Expose the mysql_set_server_option: (#943)

- Use case: I'd like to be able to do multiple statements query without having to reconnect to the db first. Without this feature, if I want to do a multi statement query **after** the connection is established without the `MULTI_STATEMENTS` flag, I'd have to set the flag on the connection and reconnect
- One of the main motivation for this is because Rails is now inserting fixtures inside a multi-statements query. We used the workaround I described above, but it would be great if we could use the mysql function mysql_set_server_option . For more context [Ref](rails/rails#31422 (comment))
- Ref https://dev.mysql.com/doc/refman/5.5/en/mysql-set-server-option.html

ext/mysql2/client.c

@@ -1098,6 +1098,23 @@ static VALUE rb_mysql_client_ping(VALUE self) {
   }
 }
 
+/* call-seq:
+ *    client.set_server_option(value)
+ *
+ * Enables or disables an option for the connection.
+ * Read https://dev.mysql.com/doc/refman/5.7/en/mysql-set-server-option.html
+ * for more information.
+ */
+static VALUE rb_mysql_client_set_server_option(VALUE self, VALUE value) {
+  GET_CLIENT(self);
+
+  if (mysql_set_server_option(wrapper->client, NUM2INT(value)) == 0) {
+    return Qtrue;
+  } else {
+    return Qfalse;
+  }
+}
+
 /* call-seq:
  *    client.more_results?
  *
@@ -1399,6 +1416,7 @@ void init_mysql2_client() {
   rb_define_method(cMysql2Client, "thread_id", rb_mysql_client_thread_id, 0);
   rb_define_method(cMysql2Client, "ping", rb_mysql_client_ping, 0);
   rb_define_method(cMysql2Client, "select_db", rb_mysql_client_select_db, 1);
+  rb_define_method(cMysql2Client, "set_server_option", rb_mysql_client_set_server_option, 1);
   rb_define_method(cMysql2Client, "more_results?", rb_mysql_client_more_results, 0);
   rb_define_method(cMysql2Client, "next_result", rb_mysql_client_next_result, 0);
   rb_define_method(cMysql2Client, "store_result", rb_mysql_client_store_result, 0);
@@ -1528,6 +1546,13 @@ void init_mysql2_client() {
   rb_const_set(cMysql2Client, rb_intern("SECURE_CONNECTION"), LONG2NUM(0));
 #endif
 
+#if MYSQL_VERSION_ID >= 40101
+  rb_const_set(cMysql2Client, rb_intern("OPTION_MULTI_STATEMENTS_ON"),
+      LONG2NUM(MYSQL_OPTION_MULTI_STATEMENTS_ON));
+  rb_const_set(cMysql2Client, rb_intern("OPTION_MULTI_STATEMENTS_OFF"),
+      LONG2NUM(MYSQL_OPTION_MULTI_STATEMENTS_OFF));
+#endif
+
 #ifdef CLIENT_MULTI_STATEMENTS
   rb_const_set(cMysql2Client, rb_intern("MULTI_STATEMENTS"),
       LONG2NUM(CLIENT_MULTI_STATEMENTS));

spec/mysql2/client_spec.rb

@@ -205,6 +205,40 @@ def run_gc
     # rubocop:enable Lint/AmbiguousBlockAssociation
   end
 
+  context "#set_server_option" do
+    let(:client) do
+      new_client.tap do |client|
+        client.set_server_option(Mysql2::Client::OPTION_MULTI_STATEMENTS_ON)
+      end
+    end
+
+    it 'returns true when multi_statements is enable' do
+      expect(client.set_server_option(Mysql2::Client::OPTION_MULTI_STATEMENTS_ON)).to be true
+    end
+
+    it 'returns true when multi_statements is disable' do
+      expect(client.set_server_option(Mysql2::Client::OPTION_MULTI_STATEMENTS_OFF)).to be true
+    end
+
+    it 'returns false when multi_statements is neither OPTION_MULTI_STATEMENTS_OFF or OPTION_MULTI_STATEMENTS_ON' do
+      expect(client.set_server_option(344)).to be false
+    end
+
+    it 'enables multiple-statement' do
+      client.query("SELECT 1;SELECT 2;")
+
+      expect(client.next_result).to be true
+      expect(client.store_result.first).to eql('2' => 2)
+      expect(client.next_result).to be false
+    end
+
+    it 'disables multiple-statement' do
+      client.set_server_option(Mysql2::Client::OPTION_MULTI_STATEMENTS_OFF)
+
+      expect { client.query("SELECT 1;SELECT 2;") }.to raise_error(Mysql2::Error)
+    end
+  end
+
   context "#automatic_close" do
     it "is enabled by default" do
       expect(new_client.automatic_close?).to be(true)