deserialize/value: accept shorter encoded tuples

When deserializing tuples, if the encoded data is shorter than expected
(i.e., it contains fewer elements than the tuple type specifies),
we should treat the missing elements as nulls instead of returning an
error. This is what other drivers (Python, Java, Node.js) do, and it
improves compatibility with ScyllaDB/Cassandra, which allow inserting
tuples with fewer elements than the defined type.

Fixes: #1452.

Author: wprzytula

scylla-cql/src/deserialize/value.rs

@@ -1340,17 +1340,26 @@ macro_rules! impl_tuple {
                 let mut v = ensure_not_null_frame_slice::<Self>(typ, v)?;
                 let ret = (
                     $(
-                        v.read_cql_bytes()
-                            .map_err(|err| DeserializationError::new(err))
-                            .and_then(|cql_bytes| <$Ti>::deserialize($idf, cql_bytes))
-                            .map_err(|err| mk_deser_err::<Self>(
-                                typ,
-                                TupleDeserializationErrorKind::FieldDeserializationFailed {
-                                    position: $idx,
-                                    err,
-                                }
-                            )
-                        )?,
+                        {
+                            let cql_bytes = if v.is_empty() {
+                                // Special case: if there are no bytes left to read, consider the tuple element as null.
+                                // This is needed to handle tuples with less elements than expected
+                                // (see https://github.com/scylladb/scylla-rust-driver/issues/1452 for context).
+                                None
+                            } else {
+                                v.read_cql_bytes().map_err(|err| DeserializationError::new(err))?
+                            };
+
+                            <$Ti>::deserialize($idf, cql_bytes)
+                                .map_err(|err| mk_deser_err::<Self>(
+                                    typ,
+                                    TupleDeserializationErrorKind::FieldDeserializationFailed {
+                                        position: $idx,
+                                        err,
+                                    }
+                                )
+                            )?
+                        },
                     )*
                 );
                 Ok(ret)