chroot: handle the error when invalid user

Currently fails with:
```
2024-12-28T14:55:18.9330231Z thread 'main' panicked at src/uu/chroot/src/chroot.rs:284:46:
2024-12-28T14:55:18.9330718Z called `Result::unwrap()` on an `Err` value: Custom { kind: NotFound, error: "Not found: nobody:+65535" }
2024-12-28T14:55:18.9331305Z note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

src/uu/chroot/src/chroot.rs

@@ -281,7 +281,8 @@ fn set_groups_from_str(groups: &str) -> UResult<()> {
 
 fn set_user(user: &str) -> UResult<()> {
     if !user.is_empty() {
-        let user_id = entries::usr2uid(user).unwrap();
+        let user_id =
+            entries::usr2uid(user).map_err(|_| ChrootError::NoSuchUser(user.to_string()))?;
         let err = unsafe { setuid(user_id as libc::uid_t) };
         if err != 0 {
             return Err(

src/uu/chroot/src/error.rs

@@ -27,6 +27,9 @@ pub enum ChrootError {
     /// The new root directory was not given.
     MissingNewRoot,
 
+    /// Failed to find the specified user.
+    NoSuchUser(String),
+
     /// Failed to find the specified group.
     NoSuchGroup(String),
 
@@ -71,6 +74,7 @@ impl Display for ChrootError {
                 "Missing operand: NEWROOT\nTry '{} --help' for more information.",
                 uucore::execution_phrase(),
             ),
+            Self::NoSuchUser(s) => write!(f, "no such user: {}", s.maybe_quote(),),
             Self::NoSuchGroup(s) => write!(f, "no such group: {}", s.maybe_quote(),),
             Self::NoSuchDirectory(s) => write!(
                 f,

tests/by-util/test_chroot.rs

@@ -64,6 +64,28 @@ fn test_invalid_user_spec() {
     assert!(result.stderr_str().starts_with("chroot: invalid userspec"));
 }
 
+#[test]
+fn test_invalid_user() {
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+
+    let dir = "CHROOT_DIR";
+    at.mkdir(dir);
+    if let Ok(result) = run_ucmd_as_root(&ts, &[dir, "whoami"]) {
+        result.success().no_stderr().stdout_is("root");
+    } else {
+        print!("Test skipped; requires root user");
+    }
+
+    if let Ok(result) = run_ucmd_as_root(&ts, &["--user=nobody:+65535", dir, "pwd"]) {
+        result
+            .failure()
+            .stderr_contains("no such user: nobody:+65535");
+    } else {
+        print!("Test skipped; requires root user");
+    }
+}
+
 #[test]
 #[cfg(not(target_os = "android"))]
 fn test_preference_of_userspec() {