Add more user-friendly message in case of hitting against runner registration limit

app/controllers/projects/runner_projects_controller.rb

@@ -17,7 +17,10 @@ class Projects::RunnerProjectsController < Projects::ApplicationController
     if @runner.assign_to(project, current_user)
       redirect_to path
     else
-      redirect_to path, alert: 'Failed adding runner to project'
+      assign_to_messages = @runner.errors.messages[:assign_to]
+      alert = assign_to_messages&.join(',') || 'Failed adding runner to project'
+
+      redirect_to path, alert: alert
     end
   end
 

app/models/ci/runner.rb

@@ -46,9 +46,9 @@ module Ci
     MINUTES_COST_FACTOR_FIELDS = %i[public_projects_minutes_cost_factor private_projects_minutes_cost_factor].freeze
 
     has_many :builds
-    has_many :runner_projects, inverse_of: :runner, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
+    has_many :runner_projects, inverse_of: :runner, autosave: true, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
     has_many :projects, through: :runner_projects
-    has_many :runner_namespaces, inverse_of: :runner
+    has_many :runner_namespaces, inverse_of: :runner, autosave: true
     has_many :groups, through: :runner_namespaces
 
     has_one :last_build, -> { order('id DESC') }, class_name: 'Ci::Build'

spec/requests/api/ci/runner/runners_post_spec.rb

@@ -102,7 +102,7 @@ RSpec.describe API::Ci::Runner, :clean_gitlab_redis_shared_state do
               request
 
               expect(response).to have_gitlab_http_status(:bad_request)
-              expect(json_response['message']).to include('runner_projects' => ['is invalid'])
+              expect(json_response['message']).to include('runner_projects.base' => ['Maximum number of ci registered project runners (1) exceeded'])
               expect(project.runners.reload.size).to eq(1)
             end
           end
@@ -143,7 +143,7 @@ RSpec.describe API::Ci::Runner, :clean_gitlab_redis_shared_state do
               request
 
               expect(response).to have_gitlab_http_status(:bad_request)
-              expect(json_response['message']).to include('runner_namespaces' => ['is invalid'])
+              expect(json_response['message']).to include('runner_namespaces.base' => ['Maximum number of ci registered group runners (1) exceeded'])
               expect(group.runners.reload.size).to eq(1)
             end
           end