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.

2 files changed, 54 insertions, 0 deletions

diff --git a/activesupport/lib/active_support.rb b/activesupport/lib/active_support.rb
index 5589c71281..40a9181493 100644
--- a/activesupport/lib/active_support.rb
+++ b/activesupport/lib/active_support.rb
@@ -33,6 +33,7 @@ require "active_support/core_ext/date_and_time/compatibility"
 module ActiveSupport
   extend ActiveSupport::Autoload
 
+  autoload :ActionableError
   autoload :Concern
   autoload :CurrentAttributes
   autoload :Dependencies
diff --git a/activesupport/lib/active_support/actionable_error.rb b/activesupport/lib/active_support/actionable_error.rb
new file mode 100644
index 0000000000..aeee2177aa
--- /dev/null
+++ b/activesupport/lib/active_support/actionable_error.rb
@@ -0,0 +1,53 @@
+# frozen_string_literal: true
+
+require "active_support/concern"
+
+module ActiveSupport
+  # Actionable errors let's you define actions to resolve an error.
+  #
+  # To make an error actionable, include the <tt>ActiveSupport::ActionableError</tt>
+  # module and invoke the +action+ class macro to define the action.
+  #
+  # An action needs a name and a procedure to execute. The name can be shown by
+  # the action dispatching mechanism.
+  module ActionableError
+    extend Concern
+
+    NonActionable = Class.new(StandardError)
+
+    included do
+      class_attribute :_actions, default: Hash.new do |_, label|
+        raise NonActionable, "Cannot find action \"#{label}\" for #{self}"
+      end
+    end
+
+    def self.===(other) # :nodoc:
+      super || Module === other && other.ancestors.include?(self)
+    end
+
+    def self.actions(error) # :nodoc:
+      error = error.constantize if String === error
+      raise NonActionable, "#{error.name} is non-actionable" unless self === error
+      error._actions
+    end
+
+    def self.dispatch(error, label) # :nodoc:
+      actions(error)[label].call
+    end
+
+    module ClassMethods
+      # Defines an action that can resolve the error.
+      #
+      #   class PendingMigrationError < MigrationError
+      #     include ActiveSupport::ActionableError
+      #
+      #     action "Run pending migrations" do
+      #       ActiveRecord::Tasks::DatabaseTasks.migrate
+      #     end
+      #   end
+      def action(label, &block)
+        _actions[label] = block
+      end
+    end
+  end
+end