DEV: Support adding a custom filter on `/filter` (#27927)

# Context

Currently there is no way to add a custom filter to the experimental `/filter` endpoint. While you can implement a custom `status:` there is no way to include the user's input in a custom query. 

# PR

This PR adds the ability to implement a custom filter. eg. `CUSTOM_FILTER:foo`

- Add `add_filter_custom_filter` for extension
- Add specs

lib/discourse_plugin_registry.rb

@@ -127,6 +127,8 @@ class DiscoursePluginRegistry
 
   define_filtered_register :flag_applies_to_types
 
+  define_filtered_register :custom_filter_mappings
+
   def self.register_auth_provider(auth_provider)
     self.auth_providers << auth_provider
   end

lib/plugin/instance.rb

@@ -239,6 +239,17 @@ class Plugin::Instance
     DiscoursePluginRegistry.register_editable_group_custom_field(field, self)
   end
 
+  # Allows to define custom filter utilizing the user's input.
+  # Ensure proper input sanitization before using it in a query.
+  #
+  # Example usage:
+  #   add_filter_custom_filter("word_count") do |scope, value|
+  #     scope.where(word_count: value)
+  #   end
+  def add_filter_custom_filter(name, &block)
+    DiscoursePluginRegistry.register_custom_filter_mapping({ name => block }, self)
+  end
+
   # Allows to define custom "status:" filter. Example usage:
   #   register_custom_filter_by_status("foobar") do |scope|
   #     scope.where("word_count = 42")

lib/topics_filter.rb

@@ -82,6 +82,11 @@ class TopicsFilter
         filter_by_number_of_views(min: filter_values)
       when "views-max"
         filter_by_number_of_views(max: filter_values)
+      else
+        if custom_filter =
+             DiscoursePluginRegistry.custom_filter_mappings.find { |hash| hash.key?(filter) }
+          @scope = custom_filter[filter].call(@scope, filter_values)
+        end
       end
     end
 

spec/lib/topics_filter_spec.rb

@@ -212,6 +212,43 @@ RSpec.describe TopicsFilter do
       end
     end
 
+    describe "when filtering with custom filters" do
+      fab!(:topic)
+      fab!(:word_count_topic) { Fabricate(:topic, word_count: 42) }
+      fab!(:word_count_topic_2) { Fabricate(:topic, word_count: 42) }
+
+      let(:word_count_block) { Proc.new { |scope, value| scope.where(word_count: value) } }
+      let(:id_block) { Proc.new { |scope, value| scope.where(id: value) } }
+      let(:plugin) { Plugin::Instance.new }
+
+      it "supports a custom filter" do
+        plugin.add_filter_custom_filter("word_count", &word_count_block)
+
+        expect(
+          TopicsFilter
+            .new(guardian: Guardian.new)
+            .filter_from_query_string("word_count:42")
+            .pluck(:id),
+        ).to contain_exactly(word_count_topic.id, word_count_topic_2.id)
+      ensure
+        DiscoursePluginRegistry.reset_register!(:custom_filter_mappings)
+      end
+
+      it "supports multiple custom filters" do
+        plugin.add_filter_custom_filter("word_count", &word_count_block)
+        plugin.add_filter_custom_filter("id", &id_block)
+
+        expect(
+          TopicsFilter
+            .new(guardian: Guardian.new)
+            .filter_from_query_string("word_count:42 id:#{word_count_topic.id}")
+            .pluck(:id),
+        ).to contain_exactly(word_count_topic.id)
+      ensure
+        DiscoursePluginRegistry.reset_register!(:custom_filter_mappings)
+      end
+    end
+
     describe "when filtering by categories" do
       fab!(:category) { Fabricate(:category, name: "category") }