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, 60 insertions, 0 deletions

diff --git a/actionpack/test/dispatch/actionable_exceptions_test.rb b/actionpack/test/dispatch/actionable_exceptions_test.rb
new file mode 100644
index 0000000000..46979ed878
--- /dev/null
+++ b/actionpack/test/dispatch/actionable_exceptions_test.rb
@@ -0,0 +1,60 @@
+# frozen_string_literal: true
+
+require "abstract_unit"
+
+class ActionableExceptionsTest < ActionDispatch::IntegrationTest
+  Actions = []
+
+  class ActionError < StandardError
+    include ActiveSupport::ActionableError
+
+    action "Successful action" do
+      Actions << "Action!"
+    end
+
+    action "Failed action" do
+      raise "Inaction!"
+    end
+  end
+
+  Noop = -> env { [200, {}, [""]] }
+
+  setup do
+    @app = ActionDispatch::ActionableExceptions.new(Noop)
+
+    Actions.clear
+  end
+
+  test "dispatches an actionable error" do
+    post ActionDispatch::ActionableExceptions.endpoint, params: {
+      error: ActionError.name,
+      action: "Successful action",
+      location: "/",
+    }
+
+    assert_equal ["Action!"], Actions
+
+    assert_equal 302, response.status
+    assert_equal "/", response.headers["Location"]
+  end
+
+  test "dispatched action can fail" do
+    assert_raise RuntimeError do
+      post ActionDispatch::ActionableExceptions.endpoint, params: {
+        error: ActionError.name,
+        action: "Failed action",
+        location: "/",
+      }
+    end
+  end
+
+  test "cannot dispatch non-actionable errors" do
+    assert_raise ActiveSupport::ActionableError::NonActionable do
+      post ActionDispatch::ActionableExceptions.endpoint, params: {
+        error: RuntimeError.name,
+        action: "Inexistent action",
+        location: "/",
+      }
+    end
+  end
+end