FIX: Support tag query param on /tag/{name} routes (#20742)

app/controllers/tags_controller.rb

@@ -545,6 +545,6 @@ class TagsController < ::ApplicationController
   end
 
   def tag_params
-    [@tag_id].concat(Array(@additional_tags))
+    Array(params[:tags]).concat(Array(@additional_tags))
   end
 end

lib/topic_query_params.rb

@@ -3,7 +3,7 @@
 module TopicQueryParams
   def build_topic_list_options
     options = {}
-    params[:tags] = [params[:tag_id]] if params[:tag_id].present? && guardian.can_tag_pms?
+    params[:tags] = [params[:tag_id], *Array(params[:tags])].uniq if params[:tag_id].present?
 
     TopicQuery.public_valid_options.each do |key|
       if params.key?(key)

spec/requests/tags_controller_spec.rb

@@ -434,6 +434,28 @@ RSpec.describe TagsController do
       expect(response.status).to eq(200)
     end
 
+    it "can handle additional tags in query params" do
+      tag2 = Fabricate(:tag)
+      topic_with_two_tags = Fabricate(:topic, tags: [tag, tag2])
+
+      get "/tag/test.json?match_all_tags=true&tags[]=#{tag2.name}"
+      expect(response.status).to eq(200)
+      expect(response.parsed_body["topic_list"]["topics"].map { |t| t["id"] }).to contain_exactly(
+        topic_with_two_tags.id,
+      )
+    end
+
+    it "can handle duplicate tags in query params" do
+      tag2 = Fabricate(:tag)
+      topic_with_two_tags = Fabricate(:topic, tags: [tag, tag2])
+
+      get "/tag/test.json?match_all_tags=true&tags[]=test&tags[]=#{tag2.name}"
+      expect(response.status).to eq(200)
+      expect(response.parsed_body["topic_list"]["topics"].map { |t| t["id"] }).to contain_exactly(
+        topic_with_two_tags.id,
+      )
+    end
+
     it "handles special tag 'none'" do
       SiteSetting.pm_tags_allowed_for_groups = "1|2|3"