mcookie - fix providing /dev/random or /dev/urandom as file leads to infinite loop (#274)

* Update mcookie.rs

* edit doc

* fmt

* fix for unix and non-unix build

* add test case

* refactor

* fmt

* fix

tests/by-util/test_mcookie.rs

@@ -64,6 +64,31 @@ fn test_seed_files_and_max_size_raw() {
     ));
 }
 
+#[test]
+#[cfg(unix)] // Character devices like /dev/zero are a Unix concept
+fn test_char_device_input() {
+    let res_no_limit = new_ucmd!().arg("-f").arg("/dev/zero").succeeds();
+
+    let stdout_no_limit = res_no_limit.no_stderr().stdout_str().trim_end();
+    assert_eq!(stdout_no_limit.len(), 32);
+    assert!(stdout_no_limit.chars().all(|c| c.is_ascii_hexdigit()));
+
+    let res_verbose = new_ucmd!()
+        .arg("--verbose")
+        .arg("-f")
+        .arg("/dev/zero")
+        .succeeds();
+
+    res_verbose.stderr_contains("Got 1024 bytes from /dev/zero");
+    res_verbose.stderr_contains("Got 128 bytes from randomness source"); // Ensure internal randomness is still added
+
+    let stdout_verbose = res_verbose.stdout_str().trim_end();
+    assert_eq!(stdout_verbose.len(), 32);
+    assert!(stdout_verbose.chars().all(|c| c.is_ascii_hexdigit()));
+
+    assert_ne!(stdout_no_limit, stdout_verbose);
+}
+
 #[test]
 fn test_seed_files_and_max_size_human_readable() {
     let mut file = NamedTempFile::new().unwrap();