feat(menu): add a simple boot menu similar to systemd-boot (#100)

The boot menu is now a BootMenu trait with two styles. The basic menu is
the existing one that prints entries and selects them by number. The new
simple menu is full-screen: entries are centered and highlighted, the
arrow keys, home, end, page up and page down move the selection, enter
boots it, and any key stops the countdown.

The style is picked with --menu-style or menu-style in [options], and
defaults to simple. The timer code used to wait for a key is shared
between both menus in menu.rs.

crates/boot/src/menu/basic.rs

@@ -0,0 +1,139 @@
+use crate::entries::BootableEntry;
+use crate::menu::{BootMenu, wait_for_key};
+use anyhow::{Context, Result};
+use core::time::Duration;
+use log::info;
+use uefi::proto::console::text::{Input, Key, ScanCode};
+
+/// The characters that can be used to select an entry from keys.
+const ENTRY_NUMBER_TABLE: &[char] = &['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'];
+
+/// A boot menu that prints the entries and selects them by number.
+pub struct BasicMenu;
+
+/// Represents the operation that can be performed by the boot menu.
+#[derive(PartialEq, Eq)]
+enum MenuOperation {
+    /// The user selected a numbered entry.
+    Number(usize),
+    /// The user selected the escape key to exit the boot menu.
+    Exit,
+    /// The user selected the enter key to display the entries again.
+    Continue,
+    /// Timeout occurred.
+    Timeout,
+    /// No operation should be performed.
+    Nop,
+}
+
+/// Read a key from the input device with a duration, returning the [MenuOperation] that was
+/// performed.
+fn read(input: &mut Input, timeout: &Duration) -> Result<MenuOperation> {
+    // If the timer triggered, the user did not select a numbered entry.
+    if !wait_for_key(input, *timeout)? {
+        return Ok(MenuOperation::Timeout);
+    }
+
+    // If we reach here, there is a key event.
+    // Some firmware signals the key event without a key being available,
+    // in which case there is nothing to do.
+    let Some(key) = input.read_key().context("unable to read key")? else {
+        return Ok(MenuOperation::Nop);
+    };
+
+    match key {
+        Key::Printable(c) => {
+            // If the key is not ascii, we can't process it.
+            if !c.is_ascii() {
+                return Ok(MenuOperation::Continue);
+            }
+            // Convert the key to a char.
+            let c: char = c.into();
+            // Find the key pressed in the entry number table or continue.
+            Ok(ENTRY_NUMBER_TABLE
+                .iter()
+                .position(|&x| x == c)
+                .map(MenuOperation::Number)
+                .unwrap_or(MenuOperation::Continue))
+        }
+
+        // The escape key is used to exit the boot menu.
+        Key::Special(ScanCode::ESCAPE) => Ok(MenuOperation::Exit),
+
+        // If the special key is unknown, do nothing.
+        Key::Special(_) => Ok(MenuOperation::Nop),
+    }
+}
+
+/// Selects an entry from the list of entries using the boot menu.
+fn select_with_input<'a>(
+    input: &mut Input,
+    timeout: Duration,
+    entries: &'a [BootableEntry],
+) -> Result<&'a BootableEntry> {
+    loop {
+        // If the timeout is not zero, let's display the boot menu.
+        if !timeout.is_zero() {
+            // Print all the entries with the number used to select them.
+            info!("Boot Menu:");
+            for (index, entry) in entries.iter().enumerate() {
+                let title = entry.context().stamp(&entry.declaration().title);
+                info!("  [{}] {}", index, title);
+            }
+        }
+
+        // Read from input until a valid operation is selected.
+        let operation = loop {
+            // If the timeout is zero, we can exit immediately because there is nothing to do.
+            if timeout.is_zero() {
+                break MenuOperation::Exit;
+            }
+
+            info!("Select a boot entry using the number keys.");
+            info!("Press Escape to exit and enter to display the entries again.");
+
+            let operation = read(input, &timeout)?;
+            if operation != MenuOperation::Nop {
+                break operation;
+            }
+        };
+
+        match operation {
+            // Entry was selected by number. If the number is invalid, we continue.
+            MenuOperation::Number(index) => {
+                let Some(entry) = entries.get(index) else {
+                    info!("invalid entry number");
+                    continue;
+                };
+                return Ok(entry);
+            }
+
+            // When the user exits the boot menu or a timeout occurs, we should
+            // boot the default entry, if any.
+            MenuOperation::Exit | MenuOperation::Timeout => {
+                return entries
+                    .iter()
+                    .find(|item| item.is_default())
+                    .context("no default entry available");
+            }
+
+            // If the operation is to continue or nop, we can just run the loop again.
+            MenuOperation::Continue | MenuOperation::Nop => {
+                continue;
+            }
+        }
+    }
+}
+
+impl BootMenu for BasicMenu {
+    /// Selects an entry using the basic menu.
+    /// The actual work is done internally in [select_with_input] which is called
+    /// within the context of the standard input device.
+    fn select<'a>(
+        &self,
+        timeout: Duration,
+        entries: &'a [BootableEntry],
+    ) -> Result<&'a BootableEntry> {
+        uefi::system::with_stdin(move |input| select_with_input(input, timeout, entries))
+    }
+}