connection: switch to the new deserialization framework

Adjusts `Connection::fetch_schema_version` to use the new
deserialization API. Connection is meant to be an internal API, so we
don't introduce a LegacyConnection for this.
`Connection::{query,execute}_iter` will be migrated in a further commit.

Co-authored-by: Wojciech Przytuła <[email protected]>

Author: piodul

scylla/src/transport/connection.rs

@@ -46,7 +46,7 @@ use std::{
     net::{Ipv4Addr, Ipv6Addr},
 };
 
-use super::errors::{ProtocolError, UseKeyspaceProtocolError};
+use super::errors::{ProtocolError, SchemaVersionFetchError, UseKeyspaceProtocolError};
 use super::iterator::{LegacyRowIterator, QueryPager};
 use super::locator::tablets::{RawTablet, TabletParsingError};
 use super::query_result::QueryResult;
@@ -1436,9 +1436,15 @@ impl Connection {
         let (version_id,) = self
             .query_unpaged(LOCAL_VERSION)
             .await?
-            .into_legacy_result()?
-            .single_row_typed()
-            .map_err(ProtocolError::SchemaVersionFetch)?;
+            .into_rows_result()?
+            .ok_or(QueryError::ProtocolError(
+                ProtocolError::SchemaVersionFetch(SchemaVersionFetchError::ResultNotRows),
+            ))?
+            .single_row::<(Uuid,)>()
+            .map_err(|err| {
+                ProtocolError::SchemaVersionFetch(SchemaVersionFetchError::SingleRowError(err))
+            })?;
+
         Ok(version_id)
     }
 

scylla/src/transport/errors.rs

@@ -32,7 +32,7 @@ use thiserror::Error;
 
 use crate::{authentication::AuthError, frame::response};
 
-use super::legacy_query_result::{RowsExpectedError, SingleRowTypedError};
+use super::{legacy_query_result::RowsExpectedError, query_result::SingleRowError};
 
 /// Error that occurred during query execution
 #[derive(Error, Debug, Clone)]
@@ -304,7 +304,7 @@ pub enum ProtocolError {
 
     /// A protocol error appeared during schema version fetch.
     #[error("Schema version fetch protocol error: {0}")]
-    SchemaVersionFetch(SingleRowTypedError),
+    SchemaVersionFetch(#[from] SchemaVersionFetchError),
 
     /// A result with nonfinished paging state received for unpaged query.
     #[error("Unpaged query returned a non-empty paging state! This is a driver-side or server-side bug.")]
@@ -345,6 +345,16 @@ pub enum UseKeyspaceProtocolError {
     UnexpectedResponse(CqlResponseKind),
 }
 
+/// A protocol error that occurred during schema version fetch.
+#[derive(Error, Debug, Clone)]
+#[non_exhaustive]
+pub enum SchemaVersionFetchError {
+    #[error("Schema version query returned non-rows result")]
+    ResultNotRows,
+    #[error(transparent)]
+    SingleRowError(SingleRowError),
+}
+
 /// A protocol error that occurred during tracing info fetch.
 #[derive(Error, Debug, Clone)]
 #[non_exhaustive]