Indy OpenSSL problems if test Connected during callback

Question

Delphi v10.1 Berlin. Indy v10.6.2 OpenSSL v1.0.2h

I have a form I set up in Delphi to test using TIdIMAP4 with an SSL connection (TIdSSLIOHandlerSocketOpenSSL). Because this was for testing, I had a checkbox set up to show me if the IMAP component was currently connected, and I connected the OnStatus and OnStatusInfo events of both the IMAP and SSL IOHandler to code that updated the checkbox. That code was simply:

IsConnectedCB.Checked := imap.Connected;

Eventually (it took me longer than I like to admit) I worked out that that is what was upsetting the SSL connection. Here is an example of what happened (a log of the status messages):

02:35:13 S: Resolving hostname localhost.
02:35:13 S: Connecting to 127.0.0.1.
02:35:13 S: Connected.
02:35:13 S: SSL status: "before/connect initialization"
02:35:13 S: SSL status: "before/connect initialization"
02:35:13 S: SSL status: "SSLv3 write client hello A"
02:35:13 S: SSL status: "SSLv3 read server hello A"
02:35:43 S: SSL status: "SSLv3 read server certificate A"
02:35:46 S: Disconnecting.
02:35:46 S: Disconnected.
02:35:48 E: SSL negotiation failed.

Notice the time difference following "server hello A" (sometimes happened after "client hello A") - this is where the connection attempt times out.

It turns out that a call to imap.Connected ends up making a query inside the IOHandler and that upsets the callback processing.

The solution is simple: Don't read the Connected property during a callback. :-)

But, I would like to be able to update my form status while testing various long running processes, and the OnStatus/OnStatusInfo events are convenient for that (rather than interspersing my own code with lots of calls to update). So...

My question is: Is there a way to tell when it is safe to test the Connected property? (In this case TIdIMAP4.Connected, but I'm sure the problem must apply to most other components using OpenSSL.)

Remy Lebeau Remy Lebeau · Accepted Answer · 2016-07-14T18:38:39

Is there a way to tell when it is safe to test the Connected property?

As you discovered, Connected performs a read operation. Performing socket I/O is the only way to determine if a blocking socket is still connected and valid. So, you can't use Connected when reading will interfere with the flow of other operations that also need to perform I/O.

In fact, you really shouldn't be using Connected directly at all in most situations. In this case, I would suggest using the TIdIMAP4.On(Dis)Connected and/or TIdIMAP4.OnStatus events to update a Boolean variable, and then have the rest of your code simply check that variable when needed.

Indy OpenSSL problems if test Connected during callback

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