[Rust - bugfix] read_record_from_slice checking wrong size of buffer …

…wrong (#961)

### Public-Facing Changes

There should be no changes in public API caused by this change.

### Description

`read_record_from_slice` was checking that buffer size is bigger than 5
but was trying to read a `u8` and `u64` which are in total 9 bytes. This
was causing panics when reading malformed mcap files or otherwise
malformed streams.

I added some simple tests as well but they aren't very robust (I don't
check parsing of records).

rust/src/read.rs

@@ -118,7 +118,7 @@ impl<'a> Iterator for LinearReader<'a> {
 
 /// Read a record and advance the slice
 fn read_record_from_slice<'a>(buf: &mut &'a [u8]) -> McapResult<records::Record<'a>> {
-    if buf.len() < 5 {
+    if buf.len() < (size_of::<u64>() + size_of::<u8>()) {
         warn!("Malformed MCAP - not enough space for record + length!");
         return Err(McapError::UnexpectedEof);
     }
@@ -1193,4 +1193,21 @@ mod test {
             LinearReader::new_with_options(MAGIC, enum_set!(Options::IgnoreEndMagic)).unwrap();
         assert!(reader.next().is_none());
     }
+
+    #[test]
+    fn test_read_record_from_slice_fails_on_too_short_chunks() {
+        let res = read_record_from_slice(&mut [0_u8; 4].as_slice());
+        assert!(matches!(res, Err(McapError::UnexpectedEof)));
+
+        let res = read_record_from_slice(&mut [0_u8; 8].as_slice());
+        assert!(matches!(res, Err(McapError::UnexpectedEof)));
+    }
+
+    #[test]
+    fn test_read_record_from_slice_parses_for_big_enough_records() {
+        let res = read_record_from_slice(&mut [0_u8; 9].as_slice());
+        assert!(res.is_ok());
+        // Not a very strong test, but we are only testing that it checks the buffer size correctly
+        assert!(matches!(res, Ok(Record::Unknown { opcode: _, data: _ })));
+    }
 }