Raise `ActionController::Redirecting::UnsafeRedirectError` for unsafe `redirect_to` redirects.

This allows `rescue_from` to be used to add a default fallback route:

```ruby
rescue_from ActionController::Redirecting::UnsafeRedirectError do
  redirect_to root_url
end
```

Co-Authored-By: Chris Oliver <excid3@gmail.com>

actionpack/CHANGELOG.md

@@ -1,3 +1,15 @@
+*   Raise `ActionController::Redirecting::UnsafeRedirectError` for unsafe `redirect_to` redirects.
+
+    This allows `rescue_from` to be used to add a default fallback route:
+
+    ```ruby
+    rescue_from ActionController::Redirecting::UnsafeRedirectError do
+      redirect_to root_url
+    end
+    ```
+
+    *Kasper Timm Hansen*, *Chris Oliver*
+
 *   Add `url_from` to verify a redirect location is internal.
 
     Takes the open redirect protection from `redirect_to` so users can wrap a

actionpack/lib/action_controller/metal/redirecting.rb

@@ -7,6 +7,8 @@ module ActionController
     include AbstractController::Logger
     include ActionController::UrlFor
 
+    class UnsafeRedirectError < StandardError; end
+
     included do
       mattr_accessor :raise_on_open_redirects, default: false
     end
@@ -70,6 +72,8 @@ module ActionController
     #
     #   redirect_to params[:redirect_url]
     #
+    # Raises <tt>ActionController::Redirecting::UnsafeRedirectError</tt> in the case of an unsafe redirect.
+    #
     # To allow any external redirects pass `allow_other_host: true`, though using a user-provided param in that case is unsafe.
     #
     #   redirect_to "https://rubyonrails.org", allow_other_host: true
@@ -186,7 +190,7 @@ module ActionController
         if allow_other_host || _url_host_allowed?(location)
           location
         else
-          raise ArgumentError, "Unsafe redirect to #{location.truncate(100).inspect}, pass allow_other_host: true to redirect anyway."
+          raise UnsafeRedirectError, "Unsafe redirect to #{location.truncate(100).inspect}, pass allow_other_host: true to redirect anyway."
         end
       end
 

actionpack/test/controller/redirect_test.rb

@@ -482,7 +482,7 @@ class RedirectTest < ActionController::TestCase
 
   def test_unsafe_redirect
     with_raise_on_open_redirects do
-      error = assert_raise(ArgumentError) do
+      error = assert_raise(ActionController::Redirecting::UnsafeRedirectError) do
         get :unsafe_redirect
       end
 
@@ -492,7 +492,7 @@ class RedirectTest < ActionController::TestCase
 
   def test_unsafe_redirect_back
     with_raise_on_open_redirects do
-      error = assert_raise(ArgumentError) do
+      error = assert_raise(ActionController::Redirecting::UnsafeRedirectError) do
         get :unsafe_redirect_back
       end