fix(mysql): type untyped expression columns by value (SELECT 1 gone-away)

crates/litewire-mysql/src/handler.rs

@@ -37,7 +37,7 @@ fn ok_response(affected_rows: u64, last_insert_id: u64, in_transaction: bool) ->
     }
 }
 
-use crate::types::sqlite_to_mysql_column_type;
+use crate::types::{mysql_type_for_value, sqlite_to_mysql_column_type};
 
 /// A cached prepared statement.
 struct PreparedStmt {
@@ -103,11 +103,29 @@ impl LiteWireHandler {
                 let columns: Vec<Column> = rs
                     .columns
                     .iter()
-                    .map(|c| Column {
-                        table: String::new(),
-                        column: c.name.clone(),
-                        coltype: sqlite_to_mysql_column_type(c.decltype.as_deref()),
-                        colflags: ColumnFlags::empty(),
+                    .enumerate()
+                    .map(|(i, c)| {
+                        // Declared type wins; for untyped expression columns
+                        // (`SELECT 1`, decltype None) infer the wire type from
+                        // the first non-NULL value so the declared column type
+                        // matches how the row writer encodes it. Scan past
+                        // leading NULLs; empty/all-NULL columns stay VAR_STRING
+                        // (NULL is valid against any column type).
+                        let coltype = if c.decltype.is_some() {
+                            sqlite_to_mysql_column_type(c.decltype.as_deref())
+                        } else {
+                            rs.rows
+                                .iter()
+                                .filter_map(|r| r.get(i))
+                                .find(|v| !matches!(v, Value::Null))
+                                .map_or(ColumnType::MYSQL_TYPE_VAR_STRING, mysql_type_for_value)
+                        };
+                        Column {
+                            table: String::new(),
+                            column: c.name.clone(),
+                            coltype,
+                            colflags: ColumnFlags::empty(),
+                        }
                     })
                     .collect();
 

crates/litewire-mysql/src/types.rs

@@ -1,7 +1,30 @@
 //! MySQL type mapping from SQLite column declarations.
 
+use litewire_backend::Value;
 use opensrv_mysql::ColumnType;
 
+/// Map a runtime [`Value`] to the MySQL column type its binary encoding
+/// requires.
+///
+/// SQLite is dynamically typed: expression columns (`SELECT 1`, `SELECT
+/// a + b`) have no declared type, so `decltype` is `None` and the
+/// declaration-based mapping would fall back to `VAR_STRING`. But the row
+/// writer emits each value in its native form (an integer as `LONGLONG`),
+/// and opensrv's binary protocol rejects a declared/actual type mismatch
+/// ("tried to use 1 as MYSQL_TYPE_VAR_STRING") and drops the connection —
+/// surfacing to the client as `2006 server has gone away`. For untyped
+/// columns we therefore derive the wire type from the actual data.
+#[must_use]
+pub fn mysql_type_for_value(value: &Value) -> ColumnType {
+    match value {
+        Value::Null => ColumnType::MYSQL_TYPE_VAR_STRING,
+        Value::Integer(_) => ColumnType::MYSQL_TYPE_LONGLONG,
+        Value::Float(_) => ColumnType::MYSQL_TYPE_DOUBLE,
+        Value::Text(_) => ColumnType::MYSQL_TYPE_VAR_STRING,
+        Value::Blob(_) => ColumnType::MYSQL_TYPE_BLOB,
+    }
+}
+
 /// Map a SQLite column declared type to a MySQL column type.
 ///
 /// Uses the declared type string from `PRAGMA table_info` or the column

crates/litewire/tests/mysql_e2e.rs

@@ -280,6 +280,31 @@ async fn prepared_select_with_param() {
     drop(conn);
 }
 
+#[tokio::test]
+async fn prepared_select_expression_no_table() {
+    // A prepared SELECT of a bare expression (no FROM clause) — e.g. the
+    // `SELECT 1` liveness check some ORMs issue with prepare-emulation off.
+    // Regressed with "2006 server has gone away": the frontend dropped the
+    // connection because on_prepare and on_execute disagreed on column count
+    // for a table-less expression column.
+    init_tracing();
+    let port = free_port().await;
+    let _server = start_litewire(port).await;
+    let mut conn = connect(port).await;
+
+    let rows: Vec<i64> = conn.exec("SELECT 1", ()).await.unwrap();
+    assert_eq!(rows, vec![1]);
+
+    // Also exercise a multi-column expression select and an aliased one.
+    let rows: Vec<(i64, i64)> = conn.exec("SELECT 1 + 1, 40 + 2", ()).await.unwrap();
+    assert_eq!(rows, vec![(2, 42)]);
+
+    let rows: Vec<i64> = conn.exec("SELECT 7 AS answer", ()).await.unwrap();
+    assert_eq!(rows, vec![7]);
+
+    drop(conn);
+}
+
 #[tokio::test]
 async fn prepared_insert() {
     init_tracing();