FEATURE: Allow searching users using a list of usernames (#23902)

app/controllers/users_controller.rb

@@ -1203,7 +1203,10 @@ class UsersController < ApplicationController
   SEARCH_USERS_LIMIT = 50
 
   def search_users
+    # the search can specify the parameter term or usernames, term will perform the classic user search algorithm while
+    # usernames will perform an exact search on the usernames passed as parameter
     term = params[:term].to_s.strip
+    usernames = params[:usernames]&.split(",")&.map { |username| username.downcase.strip }
 
     topic_id = params[:topic_id].to_i if params[:topic_id].present?
     category_id = params[:category_id].to_i if params[:category_id].present?
@@ -1232,13 +1235,18 @@ class UsersController < ApplicationController
     options[:topic_id] = topic_id if topic_id
     options[:category_id] = category_id if category_id
 
-    results = UserSearch.new(term, options).search
+    results =
+      if usernames.blank?
+        UserSearch.new(term, options).search
+      else
+        User.where(username_lower: usernames).limit(limit)
+      end
     to_render = serialize_found_users(results)
 
     # blank term is only handy for in-topic search of users after @
     # we do not want group results ever if term is blank
     groups =
-      if term.present? && current_user
+      if (term.present? || usernames.present?) && current_user
         if params[:include_groups] == "true"
           Group.visible_groups(current_user)
         elsif params[:include_mentionable_groups] == "true"
@@ -1260,7 +1268,12 @@ class UsersController < ApplicationController
       # register_modifier(:groups_for_users_search) will be evaluated without needing the
       # param.
       groups = DiscoursePluginRegistry.apply_modifier(:groups_for_users_search, groups)
-      groups = Group.search_groups(term, groups: groups, sort: :auto)
+      groups =
+        if usernames.blank?
+          Group.search_groups(term, groups: groups, sort: :auto)
+        else
+          groups.where(name: usernames).limit(limit)
+        end
 
       to_render[:groups] = groups.map { |m| { name: m.name, full_name: m.full_name } }
     end

spec/requests/users_controller_spec.rb

@@ -4901,6 +4901,35 @@ RSpec.describe UsersController do
       expect(json["users"].map { |u| u["username"] }).to include(user.username)
     end
 
+    context "when searching usernames" do
+      it "searches when provided a list of usernames" do
+        users = Fabricate.times(3, :user)
+
+        get "/u/search/users.json", params: { usernames: users.map(&:username).join(",") }
+        expect(response.status).to eq(200)
+        json = response.parsed_body
+        expect(json["users"].map { |u| u["username"] }).to match_array(users.map(&:username))
+      end
+
+      it "searches groups if include_groups = true" do
+        users = Fabricate.times(3, :user)
+        group = Fabricate(:group)
+
+        sign_in(user)
+
+        get "/u/search/users.json",
+            params: {
+              usernames: [group.name, users.first.username].join(","),
+              include_groups: true,
+            }
+        expect(response.status).to eq(200)
+        json = response.parsed_body
+
+        expect(json["users"].map { |u| u["username"] }).to contain_exactly(users.first.username)
+        expect(json["groups"].map { |u| u["name"] }).to contain_exactly(group.name)
+      end
+    end
+
     it "searches when provided the topic only" do
       get "/u/search/users.json", params: { topic_id: topic.id }
       expect(response.status).to eq(200)