switch-to-configuration-ng: improve user experience (#349834)

Author: ElvishJerricco

pkgs/by-name/sw/switch-to-configuration-ng/src/src/main.rs

@@ -937,34 +937,28 @@ fn do_user_switch(parent_exe: String) -> anyhow::Result<()> {
     Ok(())
 }
 
+fn usage(argv0: &str) -> ! {
+    eprintln!(
+        r#"Usage: {} [switch|boot|test|dry-activate]
+switch:       make the configuration the boot default and activate now
+boot:         make the configuration the boot default
+test:         activate the configuration, but don't make it the boot default
+dry-activate: show what would be done if this configuration were activated
+"#,
+        argv0
+    );
+    std::process::exit(1);
+}
+
 /// Performs switch-to-configuration functionality for the entire system
-fn do_system_switch() -> anyhow::Result<()> {
+fn do_system_switch(action: Action) -> anyhow::Result<()> {
     let out = PathBuf::from(required_env("OUT")?);
     let toplevel = PathBuf::from(required_env("TOPLEVEL")?);
     let distro_id = required_env("DISTRO_ID")?;
     let install_bootloader = required_env("INSTALL_BOOTLOADER")?;
     let locale_archive = required_env("LOCALE_ARCHIVE")?;
     let new_systemd = PathBuf::from(required_env("SYSTEMD")?);
 
-    let mut args = std::env::args();
-    let argv0 = args.next().ok_or(anyhow!("no argv[0]"))?;
-
-    let Some(Ok(action)) = args.next().map(|a| Action::from_str(&a)) else {
-        eprintln!(
-            r#"Usage: {} [switch|boot|test|dry-activate]
-switch:       make the configuration the boot default and activate now
-boot:         make the configuration the boot default
-test:         activate the configuration, but don't make it the boot default
-dry-activate: show what would be done if this configuration were activated
-"#,
-            argv0
-                .split(std::path::MAIN_SEPARATOR_STR)
-                .last()
-                .unwrap_or("switch-to-configuration")
-        );
-        std::process::exit(1);
-    };
-
     let action = ACTION.get_or_init(|| action);
 
     // The action that is to be performed (like switch, boot, test, dry-activate) Also exposed via
@@ -1923,13 +1917,26 @@ won't take effect until you reboot the system.
 }
 
 fn main() -> anyhow::Result<()> {
-    match (
-        unsafe { nix::libc::geteuid() },
-        std::env::var("__NIXOS_SWITCH_TO_CONFIGURATION_PARENT_EXE").ok(),
-    ) {
-        (0, None) => do_system_switch(),
-        (1..=u32::MAX, None) => bail!("This program does not support being ran outside of the switch-to-configuration environment"),
-        (_, Some(parent_exe)) => do_user_switch(parent_exe),
+    match std::env::var("__NIXOS_SWITCH_TO_CONFIGURATION_PARENT_EXE").ok() {
+        Some(parent_exe) => do_user_switch(parent_exe),
+        None => {
+            let mut args = std::env::args();
+            let argv0 = args.next().ok_or(anyhow!("no argv[0]"))?;
+            let argv0 = argv0
+                .split(std::path::MAIN_SEPARATOR_STR)
+                .last()
+                .unwrap_or("switch-to-configuration");
+
+            let Some(Ok(action)) = args.next().map(|a| Action::from_str(&a)) else {
+                usage(&argv0);
+            };
+
+            if unsafe { nix::libc::geteuid() } == 0 {
+                do_system_switch(action)
+            } else {
+                bail!("{} must be run as the root user", argv0);
+            }
+        }
     }
 }