fix: attempt to solve client edits race condition

basically send increments a counter and we compare that before
transforming operations, allowing to stop processing, undo and try to
receive again. This won't solve all issues but should make them much
more rare? I hope

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "codemp"
-version = "0.4.6"
+version = "0.5.0-pre"
 edition = "2021"
 
 [lib]

src/buffer/controller.rs

@@ -2,6 +2,9 @@
 //! 
 //! a controller implementation for buffer actions
 
+use std::sync::Arc;
+use std::sync::atomic::{AtomicU64, Ordering};
+
 use operational_transform::OperationSeq;
 use tokio::sync::broadcast::error::TryRecvError;
 use tokio::sync::{watch, mpsc, broadcast, Mutex};
@@ -36,6 +39,7 @@ pub struct BufferController {
 	last_op: Mutex<watch::Receiver<String>>,
 	stream: Mutex<broadcast::Receiver<TextChange>>,
 	stop: mpsc::UnboundedSender<()>,
+	operation_tick: Arc<AtomicU64>,
 }
 
 impl BufferController {
@@ -44,10 +48,12 @@ impl BufferController {
 		operations: mpsc::UnboundedSender<OperationSeq>,
 		stream: Mutex<broadcast::Receiver<TextChange>>,
 		stop: mpsc::UnboundedSender<()>,
+		operation_tick: Arc<AtomicU64>,
 	) -> Self {
 		BufferController {
 			last_op: Mutex::new(content.clone()),
 			content, operations, stream, stop,
+			operation_tick,
 		}
 	}
 }
@@ -93,6 +99,9 @@ impl Controller<TextChange> for BufferController {
 
 	/// enqueue an opseq for processing
 	fn send(&self, op: OperationSeq) -> Result<(), Error> {
-		Ok(self.operations.send(op)?)
+		let tick = self.operation_tick.load(Ordering::Acquire);
+		self.operations.send(op)?;
+		self.operation_tick.store(tick + 1, Ordering::Release);
+		Ok(())
 	}
 }

src/buffer/worker.rs

@@ -1,3 +1,4 @@
+use std::sync::atomic::{AtomicU64, Ordering};
 use std::{sync::Arc, collections::VecDeque};
 
 use operational_transform::{OperationSeq, OTError};
@@ -17,16 +18,16 @@ use super::factory::{leading_noop, tailing_noop};
 
 pub(crate) struct BufferControllerWorker {
 	uid: String,
-	pub(crate) content: watch::Sender<String>,
-	pub(crate) operations: mpsc::UnboundedReceiver<OperationSeq>,
-	pub(crate) stream: Arc<broadcast::Sender<TextChange>>,
-	pub(crate) queue: VecDeque<OperationSeq>,
+	content: watch::Sender<String>,
+	operations: mpsc::UnboundedReceiver<OperationSeq>,
+	stream: Arc<broadcast::Sender<TextChange>>,
 	receiver: watch::Receiver<String>,
 	sender: mpsc::UnboundedSender<OperationSeq>,
 	buffer: String,
 	path: String,
 	stop: mpsc::UnboundedReceiver<()>,
 	stop_control: mpsc::UnboundedSender<()>,
+	operation_tick: Arc<AtomicU64>,
 }
 
 impl BufferControllerWorker {
@@ -42,15 +43,15 @@ impl BufferControllerWorker {
 			stream: Arc::new(s_tx),
 			receiver: txt_rx,
 			sender: op_tx,
-			queue: VecDeque::new(),
 			buffer: buffer.to_string(),
 			path: path.to_string(),
 			stop: end_rx,
 			stop_control: end_tx,
+			operation_tick: Arc::new(AtomicU64::new(0)),
 		}
 	}
 
-	fn update(&mut self, op: OperationSeq) -> Result<TextChange, OTError> {
+	fn update(&mut self, op: &OperationSeq) -> Result<TextChange, OTError> {
 		let before = Arc::new(self.buffer.clone());
 		let res = op.apply(&before)?;
 		self.content.send(res.clone())
@@ -78,46 +79,90 @@ impl ControllerWorker<TextChange> for BufferControllerWorker {
 			self.sender.clone(),
 			Mutex::new(self.stream.subscribe()),
 			self.stop_control.clone(),
+			self.operation_tick.clone(),
 		)
 	}
 
 	async fn work(mut self, mut tx: Self::Tx, mut rx: Self::Rx) {
+		let mut clientside : VecDeque<OperationSeq> = VecDeque::new();
+		let mut serverside : VecDeque<OperationSeq> = VecDeque::new();
+		let mut last_seen_tick = 0;
 		loop {
+
+			// block until one of these is ready
 			tokio::select! {
 
-				Some(operation) = recv_opseq(&mut rx) => {
-					let mut out = operation;
-					for op in self.queue.iter_mut() {
-						(*op, out) = match op.transform(&out) {
-							Ok((x, y)) => (x, y),
-							Err(e) => {
-								tracing::warn!("could not transform enqueued operation: {}", e);
-								break
-							},
+				// received a new message from server (or an error)
+				res = rx.message() => {
+					match res {
+						Err(e) => return tracing::error!("error receiving op from server: {}", e),
+						Ok(None) => return tracing::warn!("server closed operation stream"),
+						Ok(Some(msg)) => match serde_json::from_str(&msg.opseq) {
+							Err(e) => return tracing::error!("error deserializing op from server: {}", e),
+							Ok(op) => serverside.push_back(op),
 						}
 					}
-					let change = self.update(out)
-						.unwrap_or_warn_default("coult not update with (transformed) remote operation");
-					self.stream.send(change)
-						.unwrap_or_warn("could not send operation to server");
 				},
 
-				Some(op) = self.operations.recv() => {
-					self.queue.push_back(op.clone());
-					self.update(op)
-						.unwrap_or_warn("could not apply enqueued operation to current buffer");
-					while let Some(op) = self.queue.get(0) {
-						if !send_opseq(&mut tx, self.uid.clone(), self.path.clone(), op.clone()).await { break }
-						self.queue.pop_front();
+				// received a new operation from client (or channel got closed)
+				res = self.operations.recv() => {
+					match res {
+						None => return tracing::warn!("client closed operation stream"),
+						Some(op) => {
+							clientside.push_back(op.clone());
+							last_seen_tick = self.operation_tick.load(Ordering::Acquire);
+						}
 					}
 				},
 
-				Some(()) = self.stop.recv() => {
-					break;
+				// received a stop request (or channel got closed)
+				res = self.stop.recv() => {
+					match res {
+						None => return tracing::warn!("stop channel closed, stopping worker"),
+						Some(()) => return tracing::debug!("buffer worker stopping cleanly"),
+					}
 				}
 
-				else => break
+			}
 
+			// we must give priority to operations received from remote server, because we can transform
+			// our ops with server's ops but server won't transform its ops with ours. We must transform
+			// ALL enqueued client ops: if a new one arrived before we could transform and update, we
+			// should discard our progress and poll again.
+			while let Some(mut operation) = serverside.get(0).cloned() {
+				let mut queued_ops = clientside.clone();
+				for op in queued_ops.iter_mut() {
+					(*op, operation) = match op.transform(&operation) {
+						Ok((x, y)) => (x, y),
+						Err(e) => {
+							tracing::warn!("could not transform enqueued operation: {}", e);
+							break
+						},
+					}
+				}
+				let tick = self.operation_tick.load(std::sync::atomic::Ordering::Acquire);
+				if tick != last_seen_tick { break } // there are more ops to see first
+				clientside = queued_ops;
+				let change = self.update(&operation)
+					.unwrap_or_warn_default("coult not update with (transformed) remote operation");
+				self.stream.send(change)
+					.unwrap_or_warn("could not send operation to server");
+				serverside.pop_front();
+			}
+
+			// if there are still serverside operations to be applied, we can't dispatch our local ones
+			// yet because we need them to transform the ones sent by the server before applying them on
+			// our local buffer. We may get here if a new local operation arrived before we could process
+			// and transform all received server ops. since the buffer is different, it isn't safe to
+			// apply them and we must transform them again. If the loop above left its queue not empty,
+			// we should be guaranteed to unblock immediately in the select above because we have a new
+			// client operation waiting for us to be enqueued
+			if serverside.is_empty() {
+				while let Some(op) = clientside.get(0) {
+					self.update(op);
+					if !send_opseq(&mut tx, self.uid.clone(), self.path.clone(), op.clone()).await { break }
+					clientside.pop_front();
+				}
 			}
 		}
 	}
@@ -145,20 +190,3 @@ async fn send_opseq(tx: &mut BufferClient<Channel>, uid: String, path: String, o
 		}
 	}
 }
-
-async fn recv_opseq(rx: &mut Streaming<RawOp>) -> Option<OperationSeq> {
-	match rx.message().await {
-		Ok(Some(op)) => match serde_json::from_str(&op.opseq) {
-			Ok(x) => Some(x),
-			Err(e) => {
-				tracing::warn!("could not deserialize opseq: {}", e);
-				None
-			}
-		},
-		Ok(None) => None,
-		Err(e) => {
-			tracing::error!("could not receive edit from server: {}", e);
-			None
-		}
-	}
-}