FIX: Perform better email validation (#12497)

Using UserEmail for validation is not sufficient because it checks the
emails of staged users too.

app/controllers/users_controller.rb

@@ -524,20 +524,38 @@ class UsersController < ApplicationController
   end
 
   def check_email
+    begin
       RateLimiter.new(nil, "check-email-#{request.remote_ip}", 10, 1.minute).performed!
-
-    if SiteSetting.hide_email_address_taken?
+    rescue RateLimiter::LimitExceeded
       return render json: success_json
     end
 
-    user_email = UserEmail.new(email: params[:email])
+    email = Email.downcase((params[:email] || "").strip)
 
-    if user_email.valid?
-      render json: success_json
-    else
-      render json: failed_json.merge(errors: user_email.errors.full_messages)
+    if email.blank? || SiteSetting.hide_email_address_taken?
+      return render json: success_json
     end
-  rescue RateLimiter::LimitExceeded
+
+    if !(email =~ EmailValidator.email_regex)
+      error = User.new.errors.full_message(:email, I18n.t(:'user.email.invalid'))
+      return render json: failed_json.merge(errors: [error])
+    end
+
+    if !EmailValidator.allowed?(email)
+      error = User.new.errors.full_message(:email, I18n.t(:'user.email.not_allowed'))
+      return render json: failed_json.merge(errors: [error])
+    end
+
+    if ScreenedEmail.should_block?(email)
+      error = User.new.errors.full_message(:email, I18n.t(:'user.email.blocked'))
+      return render json: failed_json.merge(errors: [error])
+    end
+
+    if User.where(staged: false).find_by_email(email).present?
+      error = User.new.errors.full_message(:email, I18n.t(:'errors.messages.taken'))
+      return render json: failed_json.merge(errors: [error])
+    end
+
     render json: success_json
   end
 

spec/requests/users_controller_spec.rb

@@ -1592,6 +1592,11 @@ describe UsersController do
       expect(response.parsed_body["success"]).to be_present
     end
 
+    it 'returns success if email is empty' do
+      get "/u/check_email.json"
+      expect(response.parsed_body["success"]).to be_present
+    end
+
     it 'returns failure if email is not valid' do
       get "/u/check_email.json", params: { email: "invalid" }
       expect(response.parsed_body["failed"]).to be_present
@@ -1600,12 +1605,20 @@ describe UsersController do
     it 'returns failure if email exists' do
       get "/u/check_email.json", params: { email: user.email }
       expect(response.parsed_body["failed"]).to be_present
+
+      get "/u/check_email.json", params: { email: user.email.upcase }
+      expect(response.parsed_body["failed"]).to be_present
     end
 
     it 'returns success if email does not exists' do
       get "/u/check_email.json", params: { email: "available@example.com" }
       expect(response.parsed_body["success"]).to be_present
     end
+
+    it 'return success if user email is taken by staged user' do
+      get "/u/check_email.json", params: { email: Fabricate(:staged).email }
+      expect(response.parsed_body["success"]).to be_present
+    end
   end
 
   describe '#invited' do