Make `truncate_tables` to bulk statements

Before:

```
   (16.4ms)  TRUNCATE TABLE `author_addresses`
   (20.5ms)  TRUNCATE TABLE `authors`
   (19.4ms)  TRUNCATE TABLE `posts`
```

After:

```
  Truncate Tables (19.5ms)  TRUNCATE TABLE `author_addresses`;
TRUNCATE TABLE `authors`;
TRUNCATE TABLE `posts`
```

1 files changed, 46 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb b/activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb
index 6adcc14545..421afc34bc 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb
@@ -68,6 +68,14 @@ module ActiveRecord
         end
         alias :exec_update :exec_delete
 
+        def insert_fixtures_set(fixture_set, tables_to_delete = []) # :nodoc:
+          super { discard_remaining_results }
+        end
+
+        def truncate_tables(*table_names) # :nodoc:
+          super { discard_remaining_results }
+        end
+
         private
           def default_insert_value(column)
             Arel.sql("DEFAULT") unless column.auto_increment?
@@ -85,6 +93,14 @@ module ActiveRecord
             @connection.respond_to?(:set_server_option)
           end
 
+          def build_truncate_statements(*table_names)
+            if table_names.size == 1
+              super.first
+            else
+              super
+            end
+          end
+
           def multi_statements_enabled?(flags)
             if flags.is_a?(Array)
               flags.include?("MULTI_STATEMENTS")
@@ -117,6 +133,36 @@ module ActiveRecord
             end
           end
 
+          def combine_multi_statements(total_sql)
+            total_sql.each_with_object([]) do |sql, total_sql_chunks|
+              previous_packet = total_sql_chunks.last
+              if max_allowed_packet_reached?(sql, previous_packet)
+                total_sql_chunks << +sql
+              else
+                previous_packet << ";\n"
+                previous_packet << sql
+              end
+            end
+          end
+
+          def max_allowed_packet_reached?(current_packet, previous_packet)
+            if current_packet.bytesize > max_allowed_packet
+              raise ActiveRecordError,
+                "Fixtures set is too large #{current_packet.bytesize}. Consider increasing the max_allowed_packet variable."
+            elsif previous_packet.nil?
+              true
+            else
+              (current_packet.bytesize + previous_packet.bytesize) > max_allowed_packet
+            end
+          end
+
+          def max_allowed_packet
+            @max_allowed_packet ||= begin
+              bytes_margin = 2
+              show_variable("max_allowed_packet") - bytes_margin
+            end
+          end
+
           def exec_stmt_and_free(sql, name, binds, cache_stmt: false)
             if preventing_writes? && write_query?(sql)
               raise ActiveRecord::ReadOnlyError, "Write query attempted while in readonly mode: #{sql}"