CODEWITHSUNDEEP
delphidelphi-2010indy10
Greg Ory
Greg Ory

Reputation: 21

IdTCPServer does not release clients who disconnect abruptly

I tested a simulation disconnect of multiple clients by cutting their internet connection. I found that TIdTCPServer did not discharge their threads, it did not detect their disconnect. By comparison, when I closed a client manually, the server detected the disconnection and discharged its thread.

Upvotes: 1

Views: 1322

Answers (1)

Remy Lebeau
Remy Lebeau

Reputation: 596397

Abnormal disconnects are not detected by the OS in a timely manner. It can take a considerable amount of time for a lost socket connection to timeout internally so the OS can invalidate it. Until the OS does that, Indy has no way of knowing that the client connection is gone.

To account for that, you should either:

  1. implement a timeout in your application-layer data protocol. If you are expecting a client to send something to your server, and it does not do so for a certain amount of time, assume the client is gone and close the connection. During periods of idle activity, require clients to send a heartbeat command to your server at regular intervals to keep their connections alive. You can use the AContext.Connection.IOHandler.CheckForDataOnSource() method to wait for data to arrive, or you can use the AContext.Binding.SetSockOpt() method to specify an SO_RCVTIMEO timeout on blocking reads.

  2. if you cannot change your data protocol, you can at least enable TCP-level keep-alives on the socket itself. In the server's OnConnect event, you can call the AContext.Binding.SetKeepAliveValues() method to enable keep-alives. The OS will then handle the keep-alives for you, and will invalidate the connection if the timeout elapses.

With that said, also make sure that your server event handlers are not swallowing Indy exceptions (derived from EIdException). That can also cause the server to not terminate threads correctly, if a connection is lost and Indy raises an exception about it but you are not allowing the server to process it. If you need to catch exceptions (for logging, etc), make sure to re-raise any EIdException-derived exception and let the server handle it.

Upvotes: 4

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