Extract a BufferStore object from Project (#14037)

This is a ~small~ pure refactor that's a step toward SSH remoting. I've
extracted the Project's buffer state management into a smaller, separate
struct called `BufferStore`, currently in the same crate. I did this as
a separate PR to reduce conflicts between main and `remoting-over-ssh`.

The idea is to make use of this struct (and other smaller structs that
make up `Project`) in a dedicated, simpler `HeadlessProject` type that
we will use in the SSH server to model the remote end of a project. With
this approach, as we develop the headless project, we can avoid adding
more conditional logic to `Project` itself (which is already very
complex), and actually make `Project` a bit smaller by extracting out
helper objects.

Release Notes:

- N/A

crates/project/src/buffer_store.rs

@@ -0,0 +1,928 @@
+use crate::ProjectPath;
+use anyhow::{anyhow, Context as _, Result};
+use collections::{hash_map, HashMap};
+use futures::{channel::oneshot, StreamExt as _};
+use gpui::{
+    AppContext, AsyncAppContext, Context as _, EventEmitter, Model, ModelContext, Task, WeakModel,
+};
+use language::{
+    proto::{deserialize_version, serialize_version, split_operations},
+    Buffer, Capability, Language, Operation,
+};
+use rpc::{
+    proto::{self, AnyProtoClient, PeerId},
+    ErrorExt as _, TypedEnvelope,
+};
+use std::{io, path::Path, sync::Arc};
+use text::BufferId;
+use util::{debug_panic, maybe, ResultExt as _};
+use worktree::{File, ProjectEntryId, RemoteWorktree, Worktree};
+
+/// A set of open buffers.
+pub struct BufferStore {
+    retain_buffers: bool,
+    opened_buffers: HashMap<BufferId, OpenBuffer>,
+    local_buffer_ids_by_path: HashMap<ProjectPath, BufferId>,
+    local_buffer_ids_by_entry_id: HashMap<ProjectEntryId, BufferId>,
+    #[allow(clippy::type_complexity)]
+    loading_buffers_by_path: HashMap<
+        ProjectPath,
+        postage::watch::Receiver<Option<Result<Model<Buffer>, Arc<anyhow::Error>>>>,
+    >,
+    loading_remote_buffers_by_id: HashMap<BufferId, Model<Buffer>>,
+    remote_buffer_listeners:
+        HashMap<BufferId, Vec<oneshot::Sender<Result<Model<Buffer>, anyhow::Error>>>>,
+}
+
+enum OpenBuffer {
+    Strong(Model<Buffer>),
+    Weak(WeakModel<Buffer>),
+    Operations(Vec<Operation>),
+}
+
+pub enum BufferStoreEvent {
+    BufferAdded(Model<Buffer>),
+    BufferChangedFilePath {
+        buffer: Model<Buffer>,
+        old_file: Option<Arc<File>>,
+    },
+    BufferSaved {
+        buffer: Model<Buffer>,
+        has_changed_file: bool,
+        saved_version: clock::Global,
+    },
+}
+
+impl EventEmitter<BufferStoreEvent> for BufferStore {}
+
+impl BufferStore {
+    /// Creates a buffer store, optionally retaining its buffers.
+    ///
+    /// If `retain_buffers` is `true`, then buffers are owned by the buffer store
+    /// and won't be released unless they are explicitly removed, or `retain_buffers`
+    /// is set to `false` via `set_retain_buffers`. Otherwise, buffers are stored as
+    /// weak handles.
+    pub fn new(retain_buffers: bool) -> Self {
+        Self {
+            retain_buffers,
+            opened_buffers: Default::default(),
+            remote_buffer_listeners: Default::default(),
+            loading_remote_buffers_by_id: Default::default(),
+            local_buffer_ids_by_path: Default::default(),
+            local_buffer_ids_by_entry_id: Default::default(),
+            loading_buffers_by_path: Default::default(),
+        }
+    }
+
+    pub fn open_buffer(
+        &mut self,
+        project_path: ProjectPath,
+        worktree: Model<Worktree>,
+        cx: &mut ModelContext<Self>,
+    ) -> Task<Result<Model<Buffer>>> {
+        let existing_buffer = self.get_by_path(&project_path, cx);
+        if let Some(existing_buffer) = existing_buffer {
+            return Task::ready(Ok(existing_buffer));
+        }
+
+        let loading_watch = match self.loading_buffers_by_path.entry(project_path.clone()) {
+            // If the given path is already being loaded, then wait for that existing
+            // task to complete and return the same buffer.
+            hash_map::Entry::Occupied(e) => e.get().clone(),
+
+            // Otherwise, record the fact that this path is now being loaded.
+            hash_map::Entry::Vacant(entry) => {
+                let (mut tx, rx) = postage::watch::channel();
+                entry.insert(rx.clone());
+
+                let project_path = project_path.clone();
+                let load_buffer = match worktree.read(cx) {
+                    Worktree::Local(_) => {
+                        self.open_local_buffer_internal(project_path.path.clone(), worktree, cx)
+                    }
+                    Worktree::Remote(tree) => {
+                        self.open_remote_buffer_internal(&project_path.path, tree, cx)
+                    }
+                };
+
+                cx.spawn(move |this, mut cx| async move {
+                    let load_result = load_buffer.await;
+                    *tx.borrow_mut() = Some(this.update(&mut cx, |this, _| {
+                        // Record the fact that the buffer is no longer loading.
+                        this.loading_buffers_by_path.remove(&project_path);
+                        let buffer = load_result.map_err(Arc::new)?;
+                        Ok(buffer)
+                    })?);
+                    anyhow::Ok(())
+                })
+                .detach();
+                rx
+            }
+        };
+
+        cx.background_executor().spawn(async move {
+            Self::wait_for_loading_buffer(loading_watch)
+                .await
+                .map_err(|e| e.cloned())
+        })
+    }
+
+    fn open_local_buffer_internal(
+        &mut self,
+        path: Arc<Path>,
+        worktree: Model<Worktree>,
+        cx: &mut ModelContext<Self>,
+    ) -> Task<Result<Model<Buffer>>> {
+        let load_buffer = worktree.update(cx, |worktree, cx| {
+            let load_file = worktree.load_file(path.as_ref(), cx);
+            let reservation = cx.reserve_model();
+            let buffer_id = BufferId::from(reservation.entity_id().as_non_zero_u64());
+            cx.spawn(move |_, mut cx| async move {
+                let loaded = load_file.await?;
+                let text_buffer = cx
+                    .background_executor()
+                    .spawn(async move { text::Buffer::new(0, buffer_id, loaded.text) })
+                    .await;
+                cx.insert_model(reservation, |_| {
+                    Buffer::build(
+                        text_buffer,
+                        loaded.diff_base,
+                        Some(loaded.file),
+                        Capability::ReadWrite,
+                    )
+                })
+            })
+        });
+
+        cx.spawn(move |this, mut cx| async move {
+            let buffer = match load_buffer.await {
+                Ok(buffer) => Ok(buffer),
+                Err(error) if is_not_found_error(&error) => cx.new_model(|cx| {
+                    let buffer_id = BufferId::from(cx.entity_id().as_non_zero_u64());
+                    let text_buffer = text::Buffer::new(0, buffer_id, "".into());
+                    Buffer::build(
+                        text_buffer,
+                        None,
+                        Some(Arc::new(File {
+                            worktree,
+                            path,
+                            mtime: None,
+                            entry_id: None,
+                            is_local: true,
+                            is_deleted: false,
+                            is_private: false,
+                        })),
+                        Capability::ReadWrite,
+                    )
+                }),
+                Err(e) => Err(e),
+            }?;
+            this.update(&mut cx, |this, cx| {
+                this.add_buffer(buffer.clone(), cx).log_err();
+            })?;
+            Ok(buffer)
+        })
+    }
+
+    fn open_remote_buffer_internal(
+        &self,
+        path: &Arc<Path>,
+        worktree: &RemoteWorktree,
+        cx: &ModelContext<Self>,
+    ) -> Task<Result<Model<Buffer>>> {
+        let worktree_id = worktree.id().to_proto();
+        let project_id = worktree.project_id();
+        let client = worktree.client();
+        let path_string = path.clone().to_string_lossy().to_string();
+        cx.spawn(move |this, mut cx| async move {
+            let response = client
+                .request(proto::OpenBufferByPath {
+                    project_id,
+                    worktree_id,
+                    path: path_string,
+                })
+                .await?;
+            let buffer_id = BufferId::new(response.buffer_id)?;
+            this.update(&mut cx, |this, cx| {
+                this.wait_for_remote_buffer(buffer_id, cx)
+            })?
+            .await
+        })
+    }
+
+    pub fn create_buffer(
+        &mut self,
+        remote_client: Option<(AnyProtoClient, u64)>,
+        cx: &mut ModelContext<Self>,
+    ) -> Task<Result<Model<Buffer>>> {
+        if let Some((remote_client, project_id)) = remote_client {
+            let create = remote_client.request(proto::OpenNewBuffer { project_id });
+            cx.spawn(|this, mut cx| async move {
+                let response = create.await?;
+                let buffer_id = BufferId::new(response.buffer_id)?;
+
+                this.update(&mut cx, |this, cx| {
+                    this.wait_for_remote_buffer(buffer_id, cx)
+                })?
+                .await
+            })
+        } else {
+            Task::ready(Ok(self.create_local_buffer("", None, cx)))
+        }
+    }
+
+    pub fn create_local_buffer(
+        &mut self,
+        text: &str,
+        language: Option<Arc<Language>>,
+        cx: &mut ModelContext<Self>,
+    ) -> Model<Buffer> {
+        let buffer = cx.new_model(|cx| {
+            Buffer::local(text, cx)
+                .with_language(language.unwrap_or_else(|| language::PLAIN_TEXT.clone()), cx)
+        });
+        self.add_buffer(buffer.clone(), cx).log_err();
+        buffer
+    }
+
+    pub fn save_buffer(
+        &mut self,
+        buffer: Model<Buffer>,
+        cx: &mut ModelContext<Self>,
+    ) -> Task<Result<()>> {
+        let Some(file) = File::from_dyn(buffer.read(cx).file()) else {
+            return Task::ready(Err(anyhow!("buffer doesn't have a file")));
+        };
+        match file.worktree.read(cx) {
+            Worktree::Local(_) => {
+                self.save_local_buffer(file.worktree.clone(), buffer, file.path.clone(), false, cx)
+            }
+            Worktree::Remote(tree) => self.save_remote_buffer(buffer, None, tree, cx),
+        }
+    }
+
+    pub fn save_buffer_as(
+        &mut self,
+        buffer: Model<Buffer>,
+        path: ProjectPath,
+        worktree: Model<Worktree>,
+        cx: &mut ModelContext<Self>,
+    ) -> Task<Result<()>> {
+        let old_file = File::from_dyn(buffer.read(cx).file())
+            .cloned()
+            .map(Arc::new);
+
+        let task = match worktree.read(cx) {
+            Worktree::Local(_) => {
+                self.save_local_buffer(worktree, buffer.clone(), path.path, true, cx)
+            }
+            Worktree::Remote(tree) => {
+                self.save_remote_buffer(buffer.clone(), Some(path.to_proto()), tree, cx)
+            }
+        };
+        cx.spawn(|this, mut cx| async move {
+            task.await?;
+            this.update(&mut cx, |_, cx| {
+                cx.emit(BufferStoreEvent::BufferChangedFilePath { buffer, old_file });
+            })
+        })
+    }
+
+    fn save_local_buffer(
+        &self,
+        worktree: Model<Worktree>,
+        buffer_handle: Model<Buffer>,
+        path: Arc<Path>,
+        mut has_changed_file: bool,
+        cx: &mut ModelContext<Self>,
+    ) -> Task<Result<()>> {
+        let buffer = buffer_handle.read(cx);
+        let text = buffer.as_rope().clone();
+        let line_ending = buffer.line_ending();
+        let version = buffer.version();
+        if buffer.file().is_some_and(|file| !file.is_created()) {
+            has_changed_file = true;
+        }
+
+        let save = worktree.update(cx, |worktree, cx| {
+            worktree.write_file(path.as_ref(), text, line_ending, cx)
+        });
+
+        cx.spawn(move |this, mut cx| async move {
+            let new_file = save.await?;
+            let mtime = new_file.mtime;
+            buffer_handle.update(&mut cx, |buffer, cx| {
+                if has_changed_file {
+                    buffer.file_updated(new_file, cx);
+                }
+                buffer.did_save(version.clone(), mtime, cx);
+            })?;
+            this.update(&mut cx, |_, cx| {
+                cx.emit(BufferStoreEvent::BufferSaved {
+                    buffer: buffer_handle,
+                    has_changed_file,
+                    saved_version: version,
+                })
+            })?;
+            Ok(())
+        })
+    }
+
+    fn save_remote_buffer(
+        &self,
+        buffer_handle: Model<Buffer>,
+        new_path: Option<proto::ProjectPath>,
+        tree: &RemoteWorktree,
+        cx: &ModelContext<Self>,
+    ) -> Task<Result<()>> {
+        let buffer = buffer_handle.read(cx);
+        let buffer_id = buffer.remote_id().into();
+        let version = buffer.version();
+        let rpc = tree.client();
+        let project_id = tree.project_id();
+        cx.spawn(move |_, mut cx| async move {
+            let response = rpc
+                .request(proto::SaveBuffer {
+                    project_id,
+                    buffer_id,
+                    new_path,
+                    version: serialize_version(&version),
+                })
+                .await?;
+            let version = deserialize_version(&response.version);
+            let mtime = response.mtime.map(|mtime| mtime.into());
+
+            buffer_handle.update(&mut cx, |buffer, cx| {
+                buffer.did_save(version.clone(), mtime, cx);
+            })?;
+
+            Ok(())
+        })
+    }
+
+    fn add_buffer(&mut self, buffer: Model<Buffer>, cx: &mut ModelContext<Self>) -> Result<()> {
+        let remote_id = buffer.read(cx).remote_id();
+        let is_remote = buffer.read(cx).replica_id() != 0;
+        let open_buffer = if self.retain_buffers {
+            OpenBuffer::Strong(buffer.clone())
+        } else {
+            OpenBuffer::Weak(buffer.downgrade())
+        };
+
+        match self.opened_buffers.entry(remote_id) {
+            hash_map::Entry::Vacant(entry) => {
+                entry.insert(open_buffer);
+            }
+            hash_map::Entry::Occupied(mut entry) => {
+                if let OpenBuffer::Operations(operations) = entry.get_mut() {
+                    buffer.update(cx, |b, cx| b.apply_ops(operations.drain(..), cx))?;
+                } else if entry.get().upgrade().is_some() {
+                    if is_remote {
+                        return Ok(());
+                    } else {
+                        debug_panic!("buffer {} was already registered", remote_id);
+                        Err(anyhow!("buffer {} was already registered", remote_id))?;
+                    }
+                }
+                entry.insert(open_buffer);
+            }
+        }
+
+        if let Some(senders) = self.remote_buffer_listeners.remove(&remote_id) {
+            for sender in senders {
+                sender.send(Ok(buffer.clone())).ok();
+            }
+        }
+
+        if let Some(file) = File::from_dyn(buffer.read(cx).file()) {
+            if file.is_local {
+                self.local_buffer_ids_by_path.insert(
+                    ProjectPath {
+                        worktree_id: file.worktree_id(cx),
+                        path: file.path.clone(),
+                    },
+                    remote_id,
+                );
+
+                if let Some(entry_id) = file.entry_id {
+                    self.local_buffer_ids_by_entry_id
+                        .insert(entry_id, remote_id);
+                }
+            }
+        }
+
+        cx.emit(BufferStoreEvent::BufferAdded(buffer));
+        Ok(())
+    }
+
+    pub fn buffers(&self) -> impl '_ + Iterator<Item = Model<Buffer>> {
+        self.opened_buffers
+            .values()
+            .filter_map(|buffer| buffer.upgrade())
+    }
+
+    pub fn loading_buffers(
+        &self,
+    ) -> impl Iterator<
+        Item = (
+            &ProjectPath,
+            postage::watch::Receiver<Option<Result<Model<Buffer>, Arc<anyhow::Error>>>>,
+        ),
+    > {
+        self.loading_buffers_by_path
+            .iter()
+            .map(|(path, rx)| (path, rx.clone()))
+    }
+
+    pub fn get_by_path(&self, path: &ProjectPath, cx: &AppContext) -> Option<Model<Buffer>> {
+        self.buffers().find_map(|buffer| {
+            let file = File::from_dyn(buffer.read(cx).file())?;
+            if file.worktree_id(cx) == path.worktree_id && &file.path == &path.path {
+                Some(buffer)
+            } else {
+                None
+            }
+        })
+    }
+
+    pub fn get(&self, buffer_id: BufferId) -> Option<Model<Buffer>> {
+        self.opened_buffers
+            .get(&buffer_id)
+            .and_then(|buffer| buffer.upgrade())
+    }
+
+    pub fn get_existing(&self, buffer_id: BufferId) -> Result<Model<Buffer>> {
+        self.get(buffer_id)
+            .ok_or_else(|| anyhow!("unknown buffer id {}", buffer_id))
+    }
+
+    pub fn get_possibly_incomplete(&self, buffer_id: BufferId) -> Option<Model<Buffer>> {
+        self.get(buffer_id)
+            .or_else(|| self.loading_remote_buffers_by_id.get(&buffer_id).cloned())
+    }
+
+    fn get_or_remove_by_path(
+        &mut self,
+        entry_id: ProjectEntryId,
+        project_path: &ProjectPath,
+    ) -> Option<(BufferId, Model<Buffer>)> {
+        let buffer_id = match self.local_buffer_ids_by_entry_id.get(&entry_id) {
+            Some(&buffer_id) => buffer_id,
+            None => match self.local_buffer_ids_by_path.get(project_path) {
+                Some(&buffer_id) => buffer_id,
+                None => {
+                    return None;
+                }
+            },
+        };
+        let buffer = if let Some(buffer) = self.get(buffer_id) {
+            buffer
+        } else {
+            self.opened_buffers.remove(&buffer_id);
+            self.local_buffer_ids_by_path.remove(project_path);
+            self.local_buffer_ids_by_entry_id.remove(&entry_id);
+            return None;
+        };
+        Some((buffer_id, buffer))
+    }
+
+    pub fn wait_for_remote_buffer(
+        &mut self,
+        id: BufferId,
+        cx: &mut AppContext,
+    ) -> Task<Result<Model<Buffer>>> {
+        let buffer = self.get(id);
+        if let Some(buffer) = buffer {
+            return Task::ready(Ok(buffer));
+        }
+        let (tx, rx) = oneshot::channel();
+        self.remote_buffer_listeners.entry(id).or_default().push(tx);
+        cx.background_executor().spawn(async move { rx.await? })
+    }
+
+    pub fn buffer_version_info(
+        &self,
+        cx: &AppContext,
+    ) -> (Vec<proto::BufferVersion>, Vec<BufferId>) {
+        let buffers = self
+            .buffers()
+            .map(|buffer| {
+                let buffer = buffer.read(cx);
+                proto::BufferVersion {
+                    id: buffer.remote_id().into(),
+                    version: language::proto::serialize_version(&buffer.version),
+                }
+            })
+            .collect();
+        let incomplete_buffer_ids = self
+            .loading_remote_buffers_by_id
+            .keys()
+            .copied()
+            .collect::<Vec<_>>();
+        (buffers, incomplete_buffer_ids)
+    }
+
+    pub fn disconnected_from_host(&mut self, cx: &mut AppContext) {
+        self.set_retain_buffers(false, cx);
+
+        for buffer in self.buffers() {
+            buffer.update(cx, |buffer, cx| {
+                buffer.set_capability(Capability::ReadOnly, cx)
+            });
+        }
+
+        // Wake up all futures currently waiting on a buffer to get opened,
+        // to give them a chance to fail now that we've disconnected.
+        self.remote_buffer_listeners.clear();
+    }
+
+    pub fn set_retain_buffers(&mut self, retain_buffers: bool, cx: &mut AppContext) {
+        self.retain_buffers = retain_buffers;
+        for open_buffer in self.opened_buffers.values_mut() {
+            if retain_buffers {
+                if let OpenBuffer::Weak(buffer) = open_buffer {
+                    if let Some(buffer) = buffer.upgrade() {
+                        *open_buffer = OpenBuffer::Strong(buffer);
+                    }
+                }
+            } else {
+                if let Some(buffer) = open_buffer.upgrade() {
+                    buffer.update(cx, |buffer, _| buffer.give_up_waiting());
+                }
+                if let OpenBuffer::Strong(buffer) = open_buffer {
+                    *open_buffer = OpenBuffer::Weak(buffer.downgrade());
+                }
+            }
+        }
+    }
+
+    pub fn discard_incomplete(&mut self) {
+        self.opened_buffers
+            .retain(|_, buffer| !matches!(buffer, OpenBuffer::Operations(_)));
+    }
+
+    pub fn file_changed(
+        &mut self,
+        path: Arc<Path>,
+        entry_id: ProjectEntryId,
+        worktree_handle: &Model<worktree::Worktree>,
+        snapshot: &worktree::Snapshot,
+        cx: &mut ModelContext<Self>,
+    ) -> Option<(Model<Buffer>, Arc<File>, Arc<File>)> {
+        let (buffer_id, buffer) = self.get_or_remove_by_path(
+            entry_id,
+            &ProjectPath {
+                worktree_id: snapshot.id(),
+                path,
+            },
+        )?;
+
+        let result = buffer.update(cx, |buffer, cx| {
+            let old_file = File::from_dyn(buffer.file())?;
+            if old_file.worktree != *worktree_handle {
+                return None;
+            }
+
+            let new_file = if let Some(entry) = old_file
+                .entry_id
+                .and_then(|entry_id| snapshot.entry_for_id(entry_id))
+            {
+                File {
+                    is_local: true,
+                    entry_id: Some(entry.id),
+                    mtime: entry.mtime,
+                    path: entry.path.clone(),
+                    worktree: worktree_handle.clone(),
+                    is_deleted: false,
+                    is_private: entry.is_private,
+                }
+            } else if let Some(entry) = snapshot.entry_for_path(old_file.path.as_ref()) {
+                File {
+                    is_local: true,
+                    entry_id: Some(entry.id),
+                    mtime: entry.mtime,
+                    path: entry.path.clone(),
+                    worktree: worktree_handle.clone(),
+                    is_deleted: false,
+                    is_private: entry.is_private,
+                }
+            } else {
+                File {
+                    is_local: true,
+                    entry_id: old_file.entry_id,
+                    path: old_file.path.clone(),
+                    mtime: old_file.mtime,
+                    worktree: worktree_handle.clone(),
+                    is_deleted: true,
+                    is_private: old_file.is_private,
+                }
+            };
+
+            if new_file == *old_file {
+                return None;
+            }
+
+            let old_file = Arc::new(old_file.clone());
+            let new_file = Arc::new(new_file);
+            buffer.file_updated(new_file.clone(), cx);
+            Some((cx.handle(), old_file, new_file))
+        });
+
+        if let Some((buffer, old_file, new_file)) = &result {
+            if new_file.path != old_file.path {
+                self.local_buffer_ids_by_path.remove(&ProjectPath {
+                    path: old_file.path.clone(),
+                    worktree_id: old_file.worktree_id(cx),
+                });
+                self.local_buffer_ids_by_path.insert(
+                    ProjectPath {
+                        worktree_id: new_file.worktree_id(cx),
+                        path: new_file.path.clone(),
+                    },
+                    buffer_id,
+                );
+                cx.emit(BufferStoreEvent::BufferChangedFilePath {
+                    buffer: buffer.clone(),
+                    old_file: Some(old_file.clone()),
+                });
+            }
+
+            if new_file.entry_id != old_file.entry_id {
+                if let Some(entry_id) = old_file.entry_id {
+                    self.local_buffer_ids_by_entry_id.remove(&entry_id);
+                }
+                if let Some(entry_id) = new_file.entry_id {
+                    self.local_buffer_ids_by_entry_id
+                        .insert(entry_id, buffer_id);
+                }
+            }
+        }
+
+        result
+    }
+
+    pub fn buffer_changed_file(
+        &mut self,
+        buffer: Model<Buffer>,
+        cx: &mut AppContext,
+    ) -> Option<()> {
+        let file = File::from_dyn(buffer.read(cx).file())?;
+
+        let remote_id = buffer.read(cx).remote_id();
+        if let Some(entry_id) = file.entry_id {
+            match self.local_buffer_ids_by_entry_id.get(&entry_id) {
+                Some(_) => {
+                    return None;
+                }
+                None => {
+                    self.local_buffer_ids_by_entry_id
+                        .insert(entry_id, remote_id);
+                }
+            }
+        };
+        self.local_buffer_ids_by_path.insert(
+            ProjectPath {
+                worktree_id: file.worktree_id(cx),
+                path: file.path.clone(),
+            },
+            remote_id,
+        );
+
+        Some(())
+    }
+
+    pub async fn create_buffer_for_peer(
+        this: Model<Self>,
+        peer_id: PeerId,
+        buffer_id: BufferId,
+        project_id: u64,
+        client: AnyProtoClient,
+        cx: &mut AsyncAppContext,
+    ) -> Result<()> {
+        let Some(buffer) = this.update(cx, |this, _| this.get(buffer_id))? else {
+            return Ok(());
+        };
+
+        let operations = buffer.update(cx, |b, cx| b.serialize_ops(None, cx))?;
+        let operations = operations.await;
+        let state = buffer.update(cx, |buffer, _| buffer.to_proto())?;
+
+        let initial_state = proto::CreateBufferForPeer {
+            project_id,
+            peer_id: Some(peer_id),
+            variant: Some(proto::create_buffer_for_peer::Variant::State(state)),
+        };
+
+        if client.send(initial_state).log_err().is_some() {
+            let client = client.clone();
+            cx.background_executor()
+                .spawn(async move {
+                    let mut chunks = split_operations(operations).peekable();
+                    while let Some(chunk) = chunks.next() {
+                        let is_last = chunks.peek().is_none();
+                        client.send(proto::CreateBufferForPeer {
+                            project_id,
+                            peer_id: Some(peer_id),
+                            variant: Some(proto::create_buffer_for_peer::Variant::Chunk(
+                                proto::BufferChunk {
+                                    buffer_id: buffer_id.into(),
+                                    operations: chunk,
+                                    is_last,
+                                },
+                            )),
+                        })?;
+                    }
+                    anyhow::Ok(())
+                })
+                .await
+                .log_err();
+        }
+        Ok(())
+    }
+
+    pub fn handle_update_buffer(
+        &mut self,
+        envelope: TypedEnvelope<proto::UpdateBuffer>,
+        is_remote: bool,
+        cx: &mut AppContext,
+    ) -> Result<proto::Ack> {
+        let payload = envelope.payload.clone();
+        let buffer_id = BufferId::new(payload.buffer_id)?;
+        let ops = payload
+            .operations
+            .into_iter()
+            .map(language::proto::deserialize_operation)
+            .collect::<Result<Vec<_>, _>>()?;
+        match self.opened_buffers.entry(buffer_id) {
+            hash_map::Entry::Occupied(mut e) => match e.get_mut() {
+                OpenBuffer::Strong(buffer) => {
+                    buffer.update(cx, |buffer, cx| buffer.apply_ops(ops, cx))?;
+                }
+                OpenBuffer::Operations(operations) => operations.extend_from_slice(&ops),
+                OpenBuffer::Weak(_) => {}
+            },
+            hash_map::Entry::Vacant(e) => {
+                if !is_remote {
+                    debug_panic!(
+                        "received buffer update from {:?}",
+                        envelope.original_sender_id
+                    );
+                    return Err(anyhow!("received buffer update for non-remote project"));
+                }
+                e.insert(OpenBuffer::Operations(ops));
+            }
+        }
+        Ok(proto::Ack {})
+    }
+
+    pub fn handle_create_buffer_for_peer(
+        &mut self,
+        envelope: TypedEnvelope<proto::CreateBufferForPeer>,
+        mut worktrees: impl Iterator<Item = Model<Worktree>>,
+        replica_id: u16,
+        capability: Capability,
+        cx: &mut ModelContext<Self>,
+    ) -> Result<()> {
+        match envelope
+            .payload
+            .variant
+            .ok_or_else(|| anyhow!("missing variant"))?
+        {
+            proto::create_buffer_for_peer::Variant::State(mut state) => {
+                let buffer_id = BufferId::new(state.id)?;
+
+                let buffer_result = maybe!({
+                    let mut buffer_file = None;
+                    if let Some(file) = state.file.take() {
+                        let worktree_id = worktree::WorktreeId::from_proto(file.worktree_id);
+                        let worktree = worktrees
+                            .find(|worktree| worktree.read(cx).id() == worktree_id)
+                            .ok_or_else(|| {
+                                anyhow!("no worktree found for id {}", file.worktree_id)
+                            })?;
+                        buffer_file = Some(Arc::new(File::from_proto(file, worktree.clone(), cx)?)
+                            as Arc<dyn language::File>);
+                    }
+                    Buffer::from_proto(replica_id, capability, state, buffer_file)
+                });
+
+                match buffer_result {
+                    Ok(buffer) => {
+                        let buffer = cx.new_model(|_| buffer);
+                        self.loading_remote_buffers_by_id.insert(buffer_id, buffer);
+                    }
+                    Err(error) => {
+                        if let Some(listeners) = self.remote_buffer_listeners.remove(&buffer_id) {
+                            for listener in listeners {
+                                listener.send(Err(anyhow!(error.cloned()))).ok();
+                            }
+                        }
+                    }
+                }
+            }
+            proto::create_buffer_for_peer::Variant::Chunk(chunk) => {
+                let buffer_id = BufferId::new(chunk.buffer_id)?;
+                let buffer = self
+                    .loading_remote_buffers_by_id
+                    .get(&buffer_id)
+                    .cloned()
+                    .ok_or_else(|| {
+                        anyhow!(
+                            "received chunk for buffer {} without initial state",
+                            chunk.buffer_id
+                        )
+                    })?;
+
+                let result = maybe!({
+                    let operations = chunk
+                        .operations
+                        .into_iter()
+                        .map(language::proto::deserialize_operation)
+                        .collect::<Result<Vec<_>>>()?;
+                    buffer.update(cx, |buffer, cx| buffer.apply_ops(operations, cx))
+                });
+
+                if let Err(error) = result {
+                    self.loading_remote_buffers_by_id.remove(&buffer_id);
+                    if let Some(listeners) = self.remote_buffer_listeners.remove(&buffer_id) {
+                        for listener in listeners {
+                            listener.send(Err(error.cloned())).ok();
+                        }
+                    }
+                } else if chunk.is_last {
+                    self.loading_remote_buffers_by_id.remove(&buffer_id);
+                    self.add_buffer(buffer, cx)?;
+                }
+            }
+        }
+
+        Ok(())
+    }
+
+    pub async fn handle_save_buffer(
+        this: Model<Self>,
+        project_id: u64,
+        worktree: Option<Model<Worktree>>,
+        envelope: TypedEnvelope<proto::SaveBuffer>,
+        mut cx: AsyncAppContext,
+    ) -> Result<proto::BufferSaved> {
+        let buffer_id = BufferId::new(envelope.payload.buffer_id)?;
+        let buffer = this.update(&mut cx, |this, _| this.get_existing(buffer_id))??;
+        buffer
+            .update(&mut cx, |buffer, _| {
+                buffer.wait_for_version(deserialize_version(&envelope.payload.version))
+            })?
+            .await?;
+        let buffer_id = buffer.update(&mut cx, |buffer, _| buffer.remote_id())?;
+
+        if let Some(new_path) = envelope.payload.new_path {
+            let worktree = worktree.context("no such worktree")?;
+            let new_path = ProjectPath::from_proto(new_path);
+            this.update(&mut cx, |this, cx| {
+                this.save_buffer_as(buffer.clone(), new_path, worktree, cx)
+            })?
+            .await?;
+        } else {
+            this.update(&mut cx, |this, cx| this.save_buffer(buffer.clone(), cx))?
+                .await?;
+        }
+
+        buffer.update(&mut cx, |buffer, _| proto::BufferSaved {
+            project_id,
+            buffer_id: buffer_id.into(),
+            version: serialize_version(buffer.saved_version()),
+            mtime: buffer.saved_mtime().map(|time| time.into()),
+        })
+    }
+
+    pub async fn wait_for_loading_buffer(
+        mut receiver: postage::watch::Receiver<Option<Result<Model<Buffer>, Arc<anyhow::Error>>>>,
+    ) -> Result<Model<Buffer>, Arc<anyhow::Error>> {
+        loop {
+            if let Some(result) = receiver.borrow().as_ref() {
+                match result {
+                    Ok(buffer) => return Ok(buffer.to_owned()),
+                    Err(e) => return Err(e.to_owned()),
+                }
+            }
+            receiver.next().await;
+        }
+    }
+}
+
+impl OpenBuffer {
+    fn upgrade(&self) -> Option<Model<Buffer>> {
+        match self {
+            OpenBuffer::Strong(handle) => Some(handle.clone()),
+            OpenBuffer::Weak(handle) => handle.upgrade(),
+            OpenBuffer::Operations(_) => None,
+        }
+    }
+}
+
+fn is_not_found_error(error: &anyhow::Error) -> bool {
+    error
+        .root_cause()
+        .downcast_ref::<io::Error>()
+        .is_some_and(|err| err.kind() == io::ErrorKind::NotFound)
+}