TCPServer.Contexts.LockList : is it safe to do that?-CodePudding

Until now, I was calling the following function every second :

function TForm1.ListeConnecteMaj():Boolean;
var
    i : integer;
    List : TIdContextList;
    Client : TSimpleClient;
begin
  List := TCPServer1.Contexts.LockList;
  try
    NbSConnect := List.Count;
    for i := 0 to List.Count -1 do
      begin
          Client := List[i];
          .....   // getting information from Client
      end;
  finally
    TCPServeur1.Contexts.UnlockList;
  end;
end;

As I need to support hundreds of simultaneaous connections, I want to reduce as much as possible the duration of the LockList. I've tried this. It works but is it really safe ?

function TForm1.ListeConnecteMaj():Boolean;
var
    i : integer;
    List : TIdContextList;
    Client : TSimpleClient;
begin
  List := TCPServer.Contexts.LockList;
  TCPServeur.Contexts.UnlockList;
  try
    NbSConnect := List.Count;
    for i := 0 to List.Count -1 do
      begin
          Client := List[i];
          .....   // getting information from Client
      end;
  finally
  end;
end;

CodePudding user response：

It works but is it really safe?

No, it is not thread-safe.

You can safely use List reference acquired by LockList until you call UnlockList. The moment you call UnlockList, list will no longer be protected and any List access after that point can cause concurrency issues.

Your original code is the proper way to use LockList/UnlockList.

CodePudding user response：

As @DalijaPrasnikar's answer explains, your proposal is not safe, no. As soon as the list is unlocked, the server is able to freely modify the contents of the list as clients connect and disconnect, which will cause concurrency issues with your code.

I would suggest a different approach - rather than polling the client list every second, I would use the OnConnect and OnDisconnect events to add/remove elements from your UI as needed whenever the client list changes. If you are trying to track status updates per client during socket operations, you can post notifications to the main UI thread as they happen.