fix(adb): harden built-in USB backend

Propagate shell failures and only expose usable USB devices. Use the
standard adb key path.

Handle ADB server conflicts explicitly and improve connection errors.

Author: iamanaws

crates/uad-cli/src/main.rs

@@ -11,7 +11,7 @@
 use clap::{Parser, Subcommand, ValueEnum};
 use clap_complete::Shell;
 use std::process::ExitCode;
-use uad_core::adb::AdbBackend;
+use uad_core::adb::{ACommand, AdbBackend};
 use uad_core::uad_lists::PackageState;
 
 mod commands;
@@ -58,6 +58,10 @@ pub struct Cli {
     )]
     backend: AdbBackendArg,
 
+    /// Stop the system ADB server before using the Builtin backend
+    #[arg(long, global = true)]
+    kill_adb_server: bool,
+
     #[command(subcommand)]
     command: Commands,
 }
@@ -204,8 +208,7 @@ async fn main() -> ExitCode {
 
 fn run() -> Result<(), Box<dyn std::error::Error>> {
     let cli = Cli::parse();
-    let backend: AdbBackend = cli.backend.into();
-    backend.set_current();
+    configure_adb(&cli)?;
 
     match cli.command {
         Commands::Devices => {
@@ -289,3 +292,75 @@ fn run() -> Result<(), Box<dyn std::error::Error>> {
 
     Ok(())
 }
+
+fn configure_adb(cli: &Cli) -> Result<(), Box<dyn std::error::Error>> {
+    let backend: AdbBackend = cli.backend.into();
+    #[cfg(feature = "builtin-adb")]
+    let using_builtin = backend == AdbBackend::Builtin;
+    #[cfg(not(feature = "builtin-adb"))]
+    let using_builtin = false;
+
+    if cli.kill_adb_server && !using_builtin {
+        return Err("--kill-adb-server requires --backend builtin".into());
+    }
+    if cli.kill_adb_server {
+        ACommand::kill_system_server()
+            .map_err(|err| format!("Failed to stop the system ADB server: {err}"))?;
+    }
+
+    backend.set_current();
+
+    if using_builtin && !cli.kill_adb_server && command_uses_device(&cli.command) {
+        let devices = ACommand::new().devices()?;
+        if devices.iter().any(|(_, status)| status == "busy") {
+            return Err(
+                "The system ADB server is using the USB device. Stop it with `adb kill-server` \
+                 or rerun with `--kill-adb-server`."
+                    .into(),
+            );
+        }
+    }
+
+    Ok(())
+}
+
+fn command_uses_device(command: &Commands) -> bool {
+    match command {
+        Commands::Devices
+        | Commands::List { .. }
+        | Commands::Uninstall { .. }
+        | Commands::Enable { .. }
+        | Commands::Disable { .. }
+        | Commands::Repl { .. } => true,
+        Commands::Info { device, .. } => device.is_some(),
+        Commands::Update | Commands::Adb | Commands::Completions { .. } => false,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[cfg(feature = "builtin-adb")]
+    #[test]
+    fn builtin_server_shutdown_is_explicitly_opted_in() {
+        let cli = Cli::try_parse_from([
+            "uad",
+            "--backend",
+            "builtin",
+            "--kill-adb-server",
+            "devices",
+        ])
+        .expect("valid Builtin CLI arguments");
+
+        assert_eq!(cli.backend, AdbBackendArg::Builtin);
+        assert!(cli.kill_adb_server);
+        assert!(command_uses_device(&cli.command));
+    }
+
+    #[test]
+    fn non_device_commands_skip_usb_preflight() {
+        assert!(!command_uses_device(&Commands::Update));
+        assert!(!command_uses_device(&Commands::Adb));
+    }
+}