fix: avoid char-boundary panic in NBReader::try_read

Author: SAY-5

src/reader.rs

@@ -203,11 +203,9 @@ impl NBReader {
     pub fn try_read(&mut self) -> Option<char> {
         // discard eventual errors, EOF will be handled in read_until correctly
         let _ = self.read_into_buffer();
-        if !self.buffer.is_empty() {
-            self.buffer.drain(..1).last()
-        } else {
-            None
-        }
+        let first = self.buffer.chars().next()?;
+        self.buffer.drain(..first.len_utf8());
+        Some(first)
     }
 }
 
@@ -457,4 +455,21 @@ mod tests {
         assert_eq!(None, r.try_read());
         assert_eq!(None, r.try_read());
     }
+
+    #[test]
+    fn test_try_read_multibyte_byte() {
+        // bytes >= 0x80 become two-byte chars in the internal buffer, so draining
+        // one byte at a time used to land off a char boundary and panic
+        let f = io::Cursor::new("é");
+        let mut r = NBReader::new(f, Options::default());
+        let mut count = 0;
+        loop {
+            match r.try_read() {
+                Some(_) => count += 1,
+                None if count == "é".len() => break,
+                None => thread::sleep(time::Duration::from_millis(10)),
+            }
+        }
+        assert_eq!(count, "é".len());
+    }
 }