Add pg_xmin_optimistic_locking plugin for optimistic locking for all models without database changes

This implements similar support to the optimistic_locking plugin,
but without the need for database changes to add a lock version
column.  Instead, it uses the xmin system column, which is
automatically updated every time the row is updated.

This allows you to load the plugin into a base class and have
all models under the base class support optimistic locking.

Internally, this splits off an optimistic_locking_base internal
plugin from optimistic_locking, makes optimistic_locking and
mssql_optimistic_locking use it, and uses it as the basis
for the new pg_xmin_optimistic_locking plugin.

Author: jeremyevans

CHANGELOG

@@ -1,5 +1,7 @@
 === master
 
+* Add pg_xmin_optimistic_locking plugin for optimistic locking for all models without database changes (jeremyevans)
+
 * Recognize the xid PostgreSQL type as an integer type in the jdbc/postgresql adapter (jeremyevans)
 
 * Make set_column_allow_null method reversible in migrations (enescakir) (#2060)

lib/sequel/plugins/mssql_optimistic_locking.rb

@@ -26,57 +26,27 @@ module Plugins
     module MssqlOptimisticLocking
       # Load the instance_filters plugin into the model.
       def self.apply(model, opts=OPTS)
-        model.plugin :instance_filters
+        model.plugin(:optimistic_locking_base)
       end
 
-      # Set the lock_column to the :lock_column option (default: :timestamp)
+      # Set the lock column
       def self.configure(model, opts=OPTS)
-        model.lock_column = opts[:lock_column] || :timestamp
+        model.lock_column = opts[:lock_column] || model.lock_column || :timestamp
       end
-
-      module ClassMethods
-        # The timestamp/rowversion column containing the version for the current row.
-        attr_accessor :lock_column
-
-        Plugins.inherited_instance_variables(self, :@lock_column=>nil)
-      end
-
+      
       module InstanceMethods
-        # Add the lock column instance filter to the object before destroying it.
-        def before_destroy
-          lock_column_instance_filter
-          super
-        end
-        
-        # Add the lock column instance filter to the object before updating it.
-        def before_update
-          lock_column_instance_filter
-          super
-        end
-        
         private
 
-        # Add the lock column instance filter to the object.
-        def lock_column_instance_filter
-          lc = model.lock_column
-          instance_filter(lc=>Sequel.blob(get_column_value(lc)))
-        end
-
-        # Clear the instance filters when refreshing, so that attempting to
-        # refresh after a failed save removes the previous lock column filter
-        # (the new one will be added before updating).
-        def _refresh(ds)
-          clear_instance_filters
-          super
+        # Make the instance filter value a blob.
+        def lock_column_instance_filter_value
+          Sequel.blob(super)
         end
 
         # Remove the lock column from the columns to update.
         # SQL Server automatically updates the lock column value, and does not like
         # it to be assigned.
         def _save_update_all_columns_hash
-          v = @values.dup
-          cc = changed_columns
-          Array(primary_key).each{|x| v.delete(x) unless cc.include?(x)}
+          v = super
           v.delete(model.lock_column)
           v
         end

lib/sequel/plugins/optimistic_locking.rb

@@ -12,64 +12,31 @@ module Plugins
     #   p1 = Person[1]
     #   p2 = Person[1]
     #   p1.update(name: 'Jim') # works
-    #   p2.update(name: 'Bob') # raises Sequel::Plugins::OptimisticLocking::Error
+    #   p2.update(name: 'Bob') # raises Sequel::NoExistingObject
     #
     # In order for this plugin to work, you need to make sure that the database
-    # table has a +lock_version+ column (or other column you name via the lock_column
-    # class level accessor) that defaults to 0.
+    # table has a +lock_version+ column that defaults to 0. To change the column
+    # used, provide a +:lock_column+ option when loading the plugin:
+    #
+    #     plugin :optimistic_locking, lock_column: :version
     #
     # This plugin relies on the instance_filters plugin.
     module OptimisticLocking
       # Exception class raised when trying to update or destroy a stale object.
       Error = Sequel::NoExistingObject
 
-      # Load the instance_filters plugin into the model.
       def self.apply(model, opts=OPTS)
-        model.plugin :instance_filters
+        model.plugin(:optimistic_locking_base)
       end
 
-      # Set the lock_column to the :lock_column option, or :lock_version if
-      # that option is not given.
+      # Set the lock column
       def self.configure(model, opts=OPTS)
-        model.lock_column = opts[:lock_column] || :lock_version
+        model.lock_column = opts[:lock_column] || model.lock_column || :lock_version
       end
-      
-      module ClassMethods
-        # The column holding the version of the lock
-        attr_accessor :lock_column
-        
-        Plugins.inherited_instance_variables(self, :@lock_column=>nil)
-      end
-    
+
       module InstanceMethods
-        # Add the lock column instance filter to the object before destroying it.
-        def before_destroy
-          lock_column_instance_filter
-          super
-        end
-        
-        # Add the lock column instance filter to the object before updating it.
-        def before_update
-          lock_column_instance_filter
-          super
-        end
-        
         private
 
-        # Add the lock column instance filter to the object.
-        def lock_column_instance_filter
-          lc = model.lock_column
-          instance_filter(lc=>get_column_value(lc))
-        end
-
-        # Clear the instance filters when refreshing, so that attempting to
-        # refresh after a failed save removes the previous lock column filter
-        # (the new one will be added before updating).
-        def _refresh(ds)
-          clear_instance_filters
-          super
-        end
-        
         # Only update the row if it has the same lock version, and increment the
         # lock version.
         def _update_columns(columns)

lib/sequel/plugins/optimistic_locking_base.rb

@@ -0,0 +1,55 @@
+# frozen-string-literal: true
+
+module Sequel
+  module Plugins
+    # Base for other optimistic locking plugins
+    module OptimisticLockingBase
+      # Load the instance_filters plugin into the model.
+      def self.apply(model)
+        model.plugin :instance_filters
+      end
+
+      module ClassMethods
+        # The column holding the version of the lock
+        attr_accessor :lock_column
+        
+        Plugins.inherited_instance_variables(self, :@lock_column=>nil)
+      end
+    
+      module InstanceMethods
+        # Add the lock column instance filter to the object before destroying it.
+        def before_destroy
+          lock_column_instance_filter
+          super
+        end
+        
+        # Add the lock column instance filter to the object before updating it.
+        def before_update
+          lock_column_instance_filter
+          super
+        end
+        
+        private
+        
+        # Add the lock column instance filter to the object.
+        def lock_column_instance_filter
+          instance_filter(model.lock_column=>lock_column_instance_filter_value)
+        end
+
+        # Use the current value of the lock column
+        def lock_column_instance_filter_value
+          public_send(model.lock_column)
+        end
+
+        # Clear the instance filters when refreshing, so that attempting to
+        # refresh after a failed save removes the previous lock column filter
+        # (the new one will be added before updating).
+        def _refresh(ds)
+          clear_instance_filters
+          super
+        end
+      end
+    end
+  end
+end
+

lib/sequel/plugins/pg_xmin_optimistic_locking.rb

@@ -0,0 +1,109 @@
+# frozen-string-literal: true
+
+module Sequel
+  module Plugins
+    # This plugin implements optimistic locking mechanism on PostgreSQL based
+    # on the xmin of the row. The xmin system column is automatically set to
+    # the current transaction id whenever the row is inserted or updated:
+    # 
+    #   class Person < Sequel::Model
+    #     plugin :pg_xmin_optimistic_locking
+    #   end
+    #   p1 = Person[1]
+    #   p2 = Person[1]
+    #   p1.update(name: 'Jim') # works
+    #   p2.update(name: 'Bob') # raises Sequel::NoExistingObject
+    #
+    # The advantage of pg_xmin_optimistic_locking plugin compared to the
+    # regular optimistic_locking plugin as that it does not require any
+    # additional columns setup on the model.  This allows it to be loaded
+    # in the base model and have all subclasses automatically use
+    # optimistic locking.  The disadvantage is that testing can be
+    # more difficult if you are modifying the underlying row between
+    # when a model is retrieved and when it is saved.
+    #
+    # This plugin may not work with the class_table_inheritance plugin.
+    #
+    # This plugin relies on the instance_filters plugin.
+    module PgXminOptimisticLocking
+      WILDCARD = LiteralString.new('*').freeze
+      
+      # Define the xmin column accessor
+      def self.apply(model)
+        model.instance_exec do
+          plugin(:optimistic_locking_base)
+          @lock_column = :xmin
+          def_column_accessor(:xmin)
+        end
+      end
+
+      # Update the dataset to append the xmin column if it is usable
+      # and there is a dataset for the model.
+      def self.configure(model)
+        model.instance_exec do
+          set_dataset(@dataset) if @dataset
+        end
+      end
+
+      module ClassMethods
+        private
+
+        # Ensure the dataset selects the xmin column if doing so 
+        def convert_input_dataset(ds)
+          append_xmin_column_if_usable(super)
+        end
+
+        # If the xmin column is not already selected, and selecting it does not
+        # raise an error, append it to the selections.
+        def append_xmin_column_if_usable(ds)
+          select = ds.opts[:select]
+
+          unless select && select.include?(:xmin)
+            xmin_ds = ds.select_append(:xmin)
+            begin
+              columns = xmin_ds.columns!
+            rescue Sequel::DatabaseConnectionError, Sequel::DatabaseDisconnectError
+              raise
+            rescue Sequel::DatabaseError
+              # ignore, could be view, subquery, table returning function, etc.
+            else
+              ds = xmin_ds if columns.include?(:xmin)
+            end
+          end
+
+          ds
+        end
+      end
+
+      module InstanceMethods
+        private
+
+        # Only set the lock column instance filter if there is an xmin value. 
+        def lock_column_instance_filter
+          super if @values[:xmin]
+        end
+
+        # Include xmin value when inserting initial row
+        def _insert_dataset
+          super.returning(WILDCARD, :xmin)
+        end
+        
+        # Remove the xmin from the columns to update.
+        # PostgreSQL automatically updates the xmin value, and it cannot be assigned.
+        def _save_update_all_columns_hash
+          v = super
+          v.delete(:xmin)
+          v
+        end
+
+        # Add an RETURNING clause to fetch the updated xmin when updating the row.
+        def _update_without_checking(columns)
+          ds = _update_dataset
+          rows = ds.clone(ds.send(:default_server_opts, :sql=>ds.returning(:xmin).update_sql(columns))).all
+          values[:xmin] = rows.first[:xmin] unless rows.empty?
+          rows.length
+        end
+      end
+    end
+  end
+end

spec/adapters/postgres_spec.rb

@@ -5650,3 +5650,82 @@ def c2.name; 'Bar' end
     @m1.all.must_equal [{:i1=>1, :a=>3}]
   end
 end if DB.server_version >= 150000
+
+describe "pg_xmin_optimistic_locking plugin" do
+  before do
+    @db = DB
+    @db.create_table! :items do
+      primary_key :id
+      String :name, :size => 20
+    end
+   end
+  after do
+    @db.drop_table?(:items)
+  end
+
+  it "should not allow stale updates" do
+    c = Class.new(Sequel::Model(:items))
+    c.plugin :pg_xmin_optimistic_locking
+    o = c.create(:name=>'test')
+    o2 = c.first
+    xmin = c.dataset.naked.get(:xmin)
+    xmin.wont_equal nil
+    xmin.must_equal o.xmin
+    xmin.must_equal o2.xmin
+    o.name = 'test2'
+    o.save
+    xmin.wont_equal o.xmin
+    xmin.wont_equal c.dataset.naked.get(:xmin)
+    proc{o2.save}.must_raise(Sequel::NoExistingObject)
+  end
+
+  it "should work with subclasses" do
+    c = Class.new(Sequel::Model)
+    c.plugin :pg_xmin_optimistic_locking
+    sc = c::Model(:items)
+    o = sc.create(:name=>'test')
+    o2 = sc.first
+    xmin = sc.dataset.naked.get(:xmin)
+    xmin.wont_equal nil
+    xmin.must_equal o.xmin
+    xmin.must_equal o2.xmin
+    o.name = 'test2'
+    o.save
+    xmin.wont_equal o.xmin
+    xmin.wont_equal sc.dataset.naked.get(:xmin)
+    proc{o2.save}.must_raise(Sequel::NoExistingObject)
+  end
+
+  it "should allow updates when xmin is not selected" do
+    c = Class.new(Sequel::Model(:items))
+    c.plugin :pg_xmin_optimistic_locking
+    c.create(:name=>'test')
+    o = c.select_all(:items).first
+    o2 = c.first
+    o.xmin.must_be_nil
+    o.name = 'test2'
+    o.save
+    o.xmin.wont_be_nil
+    o.xmin.must_equal c.dataset.naked.get(:xmin)
+    proc{o2.save}.must_raise(Sequel::NoExistingObject)
+  end
+
+  it "should not select xmin when selecting from subquery" do
+    c = Class.new(Sequel::Model(@db[:items].from_self))
+    c.plugin :pg_xmin_optimistic_locking
+    @db[:items].insert(:name=>'test')
+    c.first.values[:xmin].must_be_nil
+  end
+
+  it "should not select xmin when selecting from view" do
+    begin
+      @db.create_view(:items_view, @db[:items])
+      c = Class.new(Sequel::Model(:items_view))
+      c.plugin :pg_xmin_optimistic_locking
+      @db[:items].insert(:name=>'test')
+      c.first.values[:xmin].must_be_nil
+    ensure
+      @db.drop_view(:items_view)
+    end
+  end
+end