Bug#36036725 'DO 1;' forbidden by rw-splitting

"DO 1;" is treated as multi-statement which are currently forbidden by
rw-splitting.

Change
======

- allow trailing semicolons after a single query

Change-Id: If5d389b5381c63313519ca15d5eb8349a4eb9d4c

Author: weigon

router/src/routing/src/classic_query_forwarder.cc

@@ -221,8 +221,11 @@ bool contains_multiple_statements(SqlLexer &&lexer) {
       }
 
       if (begin_end_depth == 0) {
-        // semicolon outside a BEGIN...END block.
-        if (tkn.id == ';') return true;
+        if (last_tkn) {
+          // semicolon outside a BEGIN...END block.
+
+          if (last_tkn->id == ';' && tkn.id != END_OF_INPUT) return true;
+        }
       }
 
       last_tkn = tkn;

router/tests/component/test_routing_splitting.cc

@@ -2205,6 +2205,14 @@ TEST_F(RoutingSplittingTest, multi_statements_are_forbidden) {
       EXPECT_EQ(query_res.error().value(),
                 4501);  // multi-statements are forbidden.
     }
+
+    // trailing comma is ok.
+    {
+      auto query_res = cli.query("DO 2;");
+      ASSERT_ERROR(query_res);
+      EXPECT_EQ(query_res.error().value(),
+                1273);  // syntax error (from mock-server)
+    }
   }
 }