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

src/uu/mcookie/src/mcookie.rs

@@ -3,6 +3,8 @@
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
 
+#[cfg(unix)]
+use std::os::unix::fs::FileTypeExt;
 use std::{fs::File, io::Read};
 
 use clap::{crate_version, Arg, ArgAction, Command};
@@ -56,9 +58,24 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
         if let Some(max_bytes) = &max_size {
             let mut handle = f.take(*max_bytes);
             handle.read_to_end(&mut buffer)?;
+        } else {
+            #[cfg(unix)]
+            {
+                const DEFAULT_SEED_READ_BYTES: u64 = 1024;
+                let metadata = f.metadata()?;
+
+                if metadata.file_type().is_char_device() {
+                    let mut handle = f.take(DEFAULT_SEED_READ_BYTES);
+                    handle.read_to_end(&mut buffer)?;
                 } else {
                     f.read_to_end(&mut buffer)?;
                 }
+            }
+            #[cfg(not(unix))]
+            {
+                f.read_to_end(&mut buffer)?;
+            }
+        }
 
         if verbose {
             eprintln!("Got {} bytes from {}", buffer.len(), file);

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();