MySQL specific gaplock protection is configurable

Instead of depending on the name of the ActiveRecord adapter to identify if the database is MySQL, gaplock protection is configurable, as suggested by @jurre on #399

.circleci/config.yml

@@ -38,6 +38,7 @@ jobs:
           - RAILS_VERSION=5.2.4
           - DATABASE_URL=mysql2://[email protected]/statesman_test
           - DATABASE_DEPENDENCY_PORT=3306
+          - GAPLOCK_PROTECTION=true
       - image: circleci/mysql:5.7.18
         environment:
           - MYSQL_ALLOW_EMPTY_PASSWORD=true
@@ -66,6 +67,7 @@ jobs:
           - RAILS_VERSION=6.0.2
           - DATABASE_URL=mysql2://[email protected]/statesman_test
           - DATABASE_DEPENDENCY_PORT=3306
+          - GAPLOCK_PROTECTION=true
       - image: circleci/mysql:5.7.18
         environment:
           - MYSQL_ALLOW_EMPTY_PASSWORD=true
@@ -93,6 +95,7 @@ jobs:
           - RAILS_VERSION=master
           - DATABASE_URL=mysql2://[email protected]/statesman_test
           - DATABASE_DEPENDENCY_PORT=3306
+          - GAPLOCK_PROTECTION=true
       - image: circleci/mysql:5.7.18
         environment:
           - MYSQL_ALLOW_EMPTY_PASSWORD=true
@@ -122,6 +125,7 @@ jobs:
           - RAILS_VERSION=6.0.2
           - DATABASE_URL=mysql2://[email protected]/statesman_test
           - DATABASE_DEPENDENCY_PORT=3306
+          - GAPLOCK_PROTECTION=true
       - image: circleci/mysql:5.7.18
         environment:
           - MYSQL_ALLOW_EMPTY_PASSWORD=true
@@ -149,6 +153,7 @@ jobs:
           - RAILS_VERSION=master
           - DATABASE_URL=mysql2://[email protected]/statesman_test
           - DATABASE_DEPENDENCY_PORT=3306
+          - GAPLOCK_PROTECTION=true
       - image: circleci/mysql:5.7.18
         environment:
           - MYSQL_ALLOW_EMPTY_PASSWORD=true

lib/statesman.rb

@@ -25,9 +25,14 @@ module Adapters
   def self.configure(&block)
     config = Config.new(block)
     @storage_adapter = config.adapter_class
+    @mysql_gaplock_protection = config.mysql_gaplock_protection
   end
 
   def self.storage_adapter
     @storage_adapter || Adapters::Memory
   end
+
+  def self.mysql_gaplock_protection?
+    @mysql_gaplock_protection
+  end
 end

lib/statesman/adapters/active_record.rb

@@ -16,6 +16,10 @@ def self.database_supports_partial_indexes?
         end
       end
 
+      def self.adapter_name
+        ::ActiveRecord::Base.connection.adapter_name.downcase
+      end
+
       def initialize(transition_class, parent_model, observer, options = {})
         serialized = serialized?(transition_class)
         column_type = transition_class.columns_hash["metadata"].sql_type
@@ -74,7 +78,7 @@ def create_transition(from, to, metadata)
         ::ActiveRecord::Base.transaction(requires_new: true) do
           @observer.execute(:before, from, to, transition)
 
-          if db_mysql?
+          if mysql_gaplock_protection?
             # We save the transition first with most_recent falsy, then mark most_recent
             # true after to avoid letting MySQL acquire a next-key lock which can cause
             # deadlocks.
@@ -136,7 +140,7 @@ def update_most_recents(most_recent_id = nil)
         # MySQL will validate index constraints across the intermediate result of an
         # update. This means we must order our update to deactivate the previous
         # most_recent before setting the new row to be true.
-        update.order(transition_table[:most_recent].desc) if db_mysql?
+        update.order(transition_table[:most_recent].desc) if mysql_gaplock_protection?
 
         ::ActiveRecord::Base.connection.update(update.to_sql)
       end
@@ -292,8 +296,8 @@ def updated_column_and_timestamp
         ]
       end
 
-      def db_mysql?
-        ::ActiveRecord::Base.connection.adapter_name.downcase.starts_with?("mysql")
+      def mysql_gaplock_protection?
+        Statesman.mysql_gaplock_protection?
       end
 
       def db_true

lib/statesman/config.rb

@@ -5,14 +5,33 @@
 
 module Statesman
   class Config
-    attr_reader :adapter_class
+    attr_reader :adapter_class, :mysql_gaplock_protection
 
     def initialize(block = nil)
       instance_eval(&block) unless block.nil?
     end
 
     def storage_adapter(adapter_class)
       @adapter_class = adapter_class
+
+      # If our adapter class suggests we're using mysql, enable gaplock protection by
+      # default.
+      enable_mysql_gaplock_protection if mysql_adapter?(adapter_class)
+    end
+
+    def enable_mysql_gaplock_protection
+      @mysql_gaplock_protection = true
+    end
+
+    private
+
+    def mysql_adapter?(adapter_class)
+      adapter_name = adapter_name(adapter_class)
+      adapter_name&.starts_with("mysql")
+    end
+
+    def adapter_name(adapter_class)
+      adapter_class.respond_to?(:adapter_name) && adapter_class&.adapter_name
     end
   end
 end

spec/statesman/adapters/active_record_queries_spec.rb

@@ -20,7 +20,9 @@ def configure_new(klass, transition_class)
     prepare_other_model_table
     prepare_other_transitions_table
 
-    Statesman.configure { storage_adapter(Statesman::Adapters::ActiveRecord) }
+    Statesman.configure do
+      storage_adapter(Statesman::Adapters::ActiveRecord)
+    end
   end
 
   after { Statesman.configure { storage_adapter(Statesman::Adapters::Memory) } }