Desktop: ask before quitting on top of a running install (#7550)

* Desktop: ask before quitting on top of a running install

This is the trigger neither #7492 nor #7490 addresses -- both start from a venv
that is already broken. Confirmed: neither PR touches cleanup_child_processes.

Quitting runs cleanup_child_processes -> install::stop_install, which SIGTERMs the
installer's process group. In the reported session that landed at "5/10 studio
deps", so the venv kept the CLI's dependencies and lost the server stack, and the
next launch died on `import structlog`. Three minutes of installing, destroyed
with no warning and no way back.

So ask. Only from the tray Quit item -- a deliberate action with a UI present. The
RunEvent::Exit path (OS shutdown, SIGTERM) is left alone: it must never block on a
dialog nobody can answer. The call already runs off the menu callback thread,
which is also what blocking_show requires.

Closing the window was already safe (it hides to tray); this closes the remaining
way to lose an install by accident.

* Tighten comments in desktop quit-during-install guard

* Condense comments in quit-during-install guard

---------

Co-authored-by: danielhanchen <unslothai@gmail.com>

studio/src-tauri/src/install.rs

@@ -783,6 +783,14 @@ pub fn record_install_intentional_stop(state: &InstallState, diagnostics: &Diagn
     }
 }
 
+/// True while an installer runs; quitting now would leave a broken venv.
+pub fn is_install_running(state: &InstallState) -> bool {
+    state
+        .lock()
+        .map(|install| install.child.is_some())
+        .unwrap_or(false)
+}
+
 /// Stop a running install process gracefully.
 /// Unix: SIGTERM to process group -> wait up to 5s -> SIGKILL
 /// Windows: hidden taskkill /T /F to terminate the installer tree

studio/src-tauri/src/main.rs

@@ -85,6 +85,33 @@ fn setup_custom_titlebar(app: &tauri::App) -> Result<(), Box<dyn std::error::Err
     Ok(())
 }
 
+/// Ask before quitting mid-install (true to proceed): `cleanup_child_processes` SIGTERMs the
+/// installer, leaving a venv that looks healthy but cannot start. Tray Quit only, since
+/// RunEvent::Exit must never block on a dialog nobody can answer.
+fn confirm_quit_during_install(app: &tauri::AppHandle) -> bool {
+    use tauri_plugin_dialog::{DialogExt, MessageDialogButtons, MessageDialogKind};
+
+    let Some(install_state) = app.try_state::<install::InstallState>() else {
+        return true;
+    };
+    if !install::is_install_running(&install_state) {
+        return true;
+    }
+    app.dialog()
+        .message(
+            "Unsloth Studio is still installing. Quitting now stops it part-way and \
+             leaves the installation incomplete, so it will need to be repaired before \
+             it can start.",
+        )
+        .kind(MessageDialogKind::Warning)
+        .title("Installation in progress")
+        .buttons(MessageDialogButtons::OkCancelCustom(
+            "Quit anyway".to_string(),
+            "Keep installing".to_string(),
+        ))
+        .blocking_show()
+}
+
 fn cleanup_child_processes(app: &tauri::AppHandle) {
     let diagnostics_state = app
         .try_state::<diagnostics::DiagnosticsState>()
@@ -138,6 +165,9 @@ fn setup_tray(app: &tauri::App) -> Result<(), Box<dyn std::error::Error>> {
                 // leaving the backend orphaned.
                 let app_handle = app.clone();
                 std::thread::spawn(move || {
+                    if !confirm_quit_during_install(&app_handle) {
+                        return;
+                    }
                     cleanup_child_processes(&app_handle);
                     app_handle.exit(0);
                 });