[ty] Request configuration from client (#18984)

## Summary

This PR makes the necessary changes to the server that it can request
configurations from the client using the `configuration` request.
This PR doesn't make use of the request yet. It only sets up the
foundation (mainly the coordination between client and server)
so that future PRs could pull specific settings. 

I plan to use this for pulling the Python environment from the Python
extension.

Deno does something very similar to this.

## Test Plan

Tested that diagnostics are still shown.

crates/ruff_db/src/files.rs

@@ -263,12 +263,23 @@ impl Files {
 
 impl fmt::Debug for Files {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        let mut map = f.debug_map();
+        if f.alternate() {
+            let mut map = f.debug_map();
 
-        for entry in self.inner.system_by_path.iter() {
-            map.entry(entry.key(), entry.value());
+            for entry in self.inner.system_by_path.iter() {
+                map.entry(entry.key(), entry.value());
+            }
+            map.finish()
+        } else {
+            f.debug_struct("Files")
+                .field("system_by_path", &self.inner.system_by_path.len())
+                .field(
+                    "system_virtual_by_path",
+                    &self.inner.system_virtual_by_path.len(),
+                )
+                .field("vendored_by_path", &self.inner.vendored_by_path.len())
+                .finish()
         }
-        map.finish()
     }
 }
 

crates/ty_project/src/db.rs

@@ -1,3 +1,4 @@
+use std::fmt::Formatter;
 use std::panic::{AssertUnwindSafe, RefUnwindSafe};
 use std::sync::Arc;
 use std::{cmp, fmt};
@@ -146,6 +147,16 @@ impl ProjectDatabase {
     }
 }
 
+impl std::fmt::Debug for ProjectDatabase {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        f.debug_struct("ProjectDatabase")
+            .field("project", &self.project)
+            .field("files", &self.files)
+            .field("system", &self.system)
+            .finish_non_exhaustive()
+    }
+}
+
 /// Stores memory usage information.
 pub struct SalsaMemoryDump {
     total_fields: usize,

crates/ty_project/src/lib.rs

@@ -55,6 +55,7 @@ pub fn default_lints_registry() -> LintRegistry {
 ///    it remains the same project. That's why program is a narrowed view of the project only
 ///    holding on to the most fundamental settings required for checking.
 #[salsa::input]
+#[derive(Debug)]
 pub struct Project {
     /// The files that are open in the project.
     ///

crates/ty_server/src/server.rs

@@ -136,7 +136,7 @@ impl Server {
                 &client_capabilities,
                 position_encoding,
                 global_options,
-                &workspaces,
+                workspaces,
             )?,
             client_capabilities,
         })
@@ -227,12 +227,10 @@ impl ServerPanicHookHandler {
             writeln!(stderr, "{panic_info}\n{backtrace}").ok();
 
             if let Some(client) = hook_client.upgrade() {
-                client
-                    .show_message(
-                        "The ty language server exited with a panic. See the logs for more details.",
-                        MessageType::ERROR,
-                    )
-                    .ok();
+                client.show_message(
+                    "The ty language server exited with a panic. See the logs for more details.",
+                    MessageType::ERROR,
+                );
             }
         }));
 

crates/ty_server/src/server/api.rs

@@ -160,7 +160,7 @@ where
         };
 
         let db = match &path {
-            AnySystemPath::System(path) => match session.project_db_for_path(path.as_std_path()) {
+            AnySystemPath::System(path) => match session.project_db_for_path(path) {
                 Some(db) => db.clone(),
                 None => session.default_project_db().clone(),
             },
@@ -224,17 +224,14 @@ where
                         request.id,
                         request.method
                     );
-                    if client.retry(request).is_ok() {
-                        return None;
-                    }
+                    client.retry(request);
+                } else {
+                    tracing::trace!(
+                        "request id={} was cancelled by salsa, sending content modified",
+                        id
+                    );
+                    respond_silent_error(id.clone(), client, R::salsa_cancellation_error());
                 }
-
-                tracing::trace!(
-                    "request id={} was cancelled by salsa, sending content modified",
-                    id
-                );
-
-                respond_silent_error(id.clone(), client, R::salsa_cancellation_error());
                 None
             } else {
                 Some(Err(Error {
@@ -343,17 +340,13 @@ fn respond<Req>(
         tracing::error!("An error occurred with request ID {id}: {err}");
         client.show_error_message("ty encountered a problem. Check the logs for more details.");
     }
-    if let Err(err) = client.respond(id, result) {
-        tracing::error!("Failed to send response: {err}");
-    }
+    client.respond(id, result);
 }
 
 /// Sends back an error response to the server using a [`Client`] without showing a warning
 /// to the user.
 fn respond_silent_error(id: RequestId, client: &Client, error: lsp_server::ResponseError) {
-    if let Err(err) = client.respond_err(id, error) {
-        tracing::error!("Failed to send response: {err}");
-    }
+    client.respond_err(id, error);
 }
 
 /// Tries to cast a serialized request from the server into

crates/ty_server/src/server/api/diagnostics.rs

@@ -1,4 +1,3 @@
-use lsp_server::ErrorCode;
 use lsp_types::notification::PublishDiagnostics;
 use lsp_types::{
     CodeDescription, Diagnostic, DiagnosticRelatedInformation, DiagnosticSeverity, DiagnosticTag,
@@ -46,20 +45,17 @@ impl Diagnostics {
 /// This is done by notifying the client with an empty list of diagnostics for the document.
 /// For notebook cells, this clears diagnostics for the specific cell.
 /// For other document types, this clears diagnostics for the main document.
-pub(super) fn clear_diagnostics(key: &DocumentKey, client: &Client) -> Result<()> {
+pub(super) fn clear_diagnostics(key: &DocumentKey, client: &Client) {
     let Some(uri) = key.to_url() else {
         // If we can't convert to URL, we can't clear diagnostics
-        return Ok(());
+        return;
     };
 
-    client
-        .send_notification::<PublishDiagnostics>(PublishDiagnosticsParams {
-            uri,
-            diagnostics: vec![],
-            version: None,
-        })
-        .with_failure_code(ErrorCode::InternalError)?;
-    Ok(())
+    client.send_notification::<PublishDiagnostics>(PublishDiagnosticsParams {
+        uri,
+        diagnostics: vec![],
+        version: None,
+    });
 }
 
 /// Publishes the diagnostics for the given document snapshot using the [publish diagnostics
@@ -96,22 +92,20 @@ pub(super) fn publish_diagnostics(
 
     // Sends a notification to the client with the diagnostics for the document.
     let publish_diagnostics_notification = |uri: Url, diagnostics: Vec<Diagnostic>| {
-        client
-            .send_notification::<PublishDiagnostics>(PublishDiagnosticsParams {
-                uri,
-                diagnostics,
-                version: Some(snapshot.query().version()),
-            })
-            .with_failure_code(lsp_server::ErrorCode::InternalError)
+        client.send_notification::<PublishDiagnostics>(PublishDiagnosticsParams {
+            uri,
+            diagnostics,
+            version: Some(snapshot.query().version()),
+        });
     };
 
     match diagnostics {
         Diagnostics::TextDocument(diagnostics) => {
-            publish_diagnostics_notification(url, diagnostics)?;
+            publish_diagnostics_notification(url, diagnostics);
         }
         Diagnostics::NotebookDocument(cell_diagnostics) => {
             for (cell_url, diagnostics) in cell_diagnostics {
-                publish_diagnostics_notification(cell_url, diagnostics)?;
+                publish_diagnostics_notification(cell_url, diagnostics);
             }
         }
     }

crates/ty_server/src/server/api/notifications/cancel.rs

@@ -20,7 +20,7 @@ impl SyncNotificationHandler for CancelNotificationHandler {
             lsp_types::NumberOrString::String(id) => id.into(),
         };
 
-        let _ = client.cancel(session, id);
+        client.cancel(session, id);
 
         Ok(())
     }

crates/ty_server/src/server/api/notifications/did_change.rs

@@ -39,7 +39,7 @@ impl SyncNotificationHandler for DidChangeTextDocumentHandler {
 
         match key.path() {
             AnySystemPath::System(path) => {
-                let db = match session.project_db_for_path_mut(path.as_std_path()) {
+                let db = match session.project_db_for_path_mut(path) {
                     Some(db) => db,
                     None => session.default_project_db_mut(),
                 };

crates/ty_server/src/server/api/notifications/did_change_watched_files.rs

@@ -1,5 +1,4 @@
 use crate::server::Result;
-use crate::server::api::LSPResult;
 use crate::server::api::diagnostics::publish_diagnostics;
 use crate::server::api::traits::{NotificationHandler, SyncNotificationHandler};
 use crate::session::Session;
@@ -45,7 +44,7 @@ impl SyncNotificationHandler for DidChangeWatchedFiles {
                 }
             };
 
-            let Some(db) = session.project_db_for_path(system_path.as_std_path()) else {
+            let Some(db) = session.project_db_for_path(&system_path) else {
                 tracing::trace!(
                     "Ignoring change event for `{system_path}` because it's not in any workspace"
                 );
@@ -103,13 +102,11 @@ impl SyncNotificationHandler for DidChangeWatchedFiles {
 
         if project_changed {
             if client_capabilities.diagnostics_refresh {
-                client
-                    .send_request::<types::request::WorkspaceDiagnosticRefresh>(
-                        session,
-                        (),
-                        |_, ()| {},
-                    )
-                    .with_failure_code(lsp_server::ErrorCode::InternalError)?;
+                client.send_request::<types::request::WorkspaceDiagnosticRefresh>(
+                    session,
+                    (),
+                    |_, ()| {},
+                );
             } else {
                 for key in session.text_document_keys() {
                     publish_diagnostics(session, &key, client)?;
@@ -120,9 +117,7 @@ impl SyncNotificationHandler for DidChangeWatchedFiles {
         }
 
         if client_capabilities.inlay_refresh {
-            client
-                .send_request::<types::request::InlayHintRefreshRequest>(session, (), |_, ()| {})
-                .with_failure_code(lsp_server::ErrorCode::InternalError)?;
+            client.send_request::<types::request::InlayHintRefreshRequest>(session, (), |_, ()| {});
         }
 
         Ok(())

crates/ty_server/src/server/api/notifications/did_close.rs

@@ -41,6 +41,8 @@ impl SyncNotificationHandler for DidCloseTextDocumentHandler {
             );
         }
 
-        clear_diagnostics(&key, client)
+        clear_diagnostics(&key, client);
+
+        Ok(())
     }
 }

crates/ty_server/src/server/api/notifications/did_open.rs

@@ -41,7 +41,7 @@ impl SyncNotificationHandler for DidOpenTextDocumentHandler {
 
         match key.path() {
             AnySystemPath::System(system_path) => {
-                let db = match session.project_db_for_path_mut(system_path.as_std_path()) {
+                let db = match session.project_db_for_path_mut(system_path) {
                     Some(db) => db,
                     None => session.default_project_db_mut(),
                 };

crates/ty_server/src/server/api/notifications/did_open_notebook.rs

@@ -40,7 +40,7 @@ impl SyncNotificationHandler for DidOpenNotebookHandler {
 
         match &path {
             AnySystemPath::System(system_path) => {
-                let db = match session.project_db_for_path_mut(system_path.as_std_path()) {
+                let db = match session.project_db_for_path_mut(system_path) {
                     Some(db) => db,
                     None => session.default_project_db_mut(),
                 };

crates/ty_server/src/server/main_loop.rs

@@ -1,11 +1,15 @@
 use crate::server::schedule::Scheduler;
 use crate::server::{Server, api};
+use crate::session::ClientOptions;
 use crate::session::client::Client;
 use anyhow::anyhow;
 use crossbeam::select;
 use lsp_server::Message;
 use lsp_types::notification::Notification;
-use lsp_types::{DidChangeWatchedFilesRegistrationOptions, FileSystemWatcher};
+use lsp_types::{
+    ConfigurationParams, DidChangeWatchedFilesRegistrationOptions, FileSystemWatcher, Url,
+};
+use serde_json::Value;
 
 pub(crate) type MainLoopSender = crossbeam::channel::Sender<Event>;
 pub(crate) type MainLoopReceiver = crossbeam::channel::Receiver<Event>;
@@ -26,6 +30,10 @@ impl Server {
 
             match next_event {
                 Event::Message(msg) => {
+                    let Some(msg) = self.session.should_defer_message(msg) else {
+                        continue;
+                    };
+
                     let client = Client::new(
                         self.main_loop_sender.clone(),
                         self.connection.sender.clone(),
@@ -49,7 +57,7 @@ impl Server {
                                         message: "Shutdown already requested".to_owned(),
                                         data: None,
                                     },
-                                )?;
+                                );
                                 continue;
                             }
 
@@ -130,6 +138,9 @@ impl Server {
                             );
                         }
                     }
+                    Action::InitializeWorkspaces(workspaces_with_options) => {
+                        self.session.initialize_workspaces(workspaces_with_options);
+                    }
                 },
             }
         }
@@ -140,7 +151,25 @@ impl Server {
     /// Waits for the next message from the client or action.
     ///
     /// Returns `Ok(None)` if the client connection is closed.
-    fn next_event(&self) -> Result<Option<Event>, crossbeam::channel::RecvError> {
+    fn next_event(&mut self) -> Result<Option<Event>, crossbeam::channel::RecvError> {
+        // We can't queue those into the main loop because that could result in reordering if
+        // the `select` below picks a client message first.
+        if let Some(deferred) = self.session.take_deferred_messages() {
+            match &deferred {
+                Message::Request(req) => {
+                    tracing::debug!("Processing deferred request `{}`", req.method);
+                }
+                Message::Notification(notification) => {
+                    tracing::debug!("Processing deferred notification `{}`", notification.method);
+                }
+                Message::Response(response) => {
+                    tracing::debug!("Processing deferred response `{}`", response.id);
+                }
+            }
+
+            return Ok(Some(Event::Message(deferred)));
+        }
+
         select!(
             recv(self.connection.receiver) -> msg => {
                 // Ignore disconnect errors, they're handled by the main loop (it will exit).
@@ -151,6 +180,47 @@ impl Server {
     }
 
     fn initialize(&mut self, client: &Client) {
+        let urls = self
+            .session
+            .workspaces()
+            .urls()
+            .cloned()
+            .collect::<Vec<_>>();
+        let items = urls
+            .iter()
+            .map(|root| lsp_types::ConfigurationItem {
+                scope_uri: Some(root.clone()),
+                section: Some("ty".to_string()),
+            })
+            .collect();
+
+        tracing::debug!("Requesting workspace configuration for workspaces");
+        client
+            .send_request::<lsp_types::request::WorkspaceConfiguration>(
+                &self.session,
+                ConfigurationParams { items },
+                |client, result: Vec<Value>| {
+                    tracing::debug!("Received workspace configurations, initializing workspaces");
+                    assert_eq!(result.len(), urls.len());
+
+                    let workspaces_with_options: Vec<_> = urls
+                        .into_iter()
+                        .zip(result)
+                        .map(|(url, value)| {
+                            let options: ClientOptions = serde_json::from_value(value).unwrap_or_else(|err| {
+                                tracing::warn!("Failed to deserialize workspace options for {url}: {err}. Using default options.");
+                                ClientOptions::default()
+                            });
+
+                            (url, options)
+                        })
+                        .collect();
+
+
+                    client.queue_action(Action::InitializeWorkspaces(workspaces_with_options));
+                },
+            );
+
         let fs_watcher = self
             .client_capabilities
             .workspace
@@ -206,17 +276,13 @@ impl Server {
                 tracing::info!("File watcher successfully registered");
             };
 
-            if let Err(err) = client.send_request::<lsp_types::request::RegisterCapability>(
+            client.send_request::<lsp_types::request::RegisterCapability>(
                 &self.session,
                 lsp_types::RegistrationParams {
                     registrations: vec![registration],
                 },
                 response_handler,
-            ) {
-                tracing::error!(
-                    "An error occurred when trying to register the configuration file watcher: {err}"
-                );
-            }
+            );
         } else {
             tracing::warn!("The client does not support file system watching.");
         }
@@ -231,6 +297,10 @@ pub(crate) enum Action {
 
     /// Retry a request that previously failed due to a salsa cancellation.
     RetryRequest(lsp_server::Request),
+
+    /// Initialize the workspace after the server received
+    /// the options from the client.
+    InitializeWorkspaces(Vec<(Url, ClientOptions)>),
 }
 
 #[derive(Debug)]

crates/ty_server/src/server/schedule/task.rs

@@ -83,9 +83,7 @@ impl Task {
         R: Serialize + Send + 'static,
     {
         Self::sync(move |_, client| {
-            if let Err(err) = client.respond(&id, result) {
-                tracing::error!("Unable to send immediate response: {err}");
-            }
+            client.respond(&id, result);
         })
     }
 

crates/ty_server/src/session.rs

@@ -1,22 +1,24 @@
 //! Data model, state management, and configuration resolution.
 
-use std::collections::BTreeMap;
+use std::collections::{BTreeMap, VecDeque};
 use std::ops::{Deref, DerefMut};
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
 
-use anyhow::anyhow;
+use anyhow::{Context, anyhow};
+use lsp_server::Message;
 use lsp_types::{ClientCapabilities, TextDocumentContentChangeEvent, Url};
 use options::GlobalOptions;
 use ruff_db::Db;
 use ruff_db::files::{File, system_path_to_file};
-use ruff_db::system::SystemPath;
+use ruff_db::system::{System, SystemPath, SystemPathBuf};
+use ty_project::metadata::Options;
 use ty_project::{ProjectDatabase, ProjectMetadata};
 
 pub(crate) use self::capabilities::ResolvedClientCapabilities;
 pub use self::index::DocumentQuery;
 pub(crate) use self::options::{AllOptions, ClientOptions};
-use self::settings::ClientSettings;
+pub(crate) use self::settings::ClientSettings;
 use crate::document::{DocumentKey, DocumentVersion, NotebookDocument};
 use crate::session::request_queue::RequestQueue;
 use crate::system::{AnySystemPath, LSPSystem};
@@ -40,8 +42,13 @@ pub struct Session {
     /// [`index_mut`]: Session::index_mut
     index: Option<Arc<index::Index>>,
 
-    /// Maps workspace folders to their respective project databases.
-    projects_by_workspace_folder: BTreeMap<PathBuf, ProjectDatabase>,
+    /// Maps workspace folders to their respective workspace.
+    workspaces: Workspaces,
+
+    /// The projects across all workspaces.
+    projects: BTreeMap<SystemPathBuf, ProjectDatabase>,
+
+    default_project: ProjectDatabase,
 
     /// The global position encoding, negotiated during LSP initialization.
     position_encoding: PositionEncoding,
@@ -54,6 +61,8 @@ pub struct Session {
 
     /// Has the client requested the server to shutdown.
     shutdown_requested: bool,
+
+    deferred_messages: VecDeque<Message>,
 }
 
 impl Session {
@@ -61,32 +70,33 @@ impl Session {
         client_capabilities: &ClientCapabilities,
         position_encoding: PositionEncoding,
         global_options: GlobalOptions,
-        workspace_folders: &[(Url, ClientOptions)],
+        workspace_folders: Vec<(Url, ClientOptions)>,
     ) -> crate::Result<Self> {
-        let mut workspaces = BTreeMap::new();
         let index = Arc::new(index::Index::new(global_options.into_settings()));
 
-        // TODO: Consider workspace settings
-        for (url, _) in workspace_folders {
-            let path = url
-                .to_file_path()
-                .map_err(|()| anyhow!("Workspace URL is not a file or directory: {:?}", url))?;
-            let system_path = SystemPath::from_std_path(&path)
-                .ok_or_else(|| anyhow!("Workspace path is not a valid UTF-8 path: {:?}", path))?;
-            let system = LSPSystem::new(index.clone());
-
-            // TODO(dhruvmanila): Get the values from the client settings
-            let mut metadata = ProjectMetadata::discover(system_path, &system)?;
-            metadata.apply_configuration_files(&system)?;
-
-            // TODO(micha): Handle the case where the program settings are incorrect more gracefully.
-            workspaces.insert(path, ProjectDatabase::new(metadata, system)?);
+        let mut workspaces = Workspaces::default();
+        for (url, options) in workspace_folders {
+            workspaces.register(url, options)?;
         }
 
+        let default_project = {
+            let system = LSPSystem::new(index.clone());
+            let metadata = ProjectMetadata::from_options(
+                Options::default(),
+                system.current_directory().to_path_buf(),
+                None,
+            )
+            .unwrap();
+            ProjectDatabase::new(metadata, system).unwrap()
+        };
+
         Ok(Self {
             position_encoding,
-            projects_by_workspace_folder: workspaces,
+            workspaces,
+            deferred_messages: VecDeque::new(),
             index: Some(index),
+            default_project,
+            projects: BTreeMap::new(),
             resolved_client_capabilities: Arc::new(ResolvedClientCapabilities::new(
                 client_capabilities,
             )),
@@ -111,6 +121,52 @@ impl Session {
         self.shutdown_requested = requested;
     }
 
+    /// The LSP specification doesn't allow configuration requests during initialization,
+    /// but we need access to the configuration to resolve the settings in turn to create the
+    /// project databases. This will become more important in the future when we support
+    /// persistent caching. It's then crucial that we have the correct settings to select the
+    /// right cache.
+    ///
+    /// We work around this by queueing up all messages that arrive between the `initialized` notification
+    /// and the completion of workspace initialization (which waits for the client's configuration response).
+    ///
+    /// This queuing is only necessary when registering *new* workspaces. Changes to configurations
+    /// don't need to go through the same process because we can update the existing
+    /// database in place.
+    ///
+    /// See <https://github.com/Microsoft/language-server-protocol/issues/567#issuecomment-2085131917>
+    pub(crate) fn should_defer_message(&mut self, message: Message) -> Option<Message> {
+        if self.workspaces.all_initialized() {
+            Some(message)
+        } else {
+            match &message {
+                Message::Request(request) => {
+                    tracing::debug!(
+                        "Deferring `{}` request until all workspaces are initialized",
+                        request.method
+                    );
+                }
+                Message::Response(_) => {
+                    // We still want to get client responses even during workspace initialization.
+                    return Some(message);
+                }
+                Message::Notification(notification) => {
+                    tracing::debug!(
+                        "Deferring `{}` notification until all workspaces are initialized",
+                        notification.method
+                    );
+                }
+            }
+
+            self.deferred_messages.push_back(message);
+            None
+        }
+    }
+
+    pub(crate) fn workspaces(&self) -> &Workspaces {
+        &self.workspaces
+    }
+
     // TODO(dhruvmanila): Ideally, we should have a single method for `workspace_db_for_path_mut`
     // and `default_workspace_db_mut` but the borrow checker doesn't allow that.
     // https://github.com/astral-sh/ruff/pull/13041#discussion_r1726725437
@@ -119,14 +175,17 @@ impl Session {
     /// or the default project if no project is found for the path.
     pub(crate) fn project_db_or_default(&self, path: &AnySystemPath) -> &ProjectDatabase {
         path.as_system()
-            .and_then(|path| self.project_db_for_path(path.as_std_path()))
+            .and_then(|path| self.project_db_for_path(path))
             .unwrap_or_else(|| self.default_project_db())
     }
 
     /// Returns a reference to the project's [`ProjectDatabase`] corresponding to the given path, if
     /// any.
-    pub(crate) fn project_db_for_path(&self, path: impl AsRef<Path>) -> Option<&ProjectDatabase> {
-        self.projects_by_workspace_folder
+    pub(crate) fn project_db_for_path(
+        &self,
+        path: impl AsRef<SystemPath>,
+    ) -> Option<&ProjectDatabase> {
+        self.projects
             .range(..=path.as_ref().to_path_buf())
             .next_back()
             .map(|(_, db)| db)
@@ -136,9 +195,9 @@ impl Session {
     /// path, if any.
     pub(crate) fn project_db_for_path_mut(
         &mut self,
-        path: impl AsRef<Path>,
+        path: impl AsRef<SystemPath>,
     ) -> Option<&mut ProjectDatabase> {
-        self.projects_by_workspace_folder
+        self.projects
             .range_mut(..=path.as_ref().to_path_buf())
             .next_back()
             .map(|(_, db)| db)
@@ -147,23 +206,85 @@ impl Session {
     /// Returns a reference to the default project [`ProjectDatabase`]. The default project is the
     /// minimum root path in the project map.
     pub(crate) fn default_project_db(&self) -> &ProjectDatabase {
-        // SAFETY: Currently, ty only support a single project.
-        self.projects_by_workspace_folder.values().next().unwrap()
+        &self.default_project
     }
 
     /// Returns a mutable reference to the default project [`ProjectDatabase`].
     pub(crate) fn default_project_db_mut(&mut self) -> &mut ProjectDatabase {
-        // SAFETY: Currently, ty only support a single project.
-        self.projects_by_workspace_folder
+        &mut self.default_project
+    }
+
+    fn projects_mut(&mut self) -> impl Iterator<Item = &'_ mut ProjectDatabase> + '_ {
+        self.projects
             .values_mut()
-            .next()
-            .unwrap()
+            .chain(std::iter::once(&mut self.default_project))
     }
 
     pub(crate) fn key_from_url(&self, url: Url) -> crate::Result<DocumentKey> {
         self.index().key_from_url(url)
     }
 
+    pub(crate) fn initialize_workspaces(&mut self, workspace_settings: Vec<(Url, ClientOptions)>) {
+        assert!(!self.workspaces.all_initialized());
+
+        for (url, options) in workspace_settings {
+            let Some(workspace) = self.workspaces.initialize(&url, options) else {
+                continue;
+            };
+            // For now, create one project database per workspace.
+            // In the future, index the workspace directories to find all projects
+            // and create a project database for each.
+            let system = LSPSystem::new(self.index.as_ref().unwrap().clone());
+
+            let Some(system_path) = SystemPath::from_std_path(workspace.root()) else {
+                tracing::warn!(
+                    "Ignore workspace `{}` because it's root contains non UTF8 characters",
+                    workspace.root().display()
+                );
+                continue;
+            };
+
+            let root = system_path.to_path_buf();
+            let project = ProjectMetadata::discover(&root, &system)
+                .context("Failed to find project configuration")
+                .and_then(|mut metadata| {
+                    // TODO(dhruvmanila): Merge the client options with the project metadata options.
+                    metadata
+                        .apply_configuration_files(&system)
+                        .context("Failed to apply configuration files")?;
+                    ProjectDatabase::new(metadata, system)
+                        .context("Failed to create project database")
+                });
+
+            // TODO(micha): Handle the case where the program settings are incorrect more gracefully.
+            // The easiest is to ignore those projects but to show a message to the user that we do so.
+            // Ignoring the projects has the effect that we'll use the default project for those files.
+            // The only challenge with this is that we need to register the project when the configuration
+            // becomes valid again. But that's a case we need to handle anyway for good mono repository support.
+            match project {
+                Ok(project) => {
+                    self.projects.insert(root, project);
+                }
+                Err(err) => {
+                    tracing::warn!("Failed to create project database for `{root}`: {err}",);
+                }
+            }
+        }
+
+        assert!(
+            self.workspaces.all_initialized(),
+            "All workspaces should be initialized after calling `initialize_workspaces`"
+        );
+    }
+
+    pub(crate) fn take_deferred_messages(&mut self) -> Option<Message> {
+        if self.workspaces.all_initialized() {
+            self.deferred_messages.pop_front()
+        } else {
+            None
+        }
+    }
+
     /// Creates a document snapshot with the URL referencing the document to snapshot.
     ///
     /// Returns `None` if the url can't be converted to a document key or if the document isn't open.
@@ -240,7 +361,7 @@ impl Session {
     fn index_mut(&mut self) -> MutIndexGuard {
         let index = self.index.take().unwrap();
 
-        for db in self.projects_by_workspace_folder.values_mut() {
+        for db in self.projects_mut() {
             // Remove the `index` from each database. This drops the count of `Arc<Index>` down to 1
             db.system_mut()
                 .as_any_mut()
@@ -250,11 +371,11 @@ impl Session {
         }
 
         // There should now be exactly one reference to index which is self.index.
-        let index = Arc::into_inner(index);
+        let index = Arc::into_inner(index).unwrap();
 
         MutIndexGuard {
             session: self,
-            index,
+            index: Some(index),
         }
     }
 
@@ -289,7 +410,7 @@ impl Drop for MutIndexGuard<'_> {
     fn drop(&mut self) {
         if let Some(index) = self.index.take() {
             let index = Arc::new(index);
-            for db in self.session.projects_by_workspace_folder.values_mut() {
+            for db in self.session.projects_mut() {
                 db.system_mut()
                     .as_any_mut()
                     .downcast_mut::<LSPSystem>()
@@ -339,3 +460,72 @@ impl DocumentSnapshot {
         }
     }
 }
+
+#[derive(Debug, Default)]
+pub(crate) struct Workspaces {
+    workspaces: BTreeMap<Url, Workspace>,
+    uninitialized: usize,
+}
+
+impl Workspaces {
+    pub(crate) fn register(&mut self, url: Url, options: ClientOptions) -> anyhow::Result<()> {
+        let path = url
+            .to_file_path()
+            .map_err(|()| anyhow!("Workspace URL is not a file or directory: {url:?}"))?;
+
+        self.workspaces.insert(
+            url,
+            Workspace {
+                options,
+                root: path,
+            },
+        );
+
+        self.uninitialized += 1;
+
+        Ok(())
+    }
+
+    pub(crate) fn initialize(
+        &mut self,
+        url: &Url,
+        options: ClientOptions,
+    ) -> Option<&mut Workspace> {
+        if let Some(workspace) = self.workspaces.get_mut(url) {
+            workspace.options = options;
+            self.uninitialized -= 1;
+            Some(workspace)
+        } else {
+            None
+        }
+    }
+
+    pub(crate) fn urls(&self) -> impl Iterator<Item = &Url> + '_ {
+        self.workspaces.keys()
+    }
+
+    pub(crate) fn all_initialized(&self) -> bool {
+        self.uninitialized == 0
+    }
+}
+
+impl<'a> IntoIterator for &'a Workspaces {
+    type Item = (&'a Url, &'a Workspace);
+    type IntoIter = std::collections::btree_map::Iter<'a, Url, Workspace>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.workspaces.iter()
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct Workspace {
+    root: PathBuf,
+    options: ClientOptions,
+}
+
+impl Workspace {
+    pub(crate) fn root(&self) -> &Path {
+        &self.root
+    }
+}

crates/ty_server/src/session/client.rs

@@ -1,7 +1,6 @@
 use crate::Session;
 use crate::server::{Action, ConnectionSender};
 use crate::server::{Event, MainLoopSender};
-use anyhow::{Context, anyhow};
 use lsp_server::{ErrorCode, Message, Notification, RequestId, ResponseError};
 use serde_json::Value;
 use std::any::TypeId;
@@ -45,8 +44,7 @@ impl Client {
         session: &Session,
         params: R::Params,
         response_handler: impl FnOnce(&Client, R::Result) + Send + 'static,
-    ) -> crate::Result<()>
-    where
+    ) where
         R: lsp_types::request::Request,
     {
         let response_handler = Box::new(move |client: &Client, response: lsp_server::Response| {
@@ -95,60 +93,64 @@ impl Client {
             .outgoing()
             .register(response_handler);
 
-        self.client_sender
+        if let Err(err) = self
+            .client_sender
             .send(Message::Request(lsp_server::Request {
                 id,
                 method: R::METHOD.to_string(),
-                params: serde_json::to_value(params).context("Failed to serialize params")?,
+                params: serde_json::to_value(params).expect("Params to be serializable"),
             }))
-            .with_context(|| {
-                format!("Failed to send request method={method}", method = R::METHOD)
-            })?;
-
-        Ok(())
+        {
+            tracing::error!(
+                "Failed to send request `{}` because the client sender is closed: {err}",
+                R::METHOD
+            );
+        }
     }
 
     /// Sends a notification to the client.
-    pub(crate) fn send_notification<N>(&self, params: N::Params) -> crate::Result<()>
+    pub(crate) fn send_notification<N>(&self, params: N::Params)
     where
         N: lsp_types::notification::Notification,
     {
         let method = N::METHOD.to_string();
 
-        self.client_sender
-            .send(lsp_server::Message::Notification(Notification::new(
-                method, params,
-            )))
-            .map_err(|error| {
-                anyhow!(
-                    "Failed to send notification (method={method}): {error}",
-                    method = N::METHOD
-                )
-            })
+        if let Err(err) =
+            self.client_sender
+                .send(lsp_server::Message::Notification(Notification::new(
+                    method, params,
+                )))
+        {
+            tracing::error!(
+                "Failed to send notification `{}` because the client sender is closed: {err}",
+                N::METHOD
+            );
+        }
     }
 
     /// Sends a notification without any parameters to the client.
     ///
     /// This is useful for notifications that don't require any data.
     #[expect(dead_code)]
-    pub(crate) fn send_notification_no_params(&self, method: &str) -> crate::Result<()> {
-        self.client_sender
-            .send(lsp_server::Message::Notification(Notification::new(
-                method.to_string(),
-                Value::Null,
-            )))
-            .map_err(|error| anyhow!("Failed to send notification (method={method}): {error}",))
+    pub(crate) fn send_notification_no_params(&self, method: &str) {
+        if let Err(err) =
+            self.client_sender
+                .send(lsp_server::Message::Notification(Notification::new(
+                    method.to_string(),
+                    Value::Null,
+                )))
+        {
+            tracing::error!(
+                "Failed to send notification `{method}` because the client sender is closed: {err}",
+            );
+        }
     }
 
     /// Sends a response to the client for a given request ID.
     ///
     /// The response isn't sent immediately. Instead, it's queued up in the main loop
     /// and checked for cancellation (each request must have exactly one response).
-    pub(crate) fn respond<R>(
-        &self,
-        id: &RequestId,
-        result: crate::server::Result<R>,
-    ) -> crate::Result<()>
+    pub(crate) fn respond<R>(&self, id: &RequestId, result: crate::server::Result<R>)
     where
         R: serde::Serialize,
     {
@@ -161,17 +163,13 @@ impl Client {
 
         self.main_loop_sender
             .send(Event::Action(Action::SendResponse(response)))
-            .map_err(|error| anyhow!("Failed to send response for request {id}: {error}"))
+            .unwrap();
     }
 
     /// Sends an error response to the client for a given request ID.
     ///
     /// The response isn't sent immediately. Instead, it's queued up in the main loop.
-    pub(crate) fn respond_err(
-        &self,
-        id: RequestId,
-        error: lsp_server::ResponseError,
-    ) -> crate::Result<()> {
+    pub(crate) fn respond_err(&self, id: RequestId, error: lsp_server::ResponseError) {
         let response = lsp_server::Response {
             id,
             result: None,
@@ -180,23 +178,19 @@ impl Client {
 
         self.main_loop_sender
             .send(Event::Action(Action::SendResponse(response)))
-            .map_err(|error| anyhow!("Failed to send response: {error}"))
+            .unwrap();
     }
 
     /// Shows a message to the user.
     ///
     /// This opens a pop up in VS Code showing `message`.
-    pub(crate) fn show_message(
-        &self,
-        message: impl Display,
-        message_type: lsp_types::MessageType,
-    ) -> crate::Result<()> {
+    pub(crate) fn show_message(&self, message: impl Display, message_type: lsp_types::MessageType) {
         self.send_notification::<lsp_types::notification::ShowMessage>(
             lsp_types::ShowMessageParams {
                 typ: message_type,
                 message: message.to_string(),
             },
-        )
+        );
     }
 
     /// Sends a request to display a warning to the client with a formatted message. The warning is
@@ -204,11 +198,7 @@ impl Client {
     ///
     /// Logs an error if the message could not be sent.
     pub(crate) fn show_warning_message(&self, message: impl Display) {
-        let result = self.show_message(message, lsp_types::MessageType::WARNING);
-
-        if let Err(err) = result {
-            tracing::error!("Failed to send warning message to the client: {err}");
-        }
+        self.show_message(message, lsp_types::MessageType::WARNING);
     }
 
     /// Sends a request to display an error to the client with a formatted message. The error is
@@ -216,23 +206,23 @@ impl Client {
     ///
     /// Logs an error if the message could not be sent.
     pub(crate) fn show_error_message(&self, message: impl Display) {
-        let result = self.show_message(message, lsp_types::MessageType::ERROR);
-
-        if let Err(err) = result {
-            tracing::error!("Failed to send error message to the client: {err}");
-        }
+        self.show_message(message, lsp_types::MessageType::ERROR);
     }
 
     /// Re-queues this request after a salsa cancellation for a retry.
     ///
     /// The main loop will skip the retry if the client cancelled the request in the  meantime.
-    pub(crate) fn retry(&self, request: lsp_server::Request) -> crate::Result<()> {
+    pub(crate) fn retry(&self, request: lsp_server::Request) {
         self.main_loop_sender
             .send(Event::Action(Action::RetryRequest(request)))
-            .map_err(|error| anyhow!("Failed to send retry request: {error}"))
+            .unwrap();
     }
 
-    pub(crate) fn cancel(&self, session: &mut Session, id: RequestId) -> crate::Result<()> {
+    pub(crate) fn queue_action(&self, action: Action) {
+        self.main_loop_sender.send(Event::Action(action)).unwrap();
+    }
+
+    pub(crate) fn cancel(&self, session: &mut Session, id: RequestId) {
         let method_name = session.request_queue_mut().incoming_mut().cancel(&id);
 
         if let Some(method_name) = method_name {
@@ -245,14 +235,18 @@ impl Client {
 
             // Use `client_sender` here instead of `respond_err` because
             // `respond_err` filters out responses for canceled requests (which we just did!).
-            self.client_sender
+            if let Err(err) = self
+                .client_sender
                 .send(Message::Response(lsp_server::Response {
                     id,
                     result: None,
                     error: Some(error),
-                }))?;
+                }))
+            {
+                tracing::error!(
+                    "Failed to send cancellation response for request `{method_name}` because the client sender is closed: {err}",
+                );
+            }
         }
-
-        Ok(())
     }
 }

crates/ty_server/src/session/options.rs

@@ -24,14 +24,7 @@ pub(crate) struct GlobalOptions {
 
 impl GlobalOptions {
     pub(crate) fn into_settings(self) -> ClientSettings {
-        ClientSettings {
-            disable_language_services: self
-                .client
-                .python
-                .and_then(|python| python.ty)
-                .and_then(|ty| ty.disable_language_services)
-                .unwrap_or_default(),
-        }
+        self.client.into_settings()
     }
 }
 
@@ -56,6 +49,19 @@ pub(crate) struct ClientOptions {
     python: Option<Python>,
 }
 
+impl ClientOptions {
+    /// Returns the client settings that are relevant to the language server.
+    pub(crate) fn into_settings(self) -> ClientSettings {
+        ClientSettings {
+            disable_language_services: self
+                .python
+                .and_then(|python| python.ty)
+                .and_then(|ty| ty.disable_language_services)
+                .unwrap_or_default(),
+        }
+    }
+}
+
 // TODO(dhruvmanila): We need to mirror the "python.*" namespace on the server side but ideally it
 // would be useful to instead use `workspace/configuration` instead. This would be then used to get
 // all settings and not just the ones in "python.*".