Allow a 2 bytes margin:

- mysql will add a 2 bytes margin to the statement, so given a `max_allowed_packet` set to 1024 bytes, a 1024 bytes fixtures will no be inserted (mysql will throw an error)
- Preventing this by decreasing the max_allowed_packet by 2 bytes when doing the comparison with the actual statement size

2 files changed, 7 insertions, 4 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index 5e6ab1c16d..072a5337a5 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -568,7 +568,8 @@ module ActiveRecord
         end
 
         def max_allowed_packet
-          @max_allowed_packet ||= show_variable("max_allowed_packet")
+          bytes_margin = 2
+          @max_allowed_packet ||= (show_variable("max_allowed_packet") - bytes_margin)
         end
 
         def with_multi_statements
diff --git a/activerecord/test/cases/fixtures_test.rb b/activerecord/test/cases/fixtures_test.rb
index 13034eef22..de5c96daaa 100644
--- a/activerecord/test/cases/fixtures_test.rb
+++ b/activerecord/test/cases/fixtures_test.rb
@@ -119,17 +119,19 @@ class FixturesTest < ActiveRecord::TestCase
   if current_adapter?(:Mysql2Adapter)
     def test_insert_fixtures_set_raises_an_error_when_max_allowed_packet_is_smaller_than_fixtures_set_size
       conn = ActiveRecord::Base.connection
+      mysql_margin = 2
       packet_size = 1024
+      bytes_needed_to_have_a_1024_bytes_fixture = 855
       fixtures = {
         "traffic_lights" => [
-          { "location" => "US", "state" => ["NY"], "long_state" => ["a" * packet_size] },
+          { "location" => "US", "state" => ["NY"], "long_state" => ["a" * bytes_needed_to_have_a_1024_bytes_fixture] },
         ]
       }
 
-      conn.stubs(:max_allowed_packet).returns(packet_size)
+      conn.stubs(:max_allowed_packet).returns(packet_size - mysql_margin)
 
       error = assert_raises(ActiveRecord::ActiveRecordError) { conn.insert_fixtures_set(fixtures) }
-      assert_match(/Fixtures set is too large/, error.message)
+      assert_match(/Fixtures set is too large #{packet_size}\./, error.message)
     end
 
     def test_insert_fixture_set_when_max_allowed_packet_is_bigger_than_fixtures_set_size