feat(desktop): home app (system-only launch cap, LAUNCH_BADGE: apps+launch only), pm starts it with session (relaunch <=5); display keys F1/Alt+Tab next, F2 home, F4 close app (ROLE from launcher, terminal exempt), close returns to previous window; closed window -> app exits; shell 'name &'; launch by id; pm stack 8, storage 16 clients

Cargo.lock

@@ -409,6 +409,16 @@ dependencies = [
  "libkos",
 ]
 
+[[package]]
+name = "home"
+version = "0.1.0"
+dependencies = [
+ "k-ui",
+ "libkos",
+ "manifest",
+ "types",
+]
+
 [[package]]
 name = "hw-virtio"
 version = "0.1.0"

Cargo.toml

@@ -6,6 +6,7 @@ members = [
     "crates/apps/flaky",
     "crates/apps/tally",
     "crates/apps/settings",
+    "crates/apps/home",
     "crates/theme",
     "crates/apps/hello",
     "crates/apps/shell",

Makefile

@@ -54,7 +54,7 @@ kani: kani-types kani-domain kani-kernel
 PROGRAMS = -p init -p echo -p sleep -p snake \
 	-p display-qemu -p input-qemu -p storage-qemu \
 	-p log-server -p permission-manager -p session-server -p smptest -p spawntest -p hello \
-	-p terminal-server -p shell -p cat -p ls -p counter -p gatekeeper -p flaky -p tally -p settings
+	-p terminal-server -p shell -p cat -p ls -p counter -p gatekeeper -p flaky -p tally -p settings -p home
 
 # Programs first: the kernel image packs init and storage (see the
 # adapter's build.rs).

crates/adapter-riscv/src/boot/loader.rs

@@ -39,11 +39,12 @@ pub fn load_init(uart: &Uart) -> LoadedProcess {
 
 /// Storage's budget, carved from init's.
 const STORAGE: Limits = Limits {
-    pages: 256,
+    // A 16-page buffer per client (16), its image and cache.
+    pages: 512,
     endpoints: 2,
     notifications: 2,
-    // A buffer per client (8) and the disk's DMA.
-    regions: 12,
+    // A buffer per client and the disk's DMA.
+    regions: 20,
 };
 
 /// Storage, which init needs to read anything else, its budget carved

crates/apps/home/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "home"
+version = "0.1.0"
+edition = "2024"
+
+[[bin]]
+name = "home"
+test = false
+bench = false
+
+[dependencies]
+types = { workspace = true }
+libkos = { workspace = true }
+k-ui = { workspace = true }
+manifest = { workspace = true }

crates/apps/home/manifest.toml

@@ -0,0 +1,10 @@
+[app]
+id      = "kos.home"
+name    = "Home"
+version = "1.0.0"
+binary  = "home"
+author  = "K-OS"
+memory  = "3 MiB"
+
+[capabilities]
+required = ["display", "launch"]

crates/apps/home/src/main.rs

@@ -0,0 +1,118 @@
+//! Home: the session's desktop, which the permission manager starts with
+//! it. It lists every app with a window and opens one when it is pressed,
+//! as many as there is room for, each in its own window. A system app:
+//! `launch` is only ever granted to apps on the system image.
+
+#![no_std]
+#![forbid(unsafe_code)]
+#![no_main]
+
+extern crate alloc;
+extern crate libkos;
+
+use alloc::format;
+use alloc::string::{String, ToString};
+use alloc::vec::Vec;
+use k_ui::{App, Element, Spacing, TextColour, TextStyle, column, list_item, text};
+use libkos::apps::Apps;
+use manifest::{Capabilities, Capability};
+use types::address::VirtualAddress;
+use types::apps::reply;
+
+/// Where the request page for the permission manager is mapped.
+const REQUEST_PAGE_VA: VirtualAddress = VirtualAddress::from_raw(0x0190_0000);
+const LISTING: usize = 4096;
+const ME: &str = "kos.home";
+
+struct Entry {
+    id: String,
+    name: String,
+}
+
+struct Home {
+    apps: Option<Apps>,
+    entries: Vec<Entry>,
+    /// What went wrong last, shown until the next press.
+    trouble: Option<String>,
+}
+
+#[derive(Clone, Copy)]
+enum Event {
+    /// Open the `n`th app listed.
+    Open(usize),
+}
+
+impl Home {
+    /// Every installed app that has a window, but Home.
+    fn reload(&mut self) {
+        let Some(apps) = &self.apps else { return };
+        // On the heap: an app's stack is a few pages.
+        let mut buf = alloc::vec![0u8; LISTING];
+        let Ok(n) = apps.apps(&mut buf) else { return };
+        let text = core::str::from_utf8(&buf[..n]).unwrap_or("");
+        self.entries = libkos::apps::entries(text)
+            .filter(|e| e.id != ME)
+            .filter(|e| Capabilities::from_bits(e.required).union(Capabilities::from_bits(e.optional)).contains(Capability::Display))
+            .map(|e| Entry { id: e.id.to_string(), name: e.name.to_string() })
+            .collect();
+    }
+
+    fn open(&mut self, index: usize) {
+        let (Some(apps), Some(e)) = (&self.apps, self.entries.get(index)) else { return };
+        self.trouble = match apps.launch(&[e.id.as_str()], None, None) {
+            // Its window comes to the front; it runs on its own.
+            Ok((process, _)) => {
+                libkos::syscall::drop_cap(process);
+                None
+            }
+            Err(r) if r.code == reply::BUSY => Some("There is no room for another app: close one first (F4).".to_string()),
+            Err(r) => Some(format!("{} did not open (code {}).", e.name, r.code)),
+        };
+        self.reload();
+    }
+}
+
+impl App for Home {
+    type Event = Event;
+
+    fn init() -> Self {
+        let apps = libkos::start::permissions().and_then(|ep| Apps::connect(ep, REQUEST_PAGE_VA));
+        let mut home = Home { apps, entries: Vec::new(), trouble: None };
+        home.reload();
+        home
+    }
+
+    fn update(&mut self, event: Event) {
+        match event {
+            Event::Open(i) => self.open(i),
+        }
+    }
+
+    fn view(&self) -> Element<Event> {
+        let hint = "Enter opens an app. Alt+Tab or F1: the next window. F2: back here. F4: close the app in front.";
+        let mut view = column()
+            .padding(Spacing::Xl)
+            .gap(Spacing::Md)
+            .child(text("Home").style(TextStyle::Heading))
+            .child(text(hint).style(TextStyle::Caption).colour(TextColour::Secondary))
+            .children(self.entries.iter().enumerate().map(|(i, e)| list_item(e.name.clone(), Event::Open(i)).subtitle(e.id.clone())));
+        if self.entries.is_empty() {
+            view = view.child(text("No apps with a window are installed."));
+        }
+        if let Some(t) = &self.trouble {
+            view = view.child(text(t.clone()).colour(TextColour::Accent));
+        }
+        view.into()
+    }
+
+    /// Home is not left: Escape stays.
+    fn back(&mut self) -> bool {
+        true
+    }
+}
+
+libkos::entry!(main);
+
+fn main() {
+    k_ui::run::<Home>();
+}

crates/apps/shell/src/main.rs

@@ -140,8 +140,11 @@ fn refused(what: &str, r: Refused) {
 }
 
 /// Runs app `args[0]` with `args` in the current folder, and waits for it
-/// to finish or Ctrl+C.
+/// to finish or Ctrl+C; with a last argument `&`, does not wait.
 fn run_program(sh: &mut Shell, args: &[&str]) {
+    if let Some((&"&", args)) = args.split_last() {
+        return start_program(sh, args);
+    }
     let folder = sh.fs.grant(sh.cwd.as_bytes(), false).ok();
     // A Ctrl+C typed since the last program is not for this one.
     let _ = syscall::poll(sh.done);
@@ -166,6 +169,21 @@ fn run_program(sh: &mut Shell, args: &[&str]) {
     }
 }
 
+/// Starts app `args[0]` and lets it run on its own (its window, if it has
+/// one, is closed with F4). It does not get the current folder: nobody
+/// would be there to take the grant back when it is done.
+fn start_program(sh: &mut Shell, args: &[&str]) {
+    let Some(name) = args.first() else { return println!("usage: <app> [args] &") };
+    match sh.apps.launch(args, Some(TERMINAL), None) {
+        Ok((process, _)) => {
+            // Not waited on: the handle is not needed.
+            syscall::drop_cap(process);
+            println!("{} started", name);
+        }
+        Err(r) => refused(name, r),
+    }
+}
+
 /// Installs the app in folder `from` (its manifest and binary); the owner
 /// is asked on the terminal.
 fn install(sh: &mut Shell, from: &str) {
@@ -232,6 +250,7 @@ fn run(sh: &mut Shell, line: &str) {
             println!("anything else runs an app: try ls, cat readme.txt, snake");
             println!("apps see the current directory, read-only, and only what you allowed");
             println!("Ctrl+C stops a running program; snake quits with q");
+            println!("end with & to leave it running (snake &); F4 closes the app in front");
         }
         Some("apps") => list(sh),
         Some("install") => match args.get(1) {

crates/apps/snake/src/main.rs

@@ -77,25 +77,23 @@ enum Input {
     Other,
 }
 
-/// The next key, if one is waiting: q or Escape quits.
+/// The next key, if one is waiting: q or Escape quits, and so does the
+/// window going.
 fn next_input(window: CapSlot) -> Input {
-    let Ok([tag, kind, ch, _]) = syscall::request(window, [types::display::window::POLL, 0, 0, 0]) else {
-        return Input::None;
-    };
-    if tag == 0 {
-        return Input::None; // NoEvent: queue drained
-    }
-    if tag != 1 {
-        return Input::Other; // a key let go
-    }
-    // Arrows, or WASD (kind 9 is a character).
-    match (kind, ch as u8) {
-        (5, _) | (9, b'q') => Input::Quit,
-        (0, _) | (9, b'w') => Input::Turn(Direction::Up),
-        (1, _) | (9, b's') => Input::Turn(Direction::Down),
-        (2, _) | (9, b'a') => Input::Turn(Direction::Left),
-        (3, _) | (9, b'd') => Input::Turn(Direction::Right),
+    use libkos::display::Event;
+    use libkos::input::{InputEvent, Key};
+    match libkos::display::poll(window) {
+        None => Input::None,
+        Some(Event::Closed) => Input::Quit,
+        Some(Event::Input(InputEvent::KeyDown(k))) => match k {
+            Key::Escape | Key::Char(b'q') => Input::Quit,
+            Key::Up | Key::Char(b'w') => Input::Turn(Direction::Up),
+            Key::Down | Key::Char(b's') => Input::Turn(Direction::Down),
+            Key::Left | Key::Char(b'a') => Input::Turn(Direction::Left),
+            Key::Right | Key::Char(b'd') => Input::Turn(Direction::Right),
             _ => Input::Other,
+        },
+        Some(_) => Input::Other,
     }
 }
 

crates/drivers/input/virtio/src/keymap.rs

@@ -6,6 +6,16 @@ pub const LEFT_SHIFT: u16 = 42;
 pub const RIGHT_SHIFT: u16 = 54;
 pub const LEFT_CTRL: u16 = 29;
 pub const RIGHT_CTRL: u16 = 97;
+pub const LEFT_ALT: u16 = 56;
+pub const RIGHT_ALT: u16 = 100;
+/// Alt+Tab is the display's window switch, the same key as F1
+/// (`types::display::window::SWITCH_KEY`).
+const SWITCH: Key = Key::Function(1);
+
+/// What a key is with Alt held: only Alt+Tab means anything.
+pub fn with_alt(key: Key) -> Option<Key> {
+    (key == Key::Tab).then_some(SWITCH)
+}
 
 /// A letter typed with Ctrl held is its control character, as on a
 /// terminal: Ctrl+C is 3. Anything else is itself.
@@ -54,6 +64,12 @@ pub fn key(code: u16, shift: bool) -> Option<Key> {
 mod tests {
     use super::*;
 
+    #[test]
+    fn alt_tab_switches_windows_and_other_alt_keys_are_nothing() {
+        assert_eq!(with_alt(Key::Tab), Some(Key::Function(1)));
+        assert_eq!(with_alt(Key::Char(b'a')), None);
+    }
+
     #[test]
     fn function_keys_are_numbered() {
         assert_eq!(key(59, false), Some(Key::Function(1)));

crates/drivers/input/virtio/src/lib.rs

@@ -36,6 +36,7 @@ pub struct VirtioInputDevice {
     next_avail: u16,
     shift: bool,
     ctrl: bool,
+    alt: bool,
 }
 
 impl VirtioInputDevice {
@@ -61,6 +62,7 @@ impl VirtioInputDevice {
             next_avail: 0,
             shift: false,
             ctrl: false,
+            alt: false,
         };
         dev.fill_queue();
         Some(dev)
@@ -129,8 +131,13 @@ impl InputDevice for VirtioInputDevice {
                 self.ctrl = ev_value != 0;
                 continue;
             }
+            if ev_type == EV_KEY && matches!(ev_code, keymap::LEFT_ALT | keymap::RIGHT_ALT) {
+                self.alt = ev_value != 0;
+                continue;
+            }
             if ev_type == EV_KEY {
                 let key = keymap::key(ev_code, self.shift).map(|k| if self.ctrl { keymap::with_ctrl(k) } else { k });
+                let key = if self.alt { key.and_then(keymap::with_alt) } else { key };
                 if let Some(key) = key {
                     if ev_value == KEY_PRESS {
                         return Some(InputEvent::KeyDown(key));

crates/init/src/manifest.rs

@@ -157,6 +157,8 @@ pub const SESSION: &[Program] = &[Program {
     session: Session::PassOn,
     // Installing waits for the user's consent.
     pinged: false,
+    // A launch (manifest, decision, arguments) is KiBs of stack.
+    stack_pages: 8,
     ..server("permission", b"bin/permission", Limits { pages: 96, ..SMALL })
 }];
 

crates/k-ui/src/run.rs

@@ -71,6 +71,7 @@ pub fn run<A: App>() -> ! {
                     dirty = true;
                     continue;
                 }
+                Event::Closed => syscall::exit(),
                 Event::Input(InputEvent::KeyDown(k)) => k,
                 Event::Input(_) => continue,
             };

crates/libkos/src/display.rs

@@ -37,6 +37,12 @@ pub fn set_accent(window: CapSlot, rgb: u32) -> bool {
     syscall::request(window, [w::ACCENT, rgb as usize, 0, 0]).is_ok_and(|r| r[0] == w::OK)
 }
 
+/// A launcher says what the window it opened is (`window::role`), before
+/// the app runs; whether the display took it.
+pub fn set_role(window: CapSlot, role: usize) -> bool {
+    syscall::request(window, [w::ROLE, role, 0, 0]).is_ok_and(|r| r[0] == w::OK)
+}
+
 /// Closes `window`: every copy of it, and its buffer, stop working.
 pub fn close(window: CapSlot) {
     let _ = syscall::request(window, [w::CLOSE, 0, 0, 0]);
@@ -62,15 +68,20 @@ pub enum Event {
     Input(crate::input::InputEvent),
     /// Ask for the theme again (`theme`).
     ThemeChanged,
+    /// The window is gone (the user closed it, or the display went):
+    /// the app is done.
+    Closed,
 }
 
 /// `window`'s next event, if one is waiting; ones not known here are
 /// skipped.
 pub fn poll(window: CapSlot) -> Option<Event> {
     loop {
-        let words = syscall::request(window, [w::POLL, 0, 0, 0]).ok()?;
+        let Ok(words) = syscall::request(window, [w::POLL, 0, 0, 0]) else { return Some(Event::Closed) };
         match words[0] {
             0 => return None,
+            // What the display answers through a window it has closed.
+            usize::MAX => return Some(Event::Closed),
             w::THEME_CHANGED => return Some(Event::ThemeChanged),
             _ => {
                 if let Some(e) = crate::input::decode(words) {
@@ -81,11 +92,14 @@ pub fn poll(window: CapSlot) -> Option<Event> {
     }
 }
 
-/// `window`'s next input event, if one is waiting.
+/// `window`'s next input event, if one is waiting. A program whose window
+/// is gone exits here.
 pub fn poll_input(window: CapSlot) -> Option<crate::input::InputEvent> {
     loop {
-        if let Event::Input(e) = poll(window)? {
-            return Some(e);
+        match poll(window)? {
+            Event::Input(e) => return Some(e),
+            Event::Closed => syscall::exit(),
+            Event::ThemeChanged => {}
         }
     }
 }

crates/manifest/src/capability.rs

@@ -62,6 +62,7 @@ catalog! {
     // System-only (see `Capabilities::SYSTEM_ONLY`).
     Permissions = "permissions",
     Session = "session",
+    Launch = "launch",
 }
 
 impl Capability {
@@ -81,7 +82,7 @@ impl Capabilities {
     /// refused, so no prompt can talk a user into handing them over.
     /// `permissions`: changing what other apps may use. `session`: who is
     /// signed in, and signing out.
-    pub const SYSTEM_ONLY: Self = Self(1 << Capability::Permissions as u32 | 1 << Capability::Session as u32);
+    pub const SYSTEM_ONLY: Self = Self(1 << Capability::Permissions as u32 | 1 << Capability::Session as u32 | 1 << Capability::Launch as u32);
 
     /// As sent in a message word.
     pub const fn bits(self) -> u32 {

crates/servers/display-server/src/server.rs

@@ -94,6 +94,16 @@ impl<F: Framebuffer, H: Host> DisplayServer<F, H> {
         &mut self.host
     }
 
+    #[cfg(test)]
+    pub fn windows_front_for_test(&self) -> Option<usize> {
+        self.windows.front()
+    }
+
+    #[cfg(test)]
+    pub fn slot_for_test(&self, badge: usize) -> Option<usize> {
+        self.windows.slot(badge)
+    }
+
     /// A window's size: the screen below the bar.
     fn window_size(&self) -> (Dimension, Dimension) {
         (self.fb.width(), Dimension::new(self.fb.height().raw().saturating_sub(BAR_HEIGHT)))
@@ -193,6 +203,28 @@ impl<F: Framebuffer, H: Host> DisplayServer<F, H> {
         raster::draw_text(&mut self.fb, ScreenCoord::new(at), ScreenCoord::new(0), bar_fg, bar_bg, user);
     }
 
+    /// What a window key does.
+    fn window_key(&mut self, key: usize) {
+        let changed = match key {
+            w::SWITCH_KEY => self.windows.cycle().is_some(),
+            w::HOME_KEY => self.windows.with_role(w::role::HOME).is_some_and(|i| self.windows.raise(i)),
+            _ => match self.windows.front().filter(|i| self.windows.role(*i) == Some(w::role::APP)) {
+                Some(index) => {
+                    // Woken, it finds its window gone, and is done.
+                    if let Some(notify) = self.windows.notify(index) {
+                        self.host.signal(notify);
+                    }
+                    self.close(index);
+                    false
+                }
+                None => false,
+            },
+        };
+        if changed {
+            self.repaint();
+        }
+    }
+
     /// Every window is told to ask for the theme again; the bar wears it.
     fn theme_changed(&mut self) {
         let mut told = [None; MAX_WINDOWS];
@@ -220,6 +252,7 @@ impl<F: Framebuffer, H: Host> DisplayServer<F, H> {
                     None => ([MALFORMED, 0, 0, 0], None),
                 }
             }
+            (w::ROLE, _) if matches!(words[1], w::role::APP | w::role::HOME) && self.windows.set_role(badge, words[1]) => ([w::OK, 0, 0, 0], None),
             (w::ACCENT, _) if words[1] <= 0xff_ffff && self.windows.set_accent(badge, words[1] as u32) => ([w::OK, 0, 0, 0], None),
             (w::PRESENT, _) => {
                 let (x, y, width, height) = w::unpack_rect([words[1], words[2]]);
@@ -248,9 +281,11 @@ impl<F: Framebuffer, H: Host> DisplayServer<F, H> {
     }
 }
 
-fn is_switch_key(event: [usize; 4]) -> bool {
+/// The display's own key in `event`, if it is one (pressed or let go).
+fn display_key(event: [usize; 4]) -> Option<usize> {
     use types::input::event::{FUNCTION_KEY, KEY_DOWN, KEY_UP};
-    matches!(event[0], KEY_DOWN | KEY_UP) && event[1] == FUNCTION_KEY && event[2] == w::SWITCH_KEY
+    let ours = matches!(event[0], KEY_DOWN | KEY_UP) && event[1] == FUNCTION_KEY;
+    (ours && matches!(event[2], w::SWITCH_KEY | w::HOME_KEY | w::CLOSE_KEY)).then_some(event[2])
 }
 
 impl<F: Framebuffer, H: Host> Server for DisplayServer<F, H> {
@@ -306,14 +341,14 @@ impl<F: Framebuffer, H: Host> Server for DisplayServer<F, H> {
         }
     }
 
-    /// The input server has events: the switch key is the display's, the
-    /// rest go to the window in front.
+    /// The input server has events: the window keys are the display's,
+    /// the rest go to the window in front.
     fn signal(&mut self, _bits: usize) {
         let mut told = None;
         while let Some(event) = self.host.input() {
-            if is_switch_key(event) {
-                if event[0] == types::input::event::KEY_DOWN && self.windows.cycle().is_some() {
-                    self.repaint();
+            if let Some(key) = display_key(event) {
+                if event[0] == types::input::event::KEY_DOWN {
+                    self.window_key(key);
                 }
                 continue;
             }

crates/servers/display-server/src/tests/server.rs

@@ -259,3 +259,42 @@ fn only_the_launcher_sets_an_accent_and_it_is_in_the_theme() {
     assert_eq!(accent(&mut s, other), theme::Theme::WHIMSY_DARK.accent);
     assert_eq!(accent(&mut s, app), theme::with_accent(theme::Theme::WHIMSY_DARK, theme::Rgba::rgb(pink as u32)).accent);
 }
+
+#[test]
+fn window_keys_raise_home_and_close_only_apps() {
+    let key = |s: &mut Srv, k: usize| {
+        s.host_mut_for_test().events = vec![[1, 10, k, 0], [2, 10, k, 0]];
+        s.signal(0);
+    };
+    let mut s = server();
+    let terminal = open(&mut s, 1, b"Terminal");
+    let home = open(&mut s, 2, b"Home");
+    let app = open(&mut s, 2, b"Tally");
+    assert_eq!(s.handle_call(home, [w::ROLE, w::role::HOME, 0, 0], None).0[0], w::OK);
+    assert_eq!(s.handle_call(app, [w::ROLE, w::role::APP, 0, 0], None).0[0], w::OK);
+    assert_eq!(s.handle_call(app, [w::ROLE, w::role::HOME, 0, 0], None).0[0], MALFORMED, "once");
+    s.handle_call(terminal, [w::BUFFER, 0, 0, 0], None);
+    assert_eq!(s.handle_call(terminal, [w::ROLE, w::role::APP, 0, 0], None).0[0], MALFORMED, "not once it runs");
+    let notify = CapSlot::new(9).unwrap();
+    s.handle_call(app, [w::SUBSCRIBE, 0, 0, 0], Some(notify));
+
+    key(&mut s, w::HOME_KEY);
+    assert_eq!(s.windows_front_for_test(), s.slot_for_test(home));
+    key(&mut s, w::CLOSE_KEY);
+    assert!(s.slot_for_test(home).is_some(), "Home is not closed");
+    // The app in front: closed, and told.
+    s.handle_call(app, [w::POLL, 0, 0, 0], None);
+    key(&mut s, w::SWITCH_KEY);
+    assert_eq!(s.windows_front_for_test(), s.slot_for_test(app));
+    key(&mut s, w::CLOSE_KEY);
+    assert_eq!(s.slot_for_test(app), None);
+    assert!(s.host().signalled.contains(&notify));
+    assert_eq!(s.handle_call(app, [w::POLL, 0, 0, 0], None).0[0], MALFORMED, "its window is gone");
+    // The terminal in front: kept.
+    key(&mut s, w::SWITCH_KEY);
+    while s.windows_front_for_test() != s.slot_for_test(terminal) {
+        key(&mut s, w::SWITCH_KEY);
+    }
+    key(&mut s, w::CLOSE_KEY);
+    assert!(s.slot_for_test(terminal).is_some());
+}

crates/servers/display-server/src/tests/windows.rs

@@ -67,3 +67,19 @@ fn a_launchers_windows_are_found_by_its_badge() {
     let (b, _) = w.open(5, b"two").unwrap();
     assert_eq!(w.opened_by(5).collect::<Vec<_>>(), [a, b]);
 }
+
+#[test]
+fn closing_the_front_window_brings_back_the_one_before_it() {
+    let mut w: Windows<u8> = Windows::new();
+    let (t, _) = w.open(1, b"Terminal").unwrap();
+    let (home, _) = w.open(2, b"Home").unwrap();
+    let (app, _) = w.open(2, b"Tally").unwrap();
+    assert!(w.close(app));
+    assert_eq!(w.front(), Some(home), "not the next slot round");
+    w.raise(t);
+    let (app2, _) = w.open(2, b"Snake").unwrap();
+    w.raise(home);
+    w.raise(app2);
+    w.close(app2);
+    assert_eq!(w.front(), Some(home));
+}

crates/servers/display-server/src/windows.rs

@@ -1,7 +1,8 @@
 //! Which windows there are and which is in front (see `types::display::
 //! window`). Every window fills the screen below the OS bar; one is in
 //! front at a time, and only it is shown and gets input. A new window comes
-//! to the front; a key cycles through them. Pure: the server keeps the
+//! to the front; a key cycles through them. Closing the one in front brings
+//! back the one in front before it. Pure: the server keeps the
 //! pixels.
 
 use types::display::window::{BADGE, NAME_BYTES};
@@ -54,6 +55,10 @@ struct Window<N> {
     accent: Option<u32>,
     /// Its buffer was asked for: the app runs, so the accent is settled.
     started: bool,
+    /// What it is (`types::display::window::role`), if it was said.
+    role: Option<usize>,
+    /// When it last came to the front (`Windows::raises`).
+    raised: u64,
 }
 
 pub struct Windows<N: Copy> {
@@ -61,11 +66,13 @@ pub struct Windows<N: Copy> {
     front: Option<usize>,
     /// Makes each window's badge new, so a closed window's never returns.
     opened: usize,
+    /// Times a window came to the front: what was in front most lately.
+    raises: u64,
 }
 
 impl<N: Copy> Windows<N> {
     pub const fn new() -> Self {
-        Self { slots: [const { None }; MAX_WINDOWS], front: None, opened: 0 }
+        Self { slots: [const { None }; MAX_WINDOWS], front: None, opened: 0, raises: 0 }
     }
 
     /// A window for `name`, opened through `opener`'s capability, in
@@ -77,8 +84,8 @@ impl<N: Copy> Windows<N> {
         let mut n = [0u8; NAME_BYTES];
         let name_len = name.len().min(NAME_BYTES);
         n[..name_len].copy_from_slice(&name[..name_len]);
-        self.slots[index] = Some(Window { badge, opener, name: n, name_len, notify: None, queue: Queue::new(), accent: None, started: false });
-        self.front = Some(index);
+        self.slots[index] = Some(Window { badge, opener, name: n, name_len, notify: None, queue: Queue::new(), accent: None, started: false, role: None, raised: 0 });
+        self.bring_forward(index);
         Some((index, badge))
     }
 
@@ -95,8 +102,7 @@ impl<N: Copy> Windows<N> {
         if self.front != Some(index) {
             return false;
         }
-        self.front = None;
-        self.front = self.after(index);
+        self.front = self.slots.iter().enumerate().filter_map(|(i, w)| w.as_ref().map(|w| (w.raised, i))).max().map(|(_, i)| i);
         true
     }
 
@@ -114,11 +120,19 @@ impl<N: Copy> Windows<N> {
     pub fn cycle(&mut self) -> Option<usize> {
         let next = self.after(self.front?)?;
         (Some(next) != self.front).then(|| {
-            self.front = Some(next);
+            self.bring_forward(next);
             next
         })
     }
 
+    fn bring_forward(&mut self, index: usize) {
+        self.raises += 1;
+        if let Some(w) = self.slots[index].as_mut() {
+            w.raised = self.raises;
+        }
+        self.front = Some(index);
+    }
+
     pub fn front(&self) -> Option<usize> {
         self.front
     }
@@ -140,6 +154,40 @@ impl<N: Copy> Windows<N> {
         }
     }
 
+    /// Says what the window `badge` is: once, before it starts.
+    pub fn set_role(&mut self, badge: usize, role: usize) -> bool {
+        let Some(w) = self.slot(badge).and_then(|i| self.slots[i].as_mut()) else { return false };
+        let ok = !w.started && w.role.is_none();
+        if ok {
+            w.role = Some(role);
+        }
+        ok
+    }
+
+    pub fn role(&self, index: usize) -> Option<usize> {
+        self.slots.get(index)?.as_ref()?.role
+    }
+
+    /// The first open window that is `role`.
+    pub fn with_role(&self, role: usize) -> Option<usize> {
+        self.slots.iter().position(|w| w.as_ref().is_some_and(|w| w.role == Some(role)))
+    }
+
+    /// Who is told of the window in `index`'s input.
+    pub fn notify(&self, index: usize) -> Option<N> {
+        self.slots.get(index)?.as_ref()?.notify
+    }
+
+    /// Brings the window in `index` to the front; whether it changed.
+    pub fn raise(&mut self, index: usize) -> bool {
+        let open = self.slots.get(index).is_some_and(Option::is_some);
+        let changed = open && self.front != Some(index);
+        if changed {
+            self.bring_forward(index);
+        }
+        changed
+    }
+
     pub fn accent(&self, badge: usize) -> Option<u32> {
         self.slots[self.slot(badge)?].as_ref()?.accent
     }

crates/servers/permission-manager/src/lib.rs

@@ -49,6 +49,7 @@ pub fn run() -> ! {
     let consent = consent::TerminalConsent { terminal: TERMINAL };
     let ports = server::Ports { init: INIT, display: Some(DISPLAY), session: Some(SESSION), endpoint: EP };
     let mut srv = server::PermissionServer::new(store, consent, ports, deaths);
+    srv.start_home();
     libkos::println!("permission: serving");
     server_framework::serve(&mut srv, EP)
 }

crates/servers/permission-manager/src/server.rs

@@ -15,6 +15,7 @@ use types::bounded::CapSlot;
 use types::budget::Limits;
 use types::mapping::PagePerms;
 use types::permission::{Permission, PermissionSet};
+use types::display::window::role as display_role;
 use types::start_arg::StartArg;
 
 use crate::consent::{CapabilityNames, TerminalConsent, version};
@@ -27,6 +28,12 @@ const PROGRAM: Limits = Limits { pages: 0, endpoints: 1, notifications: 1, regio
 /// Apps running at once; a death is a bit each on the notification.
 const MAX_RUNNING: usize = 8;
 const MAX_ARGS: usize = 16;
+/// The session's desktop, started with it and again if it dies.
+const HOME: &str = "kos.home";
+/// Home's starts in a session at most: one that keeps dying is left down.
+const MAX_HOMES: u8 = 5;
+/// A launch that no launcher asked for (Home's): nothing staged is its.
+const NO_LAUNCHER: usize = usize::MAX;
 /// Where a client's request page is mapped during its call.
 const PAGE_VA: VirtualAddress = VirtualAddress::from_raw(0x01A0_0000);
 
@@ -66,6 +73,8 @@ pub struct PermissionServer {
     deaths: CapSlot,
     staged: Staged,
     running: [Option<Running>; MAX_RUNNING],
+    /// Home's starts this session.
+    homes: u8,
 }
 
 fn refused(r: Refusal) -> [usize; 4] {
@@ -127,12 +136,12 @@ fn parts(page: &[u8]) -> impl Iterator<Item = &str> {
 impl PermissionServer {
     pub fn new(store: DiskStore, consent: TerminalConsent, ports: Ports, deaths: CapSlot) -> Self {
         let Ports { init, display, session, endpoint } = ports;
-        Self { store, consent, init, display, session, endpoint, deaths, staged: Staged::default(), running: [const { None }; MAX_RUNNING] }
+        Self { store, consent, init, display, session, endpoint, deaths, staged: Staged::default(), running: [const { None }; MAX_RUNNING], homes: 0 }
     }
 
     /// What this device has a server for.
     fn available(&self) -> Capabilities {
-        let mut c = Capabilities::NONE.with(Capability::Storage).with(Capability::Permissions);
+        let mut c = Capabilities::NONE.with(Capability::Storage).with(Capability::Permissions).with(Capability::Launch);
         if self.display.is_some() {
             c = c.with(Capability::Display);
         }
@@ -177,7 +186,27 @@ impl PermissionServer {
             *slot = p;
             n += 1;
         }
-        let args = &args[..n];
+        self.run(badge, &args[..n])
+    }
+
+    /// Starts Home, the session's desktop, unless it runs already or has
+    /// died too often.
+    pub fn start_home(&mut self) {
+        let running = self.running.iter().flatten().any(|r| r.id.as_str() == HOME);
+        if running || self.homes >= MAX_HOMES {
+            return;
+        }
+        self.homes += 1;
+        match self.run(NO_LAUNCHER, &[HOME]) {
+            Ok((process, _)) => {
+                syscall::drop_cap(process);
+            }
+            Err(words) => libkos::println!("permission: home did not start (refused: {})", words[0]),
+        }
+    }
+
+    /// Runs `args[0]` with `args` for the launcher behind `badge`.
+    fn run(&mut self, badge: usize, args: &[&str]) -> Result<(CapSlot, Capabilities), [usize; 4]> {
         let command = *args.first().ok_or(code(reply::NOT_INSTALLED))?;
         let available = self.available();
         let plan = policy::plan_launch(&mut self.store, command, available).map_err(refused)?;
@@ -201,18 +230,27 @@ impl PermissionServer {
             true => self.display.and_then(|d| libkos::display::open(d, plan.app.name.as_str().as_bytes())).map(|o| o.window),
             false => None,
         };
-        // Its brand colour too, which the display adapts to the theme.
-        if let (Some(w), Some(accent)) = (window, plan.app.accent) {
+        // Its brand colour too, which the display adapts to the theme, and
+        // what it is: Home, or an app the user may close.
+        if let Some(w) = window {
+            if let Some(accent) = plan.app.accent {
                 libkos::display::set_accent(w, accent.0);
             }
-        // Managing others through a copy of this endpoint that can do no
-        // more (see `MANAGE_BADGE`).
-        let manage = match plan.granted.contains(Capability::Permissions) {
-            true => {
-                let perms = PermissionSet::empty().with(Permission::Execute).with(Permission::Delegate);
-                syscall::mint(self.endpoint, types::badge::Badge::new(wire::MANAGE_BADGE | index), perms).ok()
+            let role = if plan.app.id.as_str() == HOME { display_role::HOME } else { display_role::APP };
+            libkos::display::set_role(w, role);
+        }
+        // Managing others, or launching them, through a copy of this
+        // endpoint that can do no more (see `MANAGE_BADGE`, `LAUNCH_BADGE`).
+        let bits = [(Capability::Permissions, wire::MANAGE_BADGE), (Capability::Launch, wire::LAUNCH_BADGE)]
+            .iter()
+            .filter(|(c, _)| plan.granted.contains(*c))
+            .fold(0, |b, (_, bit)| b | bit);
+        let manage = match bits {
+            0 => None,
+            bits => {
+                let perms = PermissionSet::empty().with(Permission::Execute).with(Permission::Delegate);
+                syscall::mint(self.endpoint, types::badge::Badge::new(bits | index), perms).ok()
             }
-            false => None,
         };
         let session = self.session.filter(|_| plan.granted.contains(Capability::Session));
         let staged = (badge == self.staged.badge).then_some(&self.staged);
@@ -319,8 +357,9 @@ impl PermissionServer {
         code(reply::OK)
     }
 
-    /// Apps behind `bits` died: their data grants go back.
+    /// Apps behind `bits` died: their data grants go back. Home comes back.
     fn reap(&mut self, bits: usize) {
+        let home_died = self.running.iter().enumerate().any(|(i, r)| bits & (1 << i) != 0 && r.as_ref().is_some_and(|r| r.id.as_str() == HOME));
         for (i, slot) in self.running.iter_mut().enumerate() {
             if bits & (1 << i) == 0 {
                 continue;
@@ -334,6 +373,9 @@ impl PermissionServer {
                 syscall::drop_cap(r.process);
             }
         }
+        if home_died {
+            self.start_home();
+        }
     }
 }
 
@@ -407,9 +449,14 @@ fn close_window(window: Option<CapSlot>) {
 }
 
 /// Whether `op` may come through the copy of this endpoint badged `badge`:
-/// a management copy (`permissions`) decides, never installs or launches.
+/// a management copy (`permissions`) decides, never installs or launches;
+/// a launcher's (`launch`) lists and launches, passing nothing along.
 pub fn allowed(badge: usize, op: usize) -> bool {
-    badge & wire::MANAGE_BADGE == 0 || matches!(op, wire::LIST | wire::APPS | wire::REVOKE | wire::ALLOW)
+    let (manage, launch) = (badge & wire::MANAGE_BADGE != 0, badge & wire::LAUNCH_BADGE != 0);
+    if !manage && !launch {
+        return true;
+    }
+    (manage && matches!(op, wire::LIST | wire::APPS | wire::REVOKE | wire::ALLOW)) || (launch && matches!(op, wire::APPS | wire::LAUNCH))
 }
 
 fn revoke(cap: Option<CapSlot>) {

crates/servers/permission-manager/src/store.rs

@@ -202,7 +202,8 @@ impl Store for DiskStore {
                 n += 1;
             }
         });
-        ids.into_iter().flatten().find(|id| self.manifest(id).is_some_and(|m| m.command.as_str() == command))
+        // By its id, or the command the shell runs it as.
+        ids.into_iter().flatten().find(|id| id.as_str() == command || self.manifest(id).is_some_and(|m| m.command.as_str() == command))
     }
 
     fn decision(&mut self, id: &AppId) -> Option<Decision> {

crates/servers/permission-manager/src/tests/mod.rs

@@ -11,5 +11,18 @@ fn a_management_copy_only_decides() {
     for op in [LAUNCH, INSTALL, PASS_OUTPUT, PASS_FOLDER, 99] {
         assert!(!allowed(manager, op), "op {op}");
     }
-    assert!(allowed(5, LAUNCH) && allowed(5, INSTALL), "a launcher's copy does everything");
+    assert!(allowed(5, LAUNCH) && allowed(5, INSTALL), "the shell's copy does everything");
+}
+
+#[test]
+fn a_home_copy_only_lists_and_launches() {
+    use crate::server::allowed;
+    use types::apps::*;
+    let home = LAUNCH_BADGE | 2;
+    assert!(allowed(home, APPS) && allowed(home, LAUNCH));
+    for op in [PASS_OUTPUT, PASS_FOLDER, INSTALL, LIST, REVOKE, ALLOW] {
+        assert!(!allowed(home, op), "op {op}");
+    }
+    let both = LAUNCH_BADGE | MANAGE_BADGE | 2;
+    assert!(allowed(both, LAUNCH) && allowed(both, REVOKE) && !allowed(both, INSTALL));
 }

crates/servers/storage-server/src/server.rs

@@ -9,7 +9,8 @@ use crate::scope::Scope;
 
 mod session;
 
-const MAX_CLIENTS: usize = 8;
+/// Connections at once: every program reading files, apps among them.
+const MAX_CLIENTS: usize = 16;
 /// Handles at or past this are never valid (the file systems hand out
 /// fewer), so ownership fits a small table.
 const MAX_HANDLES: usize = 16;

crates/types/src/apps.rs

@@ -33,6 +33,10 @@ pub const APPS: usize = 8;
 /// an app granted `permissions`: through it only `LIST`, `APPS`, `REVOKE`
 /// and `ALLOW` work, never installing or launching.
 pub const MANAGE_BADGE: usize = 1 << 61;
+/// On the copy of the endpoint an app granted `launch` gets (Home): it may
+/// only `APPS` and `LAUNCH`, and pass nothing along (no `PASS_OUTPUT`,
+/// `PASS_FOLDER`). One copy may carry both bits.
+pub const LAUNCH_BADGE: usize = 1 << 60;
 
 /// The request page's size.
 pub const PAGE_SIZE: usize = 4096;

crates/types/src/display.rs

@@ -113,13 +113,26 @@ pub mod window {
     /// accent, `0xrrggbb`. The theme the window is handed carries it where
     /// it passes contrast (`theme::with_accent`).
     pub const ACCENT: usize = 12;
+    /// Through a window, before its buffer is asked for and once: word 1
+    /// what it is (`role`). A window given none (the terminal's) is
+    /// neither closed by `CLOSE_KEY` nor brought up by `HOME_KEY`.
+    pub const ROLE: usize = 13;
+    pub mod role {
+        /// An app's: `CLOSE_KEY` closes it.
+        pub const APP: usize = 1;
+        /// Home: `HOME_KEY` brings it to the front.
+        pub const HOME: usize = 2;
+    }
     /// The event (in place of an input event's tag) every window gets when
     /// the theme changes: ask for it again.
     pub const THEME_CHANGED: usize = 16;
 
-    /// The function key that brings the next window to the front; the
-    /// display keeps it to itself.
+    /// Function keys the display keeps to itself: the next window to the
+    /// front (Alt+Tab is the same key), Home to the front, and the app in
+    /// front closed (its window goes, so its next call to it fails).
     pub const SWITCH_KEY: usize = 1;
+    pub const HOME_KEY: usize = 2;
+    pub const CLOSE_KEY: usize = 4;
 
     /// On the badge of every window capability; launchers' badges are
     /// small, so the two never meet.

docs/apps.md

@@ -91,12 +91,23 @@ optional one is simply not granted.
 | `display` | a window of its own, named from its manifest: its pixels, and the keyboard while it is in front |
 | `storage` | the user's `data/<id>/`, read-write (its own folder only) |
 | `permissions` | a management copy of the permission manager's endpoint (below) |
-| `session` | init's session copy: who is signed in (`WHO`), and signing out |
+| `session` | init's session copy: who is signed in (`WHO`), the theme, and signing out |
+| `launch` | a launcher copy of the permission manager's endpoint: `APPS` and `LAUNCH` only |
 
-`permissions` and `session` are **system only**: only an app on the system
+`permissions`, `session` and `launch` are **system only**: only an app on the system
 image (with a factory decision) has them. Installing an app that asks for
 either, required or optional, is refused (`SYSTEM_ONLY`). Settings
-(`kos.settings`) is the app that holds them.
+(`kos.settings`) holds the first two, Home (`kos.home`) the third.
+
+Home is the session's desktop: the permission manager starts it when the
+session starts (and again if it dies, five times at most), and it lists
+every app with a window and opens the one pressed, each in its own
+window, as many as there is room for (8 windows, 24 MiB between the
+apps). Its copy of the endpoint is badged `LAUNCH_BADGE`: it may list and
+launch, but pass nothing along, so what it opens gets no output and no
+folder. The shell still runs apps and waits; `name &` starts one without
+waiting (and without the current folder, which nobody would take back).
+An app is named to `LAUNCH` by its command or its id.
 
 ## Decisions
 

docs/ipc.md

@@ -173,7 +173,16 @@ The display server alone holds the screen (`types::display::window`,
 client `libkos::display`). It draws the OS bar along the top: every
 window's name, the one in front highlighted, and who is signed in (init
 says, `SET_USER`). Below it, the window in front fills the screen; one is
-in front at a time, a new one comes to the front, and F1 brings the next.
+in front at a time, a new one comes to the front, and F1 (or Alt+Tab) brings
+the next; closing the one in front brings back the one in front before
+it. At most 8 windows are open. The display keeps three keys to itself:
+F1/Alt+Tab, F2 (Home to the front) and F4 (close the app in front). What
+a window is comes from its launcher before the app runs (`ROLE`): the
+permission manager says `HOME` for Home and `APP` for every other app; a
+window given no role (the terminal's) is never closed by F4. Closing an
+app's window revokes it and signals the app, whose next call through it
+fails: `libkos::display::poll` says `Closed`, and K-UI apps and
+`poll_input` users exit, which the permission manager sees as a death.
 A window is opened (`OPEN`, with the name the bar shows) only through a
 launcher's copy of the display endpoint, one init gave a system program:
 the terminal opens its own, the permission manager one for each app

scripts/make-disk-image.sh

@@ -56,6 +56,7 @@ app sleep
 app snake
 app tally
 app settings
+app home
 staged counter inbox/counter
 # A user's first files.
 cp -r disk/home/. "${ROOT}/factory/home/"