fix(host): don't block the event loop while a closing project shuts down

Closing a project window waited on the event-loop thread for the project's
language servers, kernels and debug sessions to shut down on the async
runtime. When the runtime workers were busy answering IPC or querying
windows, they waited for the event loop in turn and the app froze.

Window destruction and the close_project command now detach the project at
once and let its workers shut down in the background. Opening a project
waits for those shutdowns first, and exit waits for them with a bound.

Closes #1414

crates/ide-host/src/lib.rs

@@ -82,8 +82,13 @@ pub struct Host {
     pub(crate) external_logs: RwLock<HashMap<String, Vec<Arc<ide_vcs::Repo>>>>,
     requests: requests::Requests,
     project_requests: RwLock<HashMap<String, Vec<RequestId>>>,
+    /// Shutdowns of projects closed with [`Host::close_project_detached`].
+    closing: parking_lot::Mutex<Vec<std::thread::JoinHandle<()>>>,
 }
 
+/// How long exit waits for project shutdown before giving up.
+const EXIT_SHUTDOWN_WAIT: std::time::Duration = std::time::Duration::from_secs(10);
+
 impl Host {
     pub fn new(config: HostConfig) -> Self {
         let HostConfig {
@@ -126,6 +131,7 @@ impl Host {
             external_logs: RwLock::default(),
             requests: Default::default(),
             project_requests: RwLock::default(),
+            closing: Default::default(),
         };
         host.apply_proxy();
         host
@@ -179,46 +185,69 @@ impl Host {
         }
     }
 
+    /// Closes `window`'s project and waits until its language servers, kernels and
+    /// debug sessions have shut down. Never call this on the UI event-loop thread:
+    /// the shutdown runs on the async runtime, whose workers may themselves be
+    /// waiting for the event loop (IPC responses, window queries). Use
+    /// [`Self::close_project_detached`] there.
     pub fn close_project(&self, window: &str) {
+        if let Some(shutdown) = self.begin_close(window) {
+            self.wait_blocking(shutdown);
+        }
+    }
+
+    /// Like [`Self::close_project`], but returns once the project is detached from
+    /// the window; its workers finish shutting down in the background, and
+    /// [`Self::shutdown_notebooks_on_exit`] waits for them before the process exits.
+    pub fn close_project_detached(&self, window: &str) {
+        if let Some(shutdown) = self.begin_close(window) {
+            let mut closing = self.closing.lock();
+            closing.retain(|handle| !handle.is_finished());
+            closing.push(shutdown);
+        }
+    }
+
+    /// Detaches the project from `window` and stops what it started, synchronously,
+    /// so the window can open another project at once. Returns the thread that
+    /// finishes shutting down its workers.
+    fn begin_close(&self, window: &str) -> Option<std::thread::JoinHandle<()>> {
         self.external_logs.write().remove(self.owner_window(window));
         for request in self.project_requests.write().remove(self.owner_window(window)).unwrap_or_default() {
             self.cancel_request(request);
         }
-        let project = self.projects.write().remove(window);
-        if let Some(project) = project {
-            if !project.light {
-                self.environment.project_closed();
-            }
-            if let Some(bridge) = &project.kotlin_bridge {
-                bridge.stop();
-            }
-            project.documents.clear_workspace_edit_history();
-            project.agents.shutdown();
-            project.notebooks.stop_admission();
-            // What the closing window started goes with it — the builds and searches it
-            // asked for, its runs, its shells and its debug sessions — and the language
-            // servers finish shutting down before the window is gone, as IDEA closes a
-            // project's processes.
-            self.processes.kill_project(&project.root);
-            self.terminals.kill_key(self.owner_window(window));
-            self.shutdown_project_workers_blocking(
-                vec![project.notebooks.clone()],
-                project.kotlin_bridge.iter().cloned().collect(),
-                Some(project.lsp.clone()),
-                Some((self.debugger.clone(), window.to_owned())),
-            );
+        let project = self.projects.write().remove(window)?;
+        if !project.light {
+            self.environment.project_closed();
         }
+        if let Some(bridge) = &project.kotlin_bridge {
+            bridge.stop();
+        }
+        project.documents.clear_workspace_edit_history();
+        project.agents.shutdown();
+        project.notebooks.stop_admission();
+        // What the closing window started goes with it — the builds and searches it
+        // asked for, its runs, its shells and its debug sessions — and its language
+        // servers shut down.
+        self.processes.kill_project(&project.root);
+        self.terminals.kill_key(self.owner_window(window));
+        Some(self.spawn_project_shutdown(
+            vec![project.notebooks.clone()],
+            project.kotlin_bridge.iter().cloned().collect(),
+            Some(project.lsp.clone()),
+            Some((self.debugger.clone(), window.to_owned())),
+        ))
     }
 
-    /// Synchronous project/window destruction must finish owned kernel cleanup,
-    /// not enqueue work which can be lost as the desktop runtime exits.
-    fn shutdown_project_workers_blocking(
+    /// Project/window destruction must finish owned kernel cleanup, not enqueue
+    /// work which can be lost as the desktop runtime exits: the returned thread is
+    /// joined by the closer or, for a detached close, before exit.
+    fn spawn_project_shutdown(
         &self,
         notebooks: Vec<Arc<notebook::NotebookService>>,
         bridges: Vec<Arc<kotlin_bridge::KotlinBridge>>,
         lsp: Option<Arc<ide_lsp::LspManager>>,
         debug: Option<(Arc<ide_dap::DebugManager>, String)>,
-    ) {
+    ) -> std::thread::JoinHandle<()> {
         for bridge in &bridges {
             bridge.stop();
         }
@@ -226,25 +255,37 @@ impl Host {
             service.stop_admission();
         }
         let runtime = self.runtime.clone();
-        let wait = move || {
-            let worker = std::thread::spawn(move || {
-                runtime.block_on(async move {
-                    for bridge in bridges {
-                        bridge.shutdown().await;
-                    }
-                    for service in notebooks {
-                        service.shutdown().await;
-                    }
-                    if let Some(lsp) = lsp {
-                        lsp.shutdown_all().await;
-                    }
-                    if let Some((debug, key)) = debug {
-                        debug.stop_key(&key).await;
-                    }
-                });
+        std::thread::spawn(move || {
+            runtime.block_on(async move {
+                for bridge in bridges {
+                    bridge.shutdown().await;
+                }
+                for service in notebooks {
+                    service.shutdown().await;
+                }
+                if let Some(lsp) = lsp {
+                    lsp.shutdown_all().await;
+                }
+                if let Some((debug, key)) = debug {
+                    debug.stop_key(&key).await;
+                }
             });
-            if let Err(error) = worker.join() {
-                tracing::error!("notebook shutdown thread failed: {error:?}");
+        })
+    }
+
+    /// Waits for detached closes, so a project opened next never races the
+    /// language servers of one still shutting down (e.g. over a shared workspace).
+    pub(crate) fn wait_detached_closes(&self) {
+        let pending: Vec<_> = self.closing.lock().drain(..).collect();
+        for shutdown in pending {
+            self.wait_blocking(shutdown);
+        }
+    }
+
+    fn wait_blocking(&self, shutdown: std::thread::JoinHandle<()>) {
+        let wait = move || {
+            if let Err(error) = shutdown.join() {
+                tracing::error!("project shutdown thread failed: {error:?}");
             }
         };
         if tokio::runtime::Handle::try_current()
@@ -256,15 +297,32 @@ impl Host {
         }
     }
 
-    /// Called before the desktop runtime exits, including update/restart exits.
+    /// Called before the desktop runtime exits, including update/restart exits, on
+    /// the event-loop thread: waits for detached closes and the open projects'
+    /// kernels, but no longer than [`EXIT_SHUTDOWN_WAIT`], so a runtime worker
+    /// stuck on the event loop cannot keep the process from exiting.
     pub fn shutdown_notebooks_on_exit(&self) {
         let projects = self.projects();
-        self.shutdown_project_workers_blocking(
+        let mut pending: Vec<_> = self.closing.lock().drain(..).collect();
+        pending.push(self.spawn_project_shutdown(
             projects.iter().map(|project| project.notebooks.clone()).collect(),
             projects.iter().filter_map(|project| project.kotlin_bridge.clone()).collect(),
             None,
             None,
-        );
+        ));
+        let deadline = std::time::Instant::now() + EXIT_SHUTDOWN_WAIT;
+        while pending.iter().any(|handle| !handle.is_finished()) {
+            if std::time::Instant::now() >= deadline {
+                tracing::warn!("project shutdown did not finish within {EXIT_SHUTDOWN_WAIT:?}; exiting anyway");
+                return;
+            }
+            std::thread::sleep(std::time::Duration::from_millis(20));
+        }
+        for handle in pending {
+            if let Err(error) = handle.join() {
+                tracing::error!("project shutdown thread failed: {error:?}");
+            }
+        }
     }
 
     /// Called before desktop exit, which may bypass document destructors.

crates/ide-host/src/project.rs

@@ -88,6 +88,7 @@ impl Host {
             return Ok(current);
         }
         self.close_project(window);
+        self.wait_detached_closes();
         let sink: SharedSink = (self.sink_for)(window);
         // Before anything the project starts: its children get the project's environment.
         let trusted = Arc::new(AtomicBool::new(self.trust.trusted(&root)?));
@@ -567,6 +568,7 @@ impl Host {
             return Ok(current);
         }
         self.close_project(window);
+        self.wait_detached_closes();
         let sink: SharedSink = (self.sink_for)(window);
         let settings = Arc::new(SettingsStore::load(&self.config_dir, None));
         let effective = settings.effective();

crates/ide-host/tests/close_project.rs

@@ -5,6 +5,7 @@
 use ide_core::NullSink;
 use ide_host::{Host, HostConfig, ops};
 use std::sync::Arc;
+use std::time::{Duration, Instant};
 
 fn options(root: &std::path::Path) -> ide_terminal::TerminalOptions {
     ide_terminal::TerminalOptions {
@@ -52,3 +53,53 @@ fn closing_a_project_closes_its_terminals_and_keeps_other_windows() {
         host.terminals.kill_key("other");
     });
 }
+
+/// The desktop closes a project on the event-loop thread while runtime workers may be
+/// blocked waiting for that thread (IPC responses, window queries): closing must not
+/// wait for workers that need the runtime, and exit still waits for them once free.
+#[test]
+fn detached_close_returns_while_runtime_workers_wait_for_the_closing_thread() {
+    let tmp = tempfile::tempdir().unwrap();
+    let root = dunce::canonicalize(tmp.path()).unwrap().join("project");
+    std::fs::create_dir_all(&root).unwrap();
+    let runtime = tokio::runtime::Builder::new_multi_thread().worker_threads(1).enable_all().build().unwrap();
+    let _guard = runtime.enter();
+    let host = Arc::new(Host::new(HostConfig {
+        config_dir: tmp.path().join("config"),
+        data_dir: tmp.path().join("data"),
+        local_data_dir: tmp.path().join("data"),
+        languages: Arc::new(ide_language::LanguageRegistry::load(None).unwrap()),
+        syntax: Arc::default(),
+        sink_for: Box::new(|_| Arc::new(NullSink)),
+        owner: |w| w,
+        runtime: runtime.handle().clone(),
+        project_environment: false,
+    }));
+    host.trust.decide(&root, true, false).unwrap();
+    host.open_project("main", &root).unwrap();
+
+    // Occupy the only runtime worker until "the event loop" answers.
+    let (started_tx, started_rx) = std::sync::mpsc::channel();
+    let (release_tx, release_rx) = std::sync::mpsc::channel::<()>();
+    runtime.spawn(async move {
+        started_tx.send(()).unwrap();
+        let _ = release_rx.recv();
+    });
+    started_rx.recv().unwrap();
+
+    let (closed_tx, closed_rx) = std::sync::mpsc::channel();
+    let closer = host.clone();
+    std::thread::spawn(move || {
+        closer.close_project_detached("main");
+        let _ = closed_tx.send(());
+    });
+    let returned = closed_rx.recv_timeout(Duration::from_secs(5));
+    release_tx.send(()).unwrap();
+    assert!(returned.is_ok(), "closing waited for a runtime worker that waits for the closing thread");
+    assert!(host.project("main").is_err(), "the window no longer has the project");
+    host.open_project("main", &root).expect("the window opens a project once the close has finished");
+
+    let started = Instant::now();
+    host.shutdown_notebooks_on_exit();
+    assert!(started.elapsed() < Duration::from_secs(5), "exit joined the finished close instead of timing out");
+}

src-tauri/src/state.rs

@@ -154,9 +154,11 @@ impl AppState {
         self.host.open_light(window, file)
     }
 
+    /// Called on the event-loop thread (window destruction, the `close_project`
+    /// command), so it must not wait for the project's workers to shut down.
     pub fn close_project(&self, window: &str) {
         self.remote_dev.disconnect(window);
-        self.host.close_project(window);
+        self.host.close_project_detached(window);
     }
 }