Introduce Actionable Errors

Actionable errors let's you dispatch actions from Rails' error pages. This
can help you save time if you have a clear action for the resolution of
common development errors.

The de-facto example are pending migrations. Every time pending migrations
are found, a middleware raises an error. With actionable errors, you can
run the migrations right from the error page. Other examples include Rails
plugins that need to run a rake task to setup themselves. They can now
raise actionable errors to run the setup straight from the error pages.

Here is how to define an actionable error:

```ruby
class PendingMigrationError < MigrationError #:nodoc:
  include ActiveSupport::ActionableError

  action "Run pending migrations" do
    ActiveRecord::Tasks::DatabaseTasks.migrate
  end
end
```

To make an error actionable, include the `ActiveSupport::ActionableError`
module and invoke the `action` class macro to define the action. An action
needs a name and a procedure to execute. The name is shown as the name of a
button on the error pages. Once clicked, it will invoke the given
procedure.

1 files changed, 57 insertions, 0 deletions

diff --git a/activesupport/test/actionable_error_test.rb b/activesupport/test/actionable_error_test.rb
new file mode 100644
index 0000000000..66ba94e0dd
--- /dev/null
+++ b/activesupport/test/actionable_error_test.rb
@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+require "abstract_unit"
+require "active_support/actionable_error"
+
+class ActionableErrorTest < ActiveSupport::TestCase
+  class NonActionableError < StandardError
+  end
+
+  class DispatchableError < StandardError
+    include ActiveSupport::ActionableError
+
+    class_attribute :flip1, default: false
+    class_attribute :flip2, default: false
+
+    action "Flip 1" do
+      self.flip1 = true
+    end
+
+    action "Flip 2" do
+      self.flip2 = true
+    end
+  end
+
+  test "can get all action of an actionable error" do
+    assert_equal ["Flip 1", "Flip 2"], ActiveSupport::ActionableError.actions(DispatchableError).keys
+    assert_equal ["Flip 1", "Flip 2"], ActiveSupport::ActionableError.actions(DispatchableError.new).keys
+  end
+
+  test "cannot get actions from non-actionable errors" do
+    assert_raises ActiveSupport::ActionableError::NonActionable do
+      ActiveSupport::ActionableError.actions(NonActionableError)
+    end
+  end
+
+  test "dispatches actions from class and a label" do
+    assert_changes "DispatchableError.flip1", from: false, to: true do
+      ActiveSupport::ActionableError.dispatch DispatchableError, "Flip 1"
+    end
+  end
+
+  test "dispatches actions from class name and a label" do
+    assert_changes "DispatchableError.flip2", from: false, to: true do
+      ActiveSupport::ActionableError.dispatch DispatchableError.name, "Flip 2"
+    end
+  end
+
+  test "cannot dispatch errors that do not include ActiveSupport::ActionableError" do
+    err = assert_raises ActiveSupport::ActionableError::NonActionable do
+      ActiveSupport::ActionableError.dispatch NonActionableError, "action"
+    end
+
+    assert_equal <<~EXPECTED.chop, err.to_s
+      ActionableErrorTest::NonActionableError is non-actionable
+    EXPECTED
+  end
+end