1 files changed, 81 insertions, 13 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 684c7042a7..bbcdc96cdc 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb
@@ -11,7 +11,7 @@ module ActiveRecord
           else
             super
           end
-          discard_remaining_results
+          @connection.abandon_results!
           result
         end
 
@@ -19,8 +19,28 @@ module ActiveRecord
           execute(sql, name).to_a
         end
 
+        READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :commit, :explain, :select, :set, :show, :release, :savepoint, :rollback) # :nodoc:
+        private_constant :READ_QUERY
+
+        def write_query?(sql) # :nodoc:
+          !READ_QUERY.match?(sql)
+        end
+
+        def explain(arel, binds = [])
+          sql     = "EXPLAIN #{to_sql(arel, binds)}"
+          start   = Concurrent.monotonic_time
+          result  = exec_query(sql, "EXPLAIN", binds)
+          elapsed = Concurrent.monotonic_time - start
+
+          MySQL::ExplainPrettyPrinter.new.pp(result, elapsed)
+        end
+
         # Executes the SQL statement in the context of this connection.
         def execute(sql, name = nil)
+          if preventing_writes? && write_query?(sql)
+            raise ActiveRecord::ReadOnlyError, "Write query attempted while in readonly mode: #{sql}"
+          end
+
           # make sure we carry over any changes to ActiveRecord::Base.default_timezone that have been
           # made since we established the connection
           @connection.query_options[:database_timezone] = ActiveRecord::Base.default_timezone
@@ -29,24 +49,30 @@ module ActiveRecord
         end
 
         def exec_query(sql, name = "SQL", binds = [], prepare: false)
-          materialize_transactions
-
           if without_prepared_statement?(binds)
             execute_and_free(sql, name) do |result|
-              ActiveRecord::Result.new(result.fields, result.to_a) if result
+              if result
+                ActiveRecord::Result.new(result.fields, result.to_a)
+              else
+                ActiveRecord::Result.new([], [])
+              end
             end
           else
             exec_stmt_and_free(sql, name, binds, cache_stmt: prepare) do |_, result|
-              ActiveRecord::Result.new(result.fields, result.to_a) if result
+              if result
+                ActiveRecord::Result.new(result.fields, result.to_a)
+              else
+                ActiveRecord::Result.new([], [])
+              end
             end
           end
         end
 
         def exec_delete(sql, name = nil, binds = [])
-          materialize_transactions
-
           if without_prepared_statement?(binds)
-            execute_and_free(sql, name) { @connection.affected_rows }
+            @lock.synchronize do
+              execute_and_free(sql, name) { @connection.affected_rows }
+            end
           else
             exec_stmt_and_free(sql, name, binds) { |stmt| stmt.affected_rows }
           end
@@ -54,22 +80,31 @@ module ActiveRecord
         alias :exec_update :exec_delete
 
         private
+          def execute_batch(sql, name = nil)
+            super
+            @connection.abandon_results!
+          end
+
           def default_insert_value(column)
-            Arel.sql("DEFAULT") unless column.auto_increment?
+            super unless column.auto_increment?
           end
 
           def last_inserted_id(result)
             @connection.last_id
           end
 
-          def discard_remaining_results
-            @connection.abandon_results!
-          end
-
           def supports_set_server_option?
             @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")
@@ -102,7 +137,40 @@ 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 + 2) > max_allowed_packet
+            end
+          end
+
+          def max_allowed_packet
+            @max_allowed_packet ||= show_variable("max_allowed_packet")
+          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}"
+            end
+
+            materialize_transactions
+
             # make sure we carry over any changes to ActiveRecord::Base.default_timezone that have been
             # made since we established the connection
             @connection.query_options[:database_timezone] = ActiveRecord::Base.default_timezone