types: introduce read_value

Introduce the `read_value` function which is able to read a [value], as
specified in the CQL protocol. It will be used in the next commit, in
order to make the interface of the SerializedValue iterators more
correct.

scylla-cql/src/frame/types.rs

@@ -104,6 +104,23 @@ impl From<std::array::TryFromSliceError> for ParseError {
     }
 }
 
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum RawValue<'a> {
+    Null,
+    Unset,
+    Value(&'a [u8]),
+}
+
+impl<'a> RawValue<'a> {
+    #[inline]
+    pub fn as_value(&self) -> Option<&'a [u8]> {
+        match self {
+            RawValue::Value(v) => Some(v),
+            RawValue::Null | RawValue::Unset => None,
+        }
+    }
+}
+
 fn read_raw_bytes<'a>(count: usize, buf: &mut &'a [u8]) -> Result<&'a [u8], ParseError> {
     if buf.len() < count {
         return Err(ParseError::BadIncomingData(format!(
@@ -218,6 +235,22 @@ pub fn read_bytes<'a>(buf: &mut &'a [u8]) -> Result<&'a [u8], ParseError> {
     Ok(v)
 }
 
+pub fn read_value<'a>(buf: &mut &'a [u8]) -> Result<RawValue<'a>, ParseError> {
+    let len = read_int(buf)?;
+    match len {
+        -2 => Ok(RawValue::Unset),
+        -1 => Ok(RawValue::Null),
+        len if len >= 0 => {
+            let v = read_raw_bytes(len as usize, buf)?;
+            Ok(RawValue::Value(v))
+        }
+        len => Err(ParseError::BadIncomingData(format!(
+            "invalid value length: {}",
+            len,
+        ))),
+    }
+}
+
 pub fn read_short_bytes<'a>(buf: &mut &'a [u8]) -> Result<&'a [u8], ParseError> {
     let len = read_short_length(buf)?;
     let v = read_raw_bytes(len, buf)?;