FEATURE: Show live user status on inline mentions on posts (#18683)

Note that we don't have a database table and a model for post mentions yet, and I decided to implement it without adding one to avoid heavy data migrations. Still, we may want to add such a model later, that would be convenient, we have such a model for mentions in chat.

Note that status appears on all mentions on all posts in a topic except of the case when you just posted a new post, and it appeared on the bottom of the topic. On such posts, status won't be shown immediately for now (you'll need to reload the page to see the status). I'll take care of it in one of the following PRs.

spec/requests/api/schemas/json/topic_create_response.json

@@ -200,6 +200,11 @@
     },
     "reviewable_score_pending_count": {
       "type": "integer"
+    },
+    "mentioned_users": {
+      "type": "array",
+      "items": {
+      }
     }
   },
   "required": [

spec/requests/topics_controller_spec.rb

@@ -2684,6 +2684,91 @@ RSpec.describe TopicsController do
       expect(response.status).to eq(200)
     end
 
+    context "with mentions" do
+      fab!(:post) { Fabricate(:post, user: post_author1) }
+      fab!(:topic) { post.topic }
+      fab!(:post2) { Fabricate(
+        :post,
+        user: post_author2,
+        topic: topic,
+        raw: "I am mentioning @#{post_author1.username}."
+      ) }
+
+      it "returns mentions" do
+        get "/t/#{topic.slug}/#{topic.id}.json"
+
+        expect(response.status).to eq(200)
+
+        json = response.parsed_body
+        expect(json["post_stream"]["posts"][1]["mentioned_users"].length).to be(1)
+
+        mentioned_user = json["post_stream"]["posts"][1]["mentioned_users"][0]
+        expect(mentioned_user["id"]).to be(post_author1.id)
+        expect(mentioned_user["name"]).to eq(post_author1.name)
+        expect(mentioned_user["username"]).to eq(post_author1.username)
+      end
+
+      it "doesn't return status on mentions by default" do
+        post_author1.set_status!("off to dentist", "tooth")
+
+        get "/t/#{topic.slug}/#{topic.id}.json"
+
+        expect(response.status).to eq(200)
+
+        json = response.parsed_body
+        expect(json["post_stream"]["posts"][1]["mentioned_users"].length).to be(1)
+        status = json["post_stream"]["posts"][1]["mentioned_users"][0]["status"]
+        expect(status).to be_nil
+      end
+
+      it "returns mentions with status if user status is enabled" do
+        SiteSetting.enable_user_status = true
+        post_author1.set_status!("off to dentist", "tooth")
+
+        get "/t/#{topic.slug}/#{topic.id}.json"
+
+        expect(response.status).to eq(200)
+
+        json = response.parsed_body
+        expect(json["post_stream"]["posts"][1]["mentioned_users"].length).to be(1)
+
+        status = json["post_stream"]["posts"][1]["mentioned_users"][0]["status"]
+        expect(status).to be_present
+        expect(status["emoji"]).to eq(post_author1.user_status.emoji)
+        expect(status["description"]).to eq(post_author1.user_status.description)
+      end
+
+      it "returns an empty list of mentioned users if there is no mentions in a post" do
+        Fabricate(
+          :post,
+          user: post_author2,
+          topic: topic,
+          raw: "Post without mentions.")
+
+        get "/t/#{topic.slug}/#{topic.id}.json"
+
+        expect(response.status).to eq(200)
+
+        json = response.parsed_body
+        expect(json["post_stream"]["posts"][2]["mentioned_users"].length).to be(0)
+      end
+
+      it "returns an empty list of mentioned users if an unexisting user was mentioned" do
+        Fabricate(
+          :post,
+          user: post_author2,
+          topic: topic,
+          raw: "Mentioning an @unexisting_user.")
+
+        get "/t/#{topic.slug}/#{topic.id}.json"
+
+        expect(response.status).to eq(200)
+
+        json = response.parsed_body
+        expect(json["post_stream"]["posts"][2]["mentioned_users"].length).to be(0)
+      end
+    end
+
     describe "has_escaped_fragment?" do
       context "when the SiteSetting is disabled" do
         it "uses the application layout even with an escaped fragment param" do