crates/oxy-core/src/tree.rs at 992467d31d26 · andymccall/oxygen

# 992467d31d26 oxygen / crates / oxy-core / src / tree.rs
28.0 KiB · Rust · last changed A cheaper tree view; history loads and snapshots timed finer 1 month ago source blame history raw
//! The tree graph: nodes, placements, and modes as atoms (spec §7.1, §7.2),
//! with tree conflict detection and path assignment (spec §9.2).

use crate::hash::ChangeHash;
use crate::ids::NodeId;
use crate::objects::{Mode, NodeKind, OpRef, TreeOp};
use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};

/// `(change, index into that change's tree_ops)`.
pub type OpKey = (ChangeHash, u32);

#[derive(Clone, Debug, PartialEq, Eq)]
pub struct Placement {
    pub node: NodeId,
    pub parent: NodeId,
    pub name: String,
}

#[derive(Clone, Debug, Default)]
pub struct TreeGraph {
    kinds: HashMap<NodeId, NodeKind>,
    introduced: HashMap<NodeId, ChangeHash>,
    placements: HashMap<OpKey, Placement>,
    modes: HashMap<OpKey, (NodeId, Mode)>,
    /// Placement atoms retired by `RemoveNode`/`Move`, and mode atoms retired
    /// by `SetMode`. Kept apart because `AddNode` introduces one of each
    /// under the same key.
    deleted_placements: HashMap<OpKey, BTreeSet<ChangeHash>>,
    deleted_modes: HashMap<OpKey, BTreeSet<ChangeHash>>,
    placements_of: HashMap<NodeId, BTreeSet<OpKey>>,
    modes_of: HashMap<NodeId, BTreeSet<OpKey>>,
}

#[derive(Clone, Debug, PartialEq, Eq)]
pub enum TreeConflict {
    /// Two live nodes share `(parent, name)`.
    Name {
        parent: NodeId,
        name: String,
        nodes: Vec<NodeId>,
    },
    /// One node has several live placements.
    MultiPlacement {
        node: NodeId,
        placements: Vec<OpKey>,
    },
    /// A live node's parent is not live.
    Orphan { node: NodeId, parent: NodeId },
    /// A directory is placed inside its own descendant.
    Cycle { node: NodeId },
    /// Several live mode atoms.
    Mode { node: NodeId, modes: Vec<Mode> },
}

/// A node as it appears in the working copy.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct TreeEntry {
    pub path: String,
    pub node: NodeId,
    pub kind: NodeKind,
    pub mode: Mode,
    /// The placement this entry materializes (for building `RemoveNode`/`Move`).
    pub placement: OpKey,
    /// The live mode atom (for building `SetMode`).
    pub mode_atom: Option<OpKey>,
}

#[derive(Clone, Debug, Default, PartialEq, Eq)]
pub struct TreeView {
    /// Path → entry, paths use `/` and have no leading slash; conflicted
    /// entries carry a `.oxy-conflict-<n>` suffix.
    pub entries: BTreeMap<String, TreeEntry>,
    pub conflicts: Vec<TreeConflict>,
}

impl TreeView {
    pub fn is_conflicted(&self) -> bool {
        !self.conflicts.is_empty()
    }
    pub fn by_node(&self) -> HashMap<NodeId, &TreeEntry> {
        self.entries.values().map(|e| (e.node, e)).collect()
    }
}

impl TreeGraph {
    pub fn new() -> Self {
        Self::default()
    }

    /// Apply tree op `index` of `change`; `OpRef` self references must already be resolved.
    pub fn apply(&mut self, change: ChangeHash, index: u32, op: &TreeOp) {
        let key = (change, index);
        match op {
            TreeOp::AddNode {
                node,
                kind,
                mode,
                parent,
                name,
            } => {
                self.kinds.insert(*node, *kind);
                self.introduced.insert(*node, change);
                self.placements.insert(
                    key,
                    Placement {
                        node: *node,
                        parent: *parent,
                        name: name.clone(),
                    },
                );
                self.placements_of.entry(*node).or_default().insert(key);
                if *kind != NodeKind::Dir {
                    self.modes.insert(key, (*node, *mode));
                    self.modes_of.entry(*node).or_default().insert(key);
                }
            }
            TreeOp::RemoveNode { placement, .. } => {
                self.deleted_placements
                    .entry(op_key(*placement))
                    .or_default()
                    .insert(change);
            }
            TreeOp::Move {
                node,
                old,
                parent,
                name,
            } => {
                self.deleted_placements
                    .entry(op_key(*old))
                    .or_default()
                    .insert(change);
                self.placements.insert(
                    key,
                    Placement {
                        node: *node,
                        parent: *parent,
                        name: name.clone(),
                    },
                );
                self.placements_of.entry(*node).or_default().insert(key);
            }
            TreeOp::SetMode { node, old, mode } => {
                self.deleted_modes
                    .entry(op_key(*old))
                    .or_default()
                    .insert(change);
                self.modes.insert(key, (*node, *mode));
                self.modes_of.entry(*node).or_default().insert(key);
            }
        }
    }

    fn placement_live(&self, k: &OpKey) -> bool {
        self.deleted_placements.get(k).is_none_or(|d| d.is_empty())
    }
    fn mode_live(&self, k: &OpKey) -> bool {
        self.deleted_modes.get(k).is_none_or(|d| d.is_empty())
    }

    pub fn kind(&self, node: &NodeId) -> Option<NodeKind> {
        if node.is_root() {
            Some(NodeKind::Dir)
        } else {
            self.kinds.get(node).copied()
        }
    }

    /// The change whose `AddNode` introduced `node`.
    pub fn introducer(&self, node: &NodeId) -> Option<ChangeHash> {
        self.introduced.get(node).copied()
    }

    pub fn placement(&self, k: &OpKey) -> Option<&Placement> {
        self.placements.get(k)
    }

    pub fn live_placements(&self, node: &NodeId) -> Vec<(OpKey, &Placement)> {
        self.placements_of
            .get(node)
            .into_iter()
            .flat_map(|ks| ks.iter())
            .filter(|k| self.placement_live(k))
            .filter_map(|k| self.placements.get(k).map(|p| (*k, p)))
            .collect()
    }

    pub fn live_modes(&self, node: &NodeId) -> Vec<(OpKey, Mode)> {
        self.modes_of
            .get(node)
            .into_iter()
            .flat_map(|ks| ks.iter())
            .filter(|k| self.mode_live(k))
            .filter_map(|k| self.modes.get(k).map(|(_, m)| (*k, *m)))
            .collect()
    }

    /// Every node that has at least one live placement.
    pub fn placed_nodes(&self) -> impl Iterator<Item = NodeId> + '_ {
        self.placements_of
            .keys()
            .copied()
            .filter(|n| !self.live_placements(n).is_empty())
    }

    /// Resolve the tree: assign paths, find conflicts.
    pub fn view(&self) -> TreeView {
        let mut view = TreeView::default();

        // Live placements grouped by parent.
        let mut children: HashMap<NodeId, Vec<(OpKey, &Placement)>> = HashMap::new();
        for (k, p) in &self.placements {
            if self.placement_live(k) {
                children.entry(p.parent).or_default().push((*k, p));
            }
        }
        for c in children.values_mut() {
            c.sort_by(|a, b| a.1.name.cmp(&b.1.name).then(a.0.cmp(&b.0)));
        }

        // Multi-placement and mode conflicts.
        let mut nodes: Vec<NodeId> = self.placements_of.keys().copied().collect();
        nodes.sort();
        let mut multi: HashSet<NodeId> = HashSet::new();
        // Each node's mode and the atom it comes from, for the entries.
        let mut modes: HashMap<NodeId, (Option<OpKey>, Mode)> = HashMap::new();
        for n in &nodes {
            let lp = self.live_placements(n);
            // One placement (nearly every node) cannot be in two places.
            if lp.len() > 1 {
                let distinct_places: BTreeSet<(&NodeId, &str)> = lp
                    .iter()
                    .map(|(_, p)| (&p.parent, p.name.as_str()))
                    .collect();
                if distinct_places.len() > 1 {
                    multi.insert(*n);
                    view.conflicts.push(TreeConflict::MultiPlacement {
                        node: *n,
                        placements: lp.iter().map(|(k, _)| *k).collect(),
                    });
                }
            }
            let lm = self.live_modes(n);
            if lm.len() > 1 {
                let distinct_modes: BTreeSet<Mode> = lm.iter().map(|(_, m)| *m).collect();
                if distinct_modes.len() > 1 {
                    view.conflicts.push(TreeConflict::Mode {
                        node: *n,
                        modes: distinct_modes.into_iter().collect(),
                    });
                }
            }
            let mode_atom = lm.iter().map(|(k, _)| *k).max();
            let mode = mode_atom
                .and_then(|k| self.modes.get(&k).map(|(_, m)| *m))
                .unwrap_or_default();
            modes.insert(*n, (mode_atom, mode));
        }

        // BFS from the root, assigning paths.
        let mut reached: HashSet<NodeId> = HashSet::new();
        reached.insert(NodeId::ROOT);
        let mut queue: Vec<(NodeId, String)> = vec![(NodeId::ROOT, String::new())];
        let mut on_path: HashSet<NodeId> = HashSet::new();
        while let Some((dir, dir_path)) = queue.pop() {
            let Some(kids) = children.get(&dir) else {
                continue;
            };
            // Name conflicts within this directory: the children are sorted
            // by name, so each name's group is a run of them.
            let mut groups: Vec<(&str, &[(OpKey, &Placement)])> = Vec::new();
            let mut start = 0;
            for i in 1..=kids.len() {
                if i == kids.len() || kids[i].1.name != kids[start].1.name {
                    groups.push((kids[start].1.name.as_str(), &kids[start..i]));
                    start = i;
                }
            }
            for (name, group) in groups {
                let distinct: BTreeSet<NodeId> = group.iter().map(|(_, p)| p.node).collect();
                if distinct.len() > 1 {
                    view.conflicts.push(TreeConflict::Name {
                        parent: dir,
                        name: name.to_string(),
                        nodes: distinct.iter().copied().collect(),
                    });
                }
                let suffixed = distinct.len() > 1;
                for (i, (k, p)) in group.iter().enumerate() {
                    let node = p.node;
                    if node == dir || on_path.contains(&node) {
                        view.conflicts.push(TreeConflict::Cycle { node });
                        continue;
                    }
                    let needs_suffix = suffixed || multi.contains(&node);
                    let leaf = if needs_suffix {
                        format!("{name}.oxy-conflict-{}", i + 1)
                    } else {
                        name.to_string()
                    };
                    let path = if dir_path.is_empty() {
                        leaf
                    } else {
                        format!("{dir_path}/{leaf}")
                    };
                    let kind = self.kind(&node).unwrap_or(NodeKind::File);
                    let (mode_atom, mode) = modes.get(&node).copied().unwrap_or_default();
                    view.entries.insert(
                        path.clone(),
                        TreeEntry {
                            path: path.clone(),
                            node,
                            kind,
                            mode,
                            placement: *k,
                            mode_atom,
                        },
                    );
                    if kind == NodeKind::Dir && reached.insert(node) {
                        on_path.insert(dir);
                        queue.push((node, path));
                    } else {
                        reached.insert(node);
                    }
                }
            }
        }

        // Nodes with live placements that were never reached: orphans or cycles.
        for n in &nodes {
            if reached.contains(n) {
                continue;
            }
            let lp = self.live_placements(n);
            if lp.is_empty() {
                continue;
            }
            let mut seen = HashSet::new();
            let mut cur = *n;
            let conflict = loop {
                if !seen.insert(cur) {
                    break TreeConflict::Cycle { node: *n };
                }
                let lp = self.live_placements(&cur);
                let Some((_, p)) = lp.first() else {
                    break TreeConflict::Orphan {
                        node: *n,
                        parent: cur,
                    };
                };
                if reached.contains(&p.parent) {
                    // Parent reached but this node was not: the parent is not a
                    // directory, so this is a name/kind problem surfaced as orphan.
                    break TreeConflict::Orphan {
                        node: *n,
                        parent: p.parent,
                    };
                }
                cur = p.parent;
            };
            if !view.conflicts.contains(&conflict) {
                view.conflicts.push(conflict);
            }
        }
        view
    }
}

impl TreeGraph {
    /// Serialize for a checkpoint.
    pub fn to_value(&self) -> crate::cbor::Value {
        use crate::cbor::Value;
        let opkey = |k: &OpKey| vec![Value::Bytes(k.0 .0.to_vec()), Value::Uint(k.1 as u64)];
        let deleters = |m: &HashMap<OpKey, BTreeSet<ChangeHash>>| {
            let mut ks: Vec<&OpKey> = m.keys().collect();
            ks.sort();
            Value::Array(
                ks.into_iter()
                    .map(|k| {
                        let mut v = opkey(k);
                        v.push(Value::Array(
                            m[k].iter().map(|d| Value::Bytes(d.0.to_vec())).collect(),
                        ));
                        Value::Array(v)
                    })
                    .collect(),
            )
        };
        let mut nodes: Vec<&NodeId> = self.kinds.keys().collect();
        nodes.sort();
        let mut pks: Vec<&OpKey> = self.placements.keys().collect();
        pks.sort();
        let mut mks: Vec<&OpKey> = self.modes.keys().collect();
        mks.sort();
        Value::map([
            (
                0,
                Value::Array(
                    nodes
                        .into_iter()
                        .map(|n| {
                            Value::Array(vec![
                                Value::Bytes(n.0.to_vec()),
                                Value::Uint(self.kinds[n].tag()),
                                self.introduced
                                    .get(n)
                                    .map(|h| Value::Bytes(h.0.to_vec()))
                                    .unwrap_or(Value::Null),
                            ])
                        })
                        .collect(),
                ),
            ),
            (
                1,
                Value::Array(
                    pks.into_iter()
                        .map(|k| {
                            let p = &self.placements[k];
                            let mut v = opkey(k);
                            v.extend([
                                Value::Bytes(p.node.0.to_vec()),
                                Value::Bytes(p.parent.0.to_vec()),
                                Value::Text(p.name.clone()),
                            ]);
                            Value::Array(v)
                        })
                        .collect(),
                ),
            ),
            (2, deleters(&self.deleted_placements)),
            (
                3,
                Value::Array(
                    mks.into_iter()
                        .map(|k| {
                            let (n, m) = &self.modes[k];
                            let mut v = opkey(k);
                            v.extend([Value::Bytes(n.0.to_vec()), Value::Uint(m.bits())]);
                            Value::Array(v)
                        })
                        .collect(),
                ),
            ),
            (4, deleters(&self.deleted_modes)),
        ])
    }

    pub fn from_value(v: &crate::cbor::Value) -> Result<TreeGraph, crate::objects::ObjectError> {
        use crate::objects::{array, bytes, field, map, oref, text, u32_, uint, ObjectError};
        let m = map(v, "tree graph")?;
        let node = |v: &crate::cbor::Value| {
            NodeId::from_slice(bytes(v, "node")?)
                .ok_or_else(|| ObjectError::Schema("node: expected 16 bytes".into()))
        };
        let opkey = |e: &[crate::cbor::Value]| -> Result<OpKey, ObjectError> {
            Ok((oref(&e[0], "op change")?, u32_(&e[1], "op index")?))
        };
        let mut g = TreeGraph::default();
        for n in array(field(m, 0, "nodes")?, "nodes")? {
            let e = array(n, "node entry")?;
            if e.len() != 3 {
                return Err(ObjectError::Schema("node entry: expected 3 fields".into()));
            }
            let id = node(&e[0])?;
            g.kinds
                .insert(id, NodeKind::from_tag(uint(&e[1], "kind")?)?);
            if !e[2].is_null() {
                g.introduced.insert(id, oref(&e[2], "introducer")?);
            }
        }
        for p in array(field(m, 1, "placements")?, "placements")? {
            let e = array(p, "placement")?;
            if e.len() != 5 {
                return Err(ObjectError::Schema("placement: expected 5 fields".into()));
            }
            let k = opkey(e)?;
            let pl = Placement {
                node: node(&e[2])?,
                parent: node(&e[3])?,
                name: text(&e[4], "name")?.to_owned(),
            };
            g.placements_of.entry(pl.node).or_default().insert(k);
            g.placements.insert(k, pl);
        }
        let deleters = |v: &crate::cbor::Value,
                        what: &str|
         -> Result<HashMap<OpKey, BTreeSet<ChangeHash>>, ObjectError> {
            let mut out = HashMap::new();
            for d in array(v, what)? {
                let e = array(d, what)?;
                if e.len() != 3 {
                    return Err(ObjectError::Schema(format!("{what}: expected 3 fields")));
                }
                out.insert(
                    opkey(e)?,
                    array(&e[2], "deleters")?
                        .iter()
                        .map(|x| oref(x, "deleter"))
                        .collect::<Result<BTreeSet<_>, _>>()?,
                );
            }
            Ok(out)
        };
        g.deleted_placements = deleters(field(m, 2, "deleted placements")?, "deleted placement")?;
        for md in array(field(m, 3, "modes")?, "modes")? {
            let e = array(md, "mode")?;
            if e.len() != 4 {
                return Err(ObjectError::Schema("mode: expected 4 fields".into()));
            }
            let k = opkey(e)?;
            let n = node(&e[2])?;
            g.modes
                .insert(k, (n, Mode::from_bits(uint(&e[3], "mode bits")?)?));
            g.modes_of.entry(n).or_default().insert(k);
        }
        g.deleted_modes = deleters(field(m, 4, "deleted modes")?, "deleted mode")?;
        Ok(g)
    }
}

pub fn op_key(r: OpRef) -> OpKey {
    (r.change, r.index)
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::hash::ObjectRef;

    fn h(n: u8) -> ChangeHash {
        ObjectRef([n; 32])
    }
    fn nid(n: u8) -> NodeId {
        NodeId([n; 16])
    }
    fn add(g: &mut TreeGraph, c: u8, i: u32, node: u8, kind: NodeKind, parent: NodeId, name: &str) {
        g.apply(
            h(c),
            i,
            &TreeOp::AddNode {
                node: nid(node),
                kind,
                mode: Mode::TEXT,
                parent,
                name: name.into(),
            },
        );
    }
    fn paths(v: &TreeView) -> Vec<String> {
        v.entries.keys().cloned().collect()
    }

    #[test]
    fn nested_paths_and_rename() {
        let mut g = TreeGraph::new();
        add(&mut g, 1, 0, 1, NodeKind::Dir, NodeId::ROOT, "src");
        add(&mut g, 1, 1, 2, NodeKind::File, nid(1), "main.rs");
        add(&mut g, 1, 2, 3, NodeKind::File, NodeId::ROOT, "README");
        let v = g.view();
        assert_eq!(paths(&v), ["README", "src", "src/main.rs"]);
        assert!(!v.is_conflicted());

        // Rename src/main.rs → src/lib.rs, move src → lib (subtree follows).
        g.apply(
            h(2),
            0,
            &TreeOp::Move {
                node: nid(2),
                old: OpRef {
                    change: h(1),
                    index: 1,
                },
                parent: nid(1),
                name: "lib.rs".into(),
            },
        );
        g.apply(
            h(2),
            1,
            &TreeOp::Move {
                node: nid(1),
                old: OpRef {
                    change: h(1),
                    index: 0,
                },
                parent: NodeId::ROOT,
                name: "lib".into(),
            },
        );
        let v = g.view();
        assert_eq!(paths(&v), ["README", "lib", "lib/lib.rs"]);
        assert_eq!(v.entries["lib/lib.rs"].placement, (h(2), 0));

        g.apply(
            h(3),
            0,
            &TreeOp::RemoveNode {
                node: nid(3),
                placement: OpRef {
                    change: h(1),
                    index: 2,
                },
            },
        );
        assert_eq!(paths(&g.view()), ["lib", "lib/lib.rs"]);
    }

    #[test]
    fn name_conflict_gets_suffixes() {
        let mut g = TreeGraph::new();
        add(&mut g, 1, 0, 1, NodeKind::File, NodeId::ROOT, "a.txt");
        add(&mut g, 2, 0, 2, NodeKind::File, NodeId::ROOT, "a.txt");
        let v = g.view();
        assert_eq!(paths(&v), ["a.txt.oxy-conflict-1", "a.txt.oxy-conflict-2"]);
        assert_eq!(
            v.conflicts,
            vec![TreeConflict::Name {
                parent: NodeId::ROOT,
                name: "a.txt".into(),
                nodes: vec![nid(1), nid(2)]
            }]
        );
    }

    #[test]
    fn concurrent_renames_are_multi_placement() {
        let mut g = TreeGraph::new();
        add(&mut g, 1, 0, 1, NodeKind::File, NodeId::ROOT, "a");
        g.apply(
            h(2),
            0,
            &TreeOp::Move {
                node: nid(1),
                old: OpRef {
                    change: h(1),
                    index: 0,
                },
                parent: NodeId::ROOT,
                name: "b".into(),
            },
        );
        g.apply(
            h(3),
            0,
            &TreeOp::Move {
                node: nid(1),
                old: OpRef {
                    change: h(1),
                    index: 0,
                },
                parent: NodeId::ROOT,
                name: "c".into(),
            },
        );
        let v = g.view();
        assert_eq!(paths(&v), ["b.oxy-conflict-1", "c.oxy-conflict-1"]);
        assert!(
            matches!(v.conflicts[0], TreeConflict::MultiPlacement { node, .. } if node == nid(1))
        );
    }

    #[test]
    fn orphan_and_cycle() {
        let mut g = TreeGraph::new();
        add(&mut g, 1, 0, 1, NodeKind::Dir, NodeId::ROOT, "d");
        add(&mut g, 1, 1, 2, NodeKind::File, nid(1), "f");
        // Concurrently: 2 removes d, 3 adds d/g.
        g.apply(
            h(2),
            0,
            &TreeOp::RemoveNode {
                node: nid(1),
                placement: OpRef {
                    change: h(1),
                    index: 0,
                },
            },
        );
        add(&mut g, 3, 0, 3, NodeKind::File, nid(1), "g");
        let v = g.view();
        assert_eq!(paths(&v), Vec::<String>::new());
        assert!(v.conflicts.contains(&TreeConflict::Orphan {
            node: nid(2),
            parent: nid(1)
        }));
        assert!(v.conflicts.contains(&TreeConflict::Orphan {
            node: nid(3),
            parent: nid(1)
        }));

        // Cycle: a/b, then concurrently move a under b and b under a's parent... simplest: move a into b.
        let mut g = TreeGraph::new();
        add(&mut g, 1, 0, 1, NodeKind::Dir, NodeId::ROOT, "a");
        add(&mut g, 1, 1, 2, NodeKind::Dir, nid(1), "b");
        g.apply(
            h(2),
            0,
            &TreeOp::Move {
                node: nid(1),
                old: OpRef {
                    change: h(1),
                    index: 0,
                },
                parent: nid(2),
                name: "a".into(),
            },
        );
        let v = g.view();
        assert!(
            v.conflicts
                .iter()
                .any(|c| matches!(c, TreeConflict::Cycle { .. })),
            "{:?}",
            v.conflicts
        );
    }

    #[test]
    fn serialization_round_trip() {
        let mut g = TreeGraph::new();
        add(&mut g, 1, 0, 1, NodeKind::Dir, NodeId::ROOT, "src");
        add(&mut g, 1, 1, 2, NodeKind::File, nid(1), "main.rs");
        g.apply(
            h(2),
            0,
            &TreeOp::Move {
                node: nid(2),
                old: OpRef {
                    change: h(1),
                    index: 1,
                },
                parent: nid(1),
                name: "lib.rs".into(),
            },
        );
        g.apply(
            h(3),
            0,
            &TreeOp::SetMode {
                node: nid(2),
                old: OpRef {
                    change: h(1),
                    index: 1,
                },
                mode: Mode {
                    exec: true,
                    opaque: false,
                },
            },
        );
        let bytes = crate::cbor::encode(&g.to_value());
        let back = TreeGraph::from_value(&crate::cbor::decode(&bytes).unwrap()).unwrap();
        assert_eq!(back.view(), g.view());
        assert_eq!(back.introducer(&nid(2)), Some(h(1)));
        assert_eq!(crate::cbor::encode(&back.to_value()), bytes);
    }

    #[test]
    fn mode_conflict_and_resolution() {
        let mut g = TreeGraph::new();
        add(&mut g, 1, 0, 1, NodeKind::File, NodeId::ROOT, "run.sh");
        g.apply(
            h(2),
            0,
            &TreeOp::SetMode {
                node: nid(1),
                old: OpRef {
                    change: h(1),
                    index: 0,
                },
                mode: Mode {
                    exec: true,
                    opaque: false,
                },
            },
        );
        g.apply(
            h(3),
            0,
            &TreeOp::SetMode {
                node: nid(1),
                old: OpRef {
                    change: h(1),
                    index: 0,
                },
                mode: Mode {
                    exec: false,
                    opaque: true,
                },
            },
        );
        let v = g.view();
        assert!(matches!(v.conflicts[0], TreeConflict::Mode { .. }));
        // Materialized with the greatest mode atom.
        assert_eq!(
            v.entries["run.sh"].mode,
            Mode {
                exec: false,
                opaque: true
            }
        );
        // The placement introduced by the same AddNode is unaffected.
        assert_eq!(v.entries["run.sh"].placement, (h(1), 0));
        // Resolution: retire both live mode atoms; identical live values do not conflict.
        g.apply(
            h(4),
            0,
            &TreeOp::SetMode {
                node: nid(1),
                old: OpRef {
                    change: h(2),
                    index: 0,
                },
                mode: Mode {
                    exec: true,
                    opaque: false,
                },
            },
        );
        g.apply(
            h(4),
            1,
            &TreeOp::SetMode {
                node: nid(1),
                old: OpRef {
                    change: h(3),
                    index: 0,
                },
                mode: Mode {
                    exec: true,
                    opaque: false,
                },
            },
        );
        let v = g.view();
        assert!(!v.is_conflicted(), "{:?}", v.conflicts);
        assert_eq!(
            v.entries["run.sh"].mode,
            Mode {
                exec: true,
                opaque: false
            }
        );
    }
}
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