fix(menu): let a key press show a hidden menu

A menu with a zero timeout or menu-hidden now waits 100 ms for a key press
before booting the default entry, which is how the boot loader interface
lets a user interrupt a hidden menu. menu-disabled still skips the wait.

crates/boot/src/main.rs

@@ -410,6 +410,9 @@ fn run() -> Result<()> {
     // Whether the boot menu should wait for the user, instead of counting down.
     let mut wait_for_user = false;
 
+    // Whether the menu is disabled, so that a key press can't bring it up.
+    let mut menu_disabled = false;
+
     // Apply the chosen timeout.
     match timeout {
         BootloaderInterfaceTimeout::MenuForce => {
@@ -424,11 +427,17 @@ fn run() -> Result<()> {
             menu_timeout = timeout;
         }
 
-        BootloaderInterfaceTimeout::MenuHidden | BootloaderInterfaceTimeout::MenuDisabled => {
+        BootloaderInterfaceTimeout::MenuHidden => {
             // Hide the boot menu by setting the timeout to zero.
             menu_timeout = 0;
         }
 
+        BootloaderInterfaceTimeout::MenuDisabled => {
+            // Hide the boot menu, and don't let a key press show it.
+            menu_timeout = 0;
+            menu_disabled = true;
+        }
+
         BootloaderInterfaceTimeout::Timeout(timeout) => {
             // Configure the timeout to the specified value.
             menu_timeout = timeout;
@@ -593,7 +602,7 @@ fn run() -> Result<()> {
     let entry = match forced_entry {
         Some(entry) => entry,
         // Delegate to the menu to select an entry to boot.
-        None => menu::select(&timer, menu_timeout, menu_style, &entries)
+        None => menu::select(&timer, menu_timeout, menu_disabled, menu_style, &entries)
             .context("unable to select entry via boot menu")?,
     };
 

crates/boot/src/menu.rs

@@ -31,6 +31,9 @@ mod logo;
 /// simple: A full-screen menu that selects entries with the arrow keys.
 pub mod simple;
 
+/// How long a hidden menu waits for a key press before the default entry is booted.
+const HIDDEN_MENU_KEY_WAIT: Duration = Duration::from_millis(100);
+
 /// The longest duration a single timer is set for. This is well within the range of
 /// the 100ns units that timers are set in, and still over a century long.
 const MAX_TIMER_DURATION: Duration = Duration::from_secs(u32::MAX as u64);
@@ -167,13 +170,29 @@ pub fn wait_for_events(events: &[Event], timeout: Option<Duration>) -> Result<Op
 }
 
 /// Shows a boot menu of the specified `style` to select a bootable entry to boot.
-/// See [BootMenu::select] for how `timeout` is handled.
+/// See [BootMenu::select] for how `timeout` is handled. A zero `timeout` hides the menu, but
+/// the menu is still shown if a key is pressed right away, unless `menu_disabled` is set.
 pub fn select<'live>(
     timer: &'live PlatformTimer,
     timeout: Option<Duration>,
+    menu_disabled: bool,
     style: MenuStyle,
     entries: &'live [BootableEntry],
 ) -> Result<&'live BootableEntry> {
+    // A hidden menu gives the user a moment to ask for the menu with a key press.
+    let timeout = if timeout.is_some_and(|timeout| timeout.is_zero()) && !menu_disabled {
+        match uefi::system::with_stdin(|input| read_key(input, Some(HIDDEN_MENU_KEY_WAIT))) {
+            Ok(Some(_)) => None,
+            Ok(None) => timeout,
+            Err(error) => {
+                warn!("unable to check for a key press: {:#}", error);
+                timeout
+            }
+        }
+    } else {
+        timeout
+    };
+
     // Notify the bootloader interface that we are about to display the menu.
     // This is only informational, so it should not prevent booting.
     if let Err(error) = BootloaderInterface::mark_menu(timer) {