1 files changed, 144 insertions, 0 deletions
diff --git a/activerecord/test/cases/hot_compatibility_test.rb b/activerecord/test/cases/hot_compatibility_test.rb
new file mode 100644
index 0000000000..e7778af55b
--- /dev/null
+++ b/activerecord/test/cases/hot_compatibility_test.rb
@@ -0,0 +1,144 @@
+# frozen_string_literal: true
+
+require "cases/helper"
+require "support/connection_helper"
+
+class HotCompatibilityTest < ActiveRecord::TestCase
+  self.use_transactional_tests = false
+  include ConnectionHelper
+
+  setup do
+    @klass = Class.new(ActiveRecord::Base) do
+      connection.create_table :hot_compatibilities, force: true do |t|
+        t.string :foo
+        t.string :bar
+      end
+
+      def self.name; "HotCompatibility"; end
+    end
+  end
+
+  teardown do
+    ActiveRecord::Base.connection.drop_table :hot_compatibilities
+  end
+
+  test "insert after remove_column" do
+    # warm cache
+    @klass.create!
+
+    # we have 3 columns
+    assert_equal 3, @klass.columns.length
+
+    # remove one of them
+    @klass.connection.remove_column :hot_compatibilities, :bar
+
+    # we still have 3 columns in the cache
+    assert_equal 3, @klass.columns.length
+
+    # but we can successfully create a record so long as we don't
+    # reference the removed column
+    record = @klass.create! foo: "foo"
+    record.reload
+    assert_equal "foo", record.foo
+  end
+
+  test "update after remove_column" do
+    record = @klass.create! foo: "foo"
+    assert_equal 3, @klass.columns.length
+    @klass.connection.remove_column :hot_compatibilities, :bar
+    assert_equal 3, @klass.columns.length
+
+    record.reload
+    assert_equal "foo", record.foo
+    record.foo = "bar"
+    record.save!
+    record.reload
+    assert_equal "bar", record.foo
+  end
+
+  if current_adapter?(:PostgreSQLAdapter)
+    test "cleans up after prepared statement failure in a transaction" do
+      with_two_connections do |original_connection, ddl_connection|
+        record = @klass.create! bar: "bar"
+
+        # prepare the reload statement in a transaction
+        @klass.transaction do
+          record.reload
+        end
+
+        assert get_prepared_statement_cache(@klass.connection).any?,
+          "expected prepared statement cache to have something in it"
+
+        # add a new column
+        ddl_connection.add_column :hot_compatibilities, :baz, :string
+
+        assert_raise(ActiveRecord::PreparedStatementCacheExpired) do
+          @klass.transaction do
+            record.reload
+          end
+        end
+
+        assert_empty get_prepared_statement_cache(@klass.connection),
+          "expected prepared statement cache to be empty but it wasn't"
+      end
+    end
+
+    test "cleans up after prepared statement failure in nested transactions" do
+      with_two_connections do |original_connection, ddl_connection|
+        record = @klass.create! bar: "bar"
+
+        # prepare the reload statement in a transaction
+        @klass.transaction do
+          record.reload
+        end
+
+        assert get_prepared_statement_cache(@klass.connection).any?,
+          "expected prepared statement cache to have something in it"
+
+        # add a new column
+        ddl_connection.add_column :hot_compatibilities, :baz, :string
+
+        assert_raise(ActiveRecord::PreparedStatementCacheExpired) do
+          @klass.transaction do
+            @klass.transaction do
+              @klass.transaction do
+                record.reload
+              end
+            end
+          end
+        end
+
+        assert_empty get_prepared_statement_cache(@klass.connection),
+          "expected prepared statement cache to be empty but it wasn't"
+      end
+    end
+  end
+
+  private
+
+    def get_prepared_statement_cache(connection)
+      connection.instance_variable_get(:@statements)
+        .instance_variable_get(:@cache)[Process.pid]
+    end
+
+    # Rails will automatically clear the prepared statements on the connection
+    # that runs the migration, so we use two connections to simulate what would
+    # actually happen on a production system; we'd have one connection running the
+    # migration from the rake task ("ddl_connection" here), and we'd have another
+    # connection in a web worker.
+    def with_two_connections
+      run_without_connection do |original_connection|
+        ActiveRecord::Base.establish_connection(original_connection.merge(pool_size: 2))
+        begin
+          ddl_connection = ActiveRecord::Base.connection_pool.checkout
+          begin
+            yield original_connection, ddl_connection
+          ensure
+            ActiveRecord::Base.connection_pool.checkin ddl_connection
+          end
+        ensure
+          ActiveRecord::Base.clear_all_connections!
+        end
+      end
+    end
+end