Fix multi user undo model (#4639)

* Undo by restoring each step's exact prior state, and only this user's own steps

* Split up prior variants

Author: TrueDoctor

document/graph-storage/src/attributes.rs

@@ -102,10 +102,10 @@ impl Attributes {
 
 	/// Lands a single-key write if it is newer than the key's value, or than the floor for a key the map lacks. A
 	/// deletion leaves a tombstone.
-	pub(crate) fn apply_delta(&mut self, delta: AttributeDelta, timestamp: TimeStamp, force: bool) {
+	pub(crate) fn apply_delta(&mut self, delta: AttributeDelta, timestamp: TimeStamp) {
 		let AttributeDelta { key, value } = delta;
 		let decided = self.get(&key).map_or(self.floor, |existing| existing.timestamp);
-		if !force && timestamp <= decided {
+		if timestamp <= decided {
 			return;
 		}
 		let entry = match value {

document/graph-storage/src/crdt.rs

@@ -1,4 +1,6 @@
-use crate::{Attributes, Implementation, InputSlot, Network, NetworkId, Node, NodeId, NodeInput, PeerId, ResourceEntry, ResourceId, Rev, SourceKey, TimeStamp, UserId, Value, attr, compute_rev};
+use crate::{
+	Attributes, Implementation, InputSlot, Network, NetworkId, Node, NodeId, NodeInput, PeerId, Prior, ResourceEntry, ResourceId, Rev, SourceKey, TimeStamp, UserId, Value, attr, compute_rev,
+};
 use graphene_resource::ResourceHash;
 use serde::{Deserialize, Serialize};
 
@@ -13,7 +15,8 @@ pub struct Delta {
 	pub author: PeerId,
 	pub timestamp: TimeStamp,
 	pub kind: RegistryDelta,
-	pub reverse: RegistryDelta,
+	/// What every slot `kind` wrote held before it, stamps included, in write order. See [`Prior`].
+	pub reverse: Vec<Prior>,
 	/// Local, mutable annotations on this commit (interaction-end marker, future commit messages / labels).
 	/// Deliberately excluded from `compute_rev`: relabeling a commit must not change its content-addressed
 	/// identity, and two peers annotating the same op differently must still dedup to one `Rev`.
@@ -26,7 +29,7 @@ pub struct Delta {
 }
 
 impl Delta {
-	pub fn new(parent: Option<Rev>, author: PeerId, timestamp: TimeStamp, kind: RegistryDelta, reverse: RegistryDelta) -> Self {
+	pub fn new(parent: Option<Rev>, author: PeerId, timestamp: TimeStamp, kind: RegistryDelta, reverse: Vec<Prior>) -> Self {
 		let id = compute_rev(parent, author, timestamp, &kind);
 		Self {
 			id,
@@ -54,7 +57,7 @@ impl Delta {
 			parent,
 			author,
 			timestamp,
-			reverse: kind.clone(),
+			reverse: Vec::new(),
 			kind,
 			attributes: Attributes::default(),
 			retired_at_ms: 0,
@@ -219,26 +222,13 @@ pub struct AttributeDelta {
 	pub value: Option<Value>,
 }
 
-pub(crate) fn reverse_attribute_delta(delta: &AttributeDelta, attributes: &Attributes) -> AttributeDelta {
-	AttributeDelta {
-		key: delta.key.clone(),
-		value: attributes.get(&delta.key).filter(|previous| !previous.deleted).map(|previous| previous.value.clone()),
-	}
-}
-
 #[cfg(test)]
 mod tests {
 	use super::*;
 
 	#[test]
 	fn unrecorded_retired_at_costs_one_byte_and_round_trips() {
-		let base = Delta::new(
-			None,
-			PeerId(1),
-			TimeStamp { counter: 1, peer: PeerId(1) },
-			RegistryDelta::Other(Value::None),
-			RegistryDelta::Other(Value::None),
-		);
+		let base = Delta::new(None, PeerId(1), TimeStamp { counter: 1, peer: PeerId(1) }, RegistryDelta::Other(Value::None), Vec::new());
 		let mut stamped = base.clone();
 		stamped.retired_at_ms = 1_790_000_000_000;