FIX: Ensure MessageIdService can handle hostname changes and multisite (#15231)

lib/email/message_id_service.rb

@@ -80,11 +80,11 @@ module Email
       end
 
       def message_id_post_id_regexp
-        @message_id_post_id_regexp ||= Regexp.new "topic/\\d+/(\\d+|\\d+\.\\w+)@#{Regexp.escape(host)}"
+        Regexp.new "topic/\\d+/(\\d+|\\d+\.\\w+)@#{Regexp.escape(host)}"
       end
 
       def message_id_topic_id_regexp
-        @message_id_topic_id_regexp ||= Regexp.new "topic/(\\d+|\\d+\.\\w+)@#{Regexp.escape(host)}"
+        Regexp.new "topic/(\\d+|\\d+\.\\w+)@#{Regexp.escape(host)}"
       end
 
       def message_id_rfc_format(message_id)
@@ -100,7 +100,7 @@ module Email
       end
 
       def host
-        @host ||= Email::Sender.host_for(Discourse.base_url)
+        Email::Sender.host_for(Discourse.base_url)
       end
     end
   end

spec/lib/message_id_service_spec.rb

@@ -120,4 +120,12 @@ describe Email::MessageIdService do
       expect(Email::MessageIdService.message_id_clean(message_id)).to eq(message_id)
     end
   end
+
+  describe "#host" do
+    it "handles hostname changes at runtime" do
+      expect(Email::MessageIdService.host).to eq("test.localhost")
+      SiteSetting.force_hostname = "other.domain.example.com"
+      expect(Email::MessageIdService.host).to eq("other.domain.example.com")
+    end
+  end
 end