17 +1,20 @@
-use indexmap::IndexSet;
-use petgraph::{
-    graph::{Graph, NodeIndex},
-    Directed, Direction,
-};
-use rustc_hash::{FxBuildHasher, FxHashMap, FxHashSet};
-
 use distribution_types::{
     Dist, DistributionMetadata, Name, ResolutionDiagnostic, ResolvedDist, VersionId,
     VersionOrUrlRef,
 };
+use indexmap::IndexSet;
 use pep440_rs::{Version, VersionSpecifier};
 use pep508_rs::{MarkerEnvironment, MarkerTree, MarkerTreeKind, VerbatimUrl};
+use petgraph::prelude::EdgeRef;
+use petgraph::{
+    graph::{Graph, NodeIndex},
+    Directed, Direction,
+};
 use pypi_types::{HashDigest, ParsedUrlError, Requirement, VerbatimParsedUrl, Yanked};
+use rustc_hash::{FxBuildHasher, FxHashMap, FxHashSet};
+use std::collections::hash_map::Entry;
+use std::collections::HashMap;
+use std::fmt::{Display, Formatter};
 use uv_configuration::{Constraints, Overrides};
 use uv_distribution::Metadata;
 use uv_git::GitResolver;
@@ -54,6 +57,8 @@ pub struct ResolutionGraph {
     /// If there are multiple options for a package, track which fork they belong to so we
     /// can write that to the lockfile and later get the correct preference per fork back.
     pub(crate) package_markers: FxHashMap<PackageName, MarkersForDistribution>,
+    /// The markers under which a package is reachable in the dependency tree.
+    pub(crate) reachability: FxHashMap<NodeIndex, MarkerTree>,
 }
 
 #[derive(Debug)]
@@ -62,6 +67,15 @@ pub(crate) enum ResolutionGraphNode {
     Dist(AnnotatedDist),
 }
 
+impl Display for ResolutionGraphNode {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        match self {
+            ResolutionGraphNode::Root => f.write_str("root"),
+            ResolutionGraphNode::Dist(dist) => Display::fmt(dist, f),
+        }
+    }
+}
+
 #[derive(Eq, PartialEq, Hash)]
 struct PackageRef<'a> {
     package_name: &'a PackageName,
@@ -197,6 +211,8 @@ impl ResolutionGraph {
                 .collect()
         };
 
+        let reachability = Self::reachability(&petgraph, &fork_markers);
+
         if matches!(resolution_strategy, ResolutionStrategy::Lowest) {
             report_missing_lower_bounds(&petgraph, &mut diagnostics);
         }
@@ -211,9 +227,84 @@ impl ResolutionGraph {
             overrides: overrides.clone(),
             options,
             fork_markers,
+            reachability,
         })
     }
 
+    /// Determine the markers under which a package is reachable in the dependency tree.
+    ///
+    /// The algorithm is a variant of Dijkstra's algorithm for not totally ordered distances:
+    /// Whenever we find a shorter distance to a node (a marker that is not a subset of the existing
+    /// marker), we re-queue the node and update all its children. This implicitly handles cycles,
+    /// whenever we re-reach a node through a cycle the marker we have is a more
+    /// specific marker/longer path, so we don't update the node and don't re-queue it.
+    fn reachability(
+        petgraph: &Graph<ResolutionGraphNode, MarkerTree>,
+        fork_markers: &[MarkerTree],
+    ) -> HashMap<NodeIndex, MarkerTree, FxBuildHasher> {
+        // Note that we build including the virtual packages due to how we propagate markers through
+        // the graph, even though we then only read the markers for base packages.
+        let mut reachability = FxHashMap::default();
+
+        // Collect the root nodes.
+        //
+        // Besides the actual virtual root node, virtual dev dependencies packages are also root
+        // nodes since the edges don't cover dev dependencies.
+        let mut queue: Vec<_> = petgraph
+            .node_indices()
+            .filter(|node_index| {
+                petgraph
+                    .edges_directed(*node_index, Direction::Incoming)
+                    .next()
+                    .is_none()
+            })
+            .collect();
+
+        // The root nodes are always applicable, unless the user has restricted resolver
+        // environments with `tool.uv.environments`.
+        let root_markers: MarkerTree = if fork_markers.is_empty() {
+            MarkerTree::TRUE
+        } else {
+            fork_markers
+                .iter()
+                .fold(MarkerTree::FALSE, |mut acc, marker| {
+                    acc.or(marker.clone());
+                    acc
+                })
+        };
+        for root_index in &queue {
+            reachability.insert(*root_index, root_markers.clone());
+        }
+
+        // Propagate all markers through the graph, so that the eventual marker for each node is the
+        // union of the markers of each path we can reach the node by.
+        while let Some(parent_index) = queue.pop() {
+            let marker = reachability[&parent_index].clone();
+            for child_edge in petgraph.edges_directed(parent_index, Direction::Outgoing) {
+                // The marker for all paths to the child through the parent.
+                let mut child_marker = child_edge.weight().clone();
+                child_marker.and(marker.clone());
+                match reachability.entry(child_edge.target()) {
+                    Entry::Occupied(mut existing) => {
+                        // If the marker is a subset of the existing marker (A ⊆ B exactly if
+                        // A ∪ B = A), updating the child wouldn't change child's marker.
+                        child_marker.or(existing.get().clone());
+                        if &child_marker != existing.get() {
+                            existing.insert(child_marker);
+                            queue.push(child_edge.target());
+                        }
+                    }
+                    Entry::Vacant(vacant) => {
+                        vacant.insert(child_marker.clone());
+                        queue.push(child_edge.target());
+                    }
+                }
+            }
+        }
+
+        reachability
+    }
+
     fn add_edge(
         petgraph: &mut Graph<ResolutionGraphNode, MarkerTree>,
         inverse: &mut FxHashMap<PackageRef<'_>, NodeIndex>,