Always return non-nil *Rows from Query to fix QueryRow

Since QueryRow delegates to Query, it needs Query to always return
non-nil *Rows to prevent a nil pointer deference when the QueryRow
caller calls Scan(). This commit fixes the few returns in QueryEx that
return nil on errors rather than *Rows with its err field set.

query.go

@@ -364,19 +364,21 @@ type QueryExOptions struct {
 }
 
 func (c *Conn) QueryEx(ctx context.Context, sql string, options *QueryExOptions, args ...interface{}) (rows *Rows, err error) {
+	rows = c.getRows(sql, args)
+
 	err = c.waitForPreviousCancelQuery(ctx)
 	if err != nil {
-		return nil, err
+		rows.fatal(err)
+		return rows, err
 	}
 
 	if err := c.ensureConnectionReadyForQuery(); err != nil {
-		return nil, err
+		rows.fatal(err)
+		return rows, err
 	}
 
 	c.lastActivityTime = time.Now()
 
-	rows = c.getRows(sql, args)
-
 	if err := c.lock(); err != nil {
 		rows.fatal(err)
 		return rows, err
@@ -413,14 +415,14 @@ func (c *Conn) QueryEx(ctx context.Context, sql string, options *QueryExOptions,
 		if err != nil && fatalWriteErr(n, err) {
 			rows.fatal(err)
 			c.die(err)
-			return nil, err
+			return rows, err
 		}
 		c.pendingReadyForQueryCount++
 
 		fieldDescriptions, err := c.readUntilRowDescription()
 		if err != nil {
 			rows.fatal(err)
-			return nil, err
+			return rows, err
 		}
 
 		if len(options.ResultFormatCodes) == 0 {

query_test.go

@@ -835,6 +835,43 @@ func TestQueryRowErrors(t *testing.T) {
 	}
 }
 
+func TestQueryRowExErrorsWrongParameterOIDs(t *testing.T) {
+	t.Parallel()
+
+	conn := mustConnect(t, *defaultConnConfig)
+	defer closeConn(t, conn)
+
+	sql := `
+		with t as (
+			select 1::int8 as some_int, 'foo'::text as some_text
+		)
+		select some_int from t where some_text = $1`
+	paramOIDs := []pgtype.OID{pgtype.TextArrayOID}
+	queryArgs := []interface{}{"bar"}
+	expectedErr := "operator does not exist: text = text[] (SQLSTATE 42883)"
+	var result int64
+
+	err := conn.QueryRowEx(
+		context.Background(),
+		sql,
+		&pgx.QueryExOptions{
+			ParameterOIDs: paramOIDs,
+			ResultFormatCodes: []int16{pgx.BinaryFormatCode},
+		},
+		queryArgs...,
+	).Scan(&result)
+
+	if err == nil {
+		t.Errorf("Unexpected success (sql -> %v, paramOIDs -> %v, queryArgs -> %v)", sql, paramOIDs, queryArgs)
+	}
+	if err != nil && !strings.Contains(err.Error(), expectedErr) {
+		t.Errorf("Expected error to contain %s, but got %v (sql -> %v, paramOIDs -> %v, queryArgs -> %v)",
+			expectedErr, err, sql, paramOIDs, queryArgs)
+	}
+
+	ensureConnValid(t, conn)
+}
+
 func TestQueryRowNoResults(t *testing.T) {
 	t.Parallel()