feat: terminate when init process exits (--kernel)

Add an init script to the initramfs that traps shell exit and triggers
poweroff via /proc/sysrq-trigger. This ensures QEMU terminates cleanly
when the user exits the shell, rather than hanging indefinitely.

Changes:
- Add /init script to initramfs that runs as PID 1
- Use -no-reboot QEMU flag to exit on guest poweroff
- Simplify kernel command line using environment variables
- Init script handles hostname, device creation, and poweroff on exit

src/qemu_vm.rs

@@ -69,20 +69,17 @@ pub fn launch_qemu(config: QemuConfig) -> Result<()> {
     // For initramfs boot, use rdinit= instead of init=
     // No root= needed as initramfs becomes the rootfs
     // 'quiet' suppresses kernel log messages for a cleaner console
-    // Use setsid with redirected stdio to get proper job control
-    // /dev/ttyS0 is created in the initramfs for serial console
-    // Set hostname before spawning shell
+    // The init script (added to initramfs) handles hostname, shell, and poweroff
     let kernel_append = if let Some(ref cmd) = config.command {
         let cmd_str = cmd.join(" ");
         format!(
-            "console=ttyS0 quiet rdinit=/bin/sh -- -c \"echo {} >/etc/hostname; hostname {}; setsid sh -c 'exec {} </dev/ttyS0 >/dev/ttyS0 2>&1 -c {}'\"",
-            hostname, hostname, shell, cmd_str
+            "console=ttyS0 quiet ECR_SHELL={} ECR_CMD=\"{}\" ECR_HOSTNAME={}",
+            shell, cmd_str, hostname
         )
     } else {
-        // Default to interactive shell with proper job control
         format!(
-            "console=ttyS0 quiet rdinit=/bin/sh -- -c \"echo {} >/etc/hostname; hostname {}; setsid sh -c 'exec {} </dev/ttyS0 >/dev/ttyS0 2>&1'\"",
-            hostname, hostname, shell
+            "console=ttyS0 quiet ECR_SHELL={} ECR_HOSTNAME={}",
+            shell, hostname
         )
     };
 
@@ -95,6 +92,7 @@ pub fn launch_qemu(config: QemuConfig) -> Result<()> {
     // Build QEMU arguments
     // -display none suppresses VGA/BIOS output
     // -serial mon:stdio connects serial console to terminal with QEMU monitor muxed
+    // -no-reboot makes QEMU exit when the guest requests poweroff/reboot
     let args = vec![
         "-kernel".to_string(),
         config.kernel_path.to_string_lossy().to_string(),
@@ -108,6 +106,7 @@ pub fn launch_qemu(config: QemuConfig) -> Result<()> {
         "none".to_string(),
         "-serial".to_string(),
         "mon:stdio".to_string(),
+        "-no-reboot".to_string(),
         "-netdev".to_string(),
         "user,id=net0".to_string(),
         "-device".to_string(),
@@ -285,6 +284,64 @@ fn create_cpio_archive(rootfs: &Path, pb: &ProgressBar) -> Result<Vec<u8>> {
             .context("Failed to finish device node entry")?;
     }
     
+    // Add the /init script that will be run as PID 1
+    // This script handles hostname setup, shell execution, and poweroff on exit
+    // Uses /proc/sysrq-trigger for poweroff since poweroff command may not be available
+    let init_script = r#"#!/bin/sh
+# ECR init script - runs as PID 1
+
+# Mount essential filesystems
+mount -t proc proc /proc
+mount -t sysfs sysfs /sys
+mount -t devtmpfs devtmpfs /dev 2>/dev/null || true
+
+# Set hostname from kernel cmdline
+if [ -n "$ECR_HOSTNAME" ]; then
+    echo "$ECR_HOSTNAME" > /etc/hostname
+    hostname "$ECR_HOSTNAME"
+fi
+
+# Create console device if missing
+mknod -m 600 /dev/console c 5 1 2>/dev/null || true
+mknod -m 666 /dev/ttyS0 c 4 64 2>/dev/null || true
+
+# Function to poweroff - use sysrq-trigger which works without external binaries
+do_poweroff() {
+    # Silence kernel printk to suppress shutdown messages
+    echo 0 > /proc/sys/kernel/printk
+    # 'o' means power off, see Documentation/admin-guide/sysrq.rst
+    echo o > /proc/sysrq-trigger
+    # Fallback: infinite loop to prevent kernel panic
+    while true; do sleep 1; done
+}
+
+# Trap exit to ensure poweroff runs
+trap do_poweroff EXIT
+
+# Run the shell or command
+if [ -n "$ECR_CMD" ]; then
+    # Run command with the specified shell
+    setsid sh -c "exec $ECR_SHELL </dev/ttyS0 >/dev/ttyS0 2>&1 -c '$ECR_CMD'"
+else
+    # Run interactive shell
+    setsid sh -c "exec $ECR_SHELL </dev/ttyS0 >/dev/ttyS0 2>&1"
+fi
+"#;
+
+    let init_data = init_script.as_bytes();
+    let init_builder = NewcBuilder::new("init")
+        .mode(0o100755) // executable
+        .uid(0)
+        .gid(0)
+        .nlink(1)
+        .mtime(0);
+    
+    let mut init_writer = init_builder.write(&mut archive, init_data.len() as u32);
+    init_writer.write_all(init_data)
+        .context("Failed to write init script to cpio archive")?;
+    init_writer.finish()
+        .context("Failed to finish init script entry")?;
+    
     // Write the trailer (takes ownership and returns the writer)
     archive = newc::trailer(archive)
         .context("Failed to write cpio trailer")?;