Synchronous VB6 apparently behaving asynchronously! Crash

user_4121869 0
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We have a legacy VB6 app which has started, from time to time, hangs. We thought it may be to do with a shift to Citrix, but can now replicate the behaviour on a thick client on Win10. We don't think that we have seen this before on earlier Windows versions, but are still checking logs to confirm that.

We experience the behaviour when tabbing into a text box and then tabbing out. As we pass through it, we are making a simple ado call to lookup/validate some data in a text box. As part of the correct program running we are logging

“Opening Dataset: SELECT ... FROM ... ”
“Opened Dataset”

Between these 2 log statements is simple ado data retrieval code with which we have had no problems previously. It is in an ActiveX dll and is running synchronously. Most importantly is that between these 2 log statements there is no DoEvents or api call which would yield control. As far as we can see, it should be a purely synchronous operation.

When the system crashes, which happens sporadically, we can see other logging statements appear between these 2 which can be either resource status (e.g. how much memory, gdi/user objects - which would usually be found because a timer has triggered in the main form) or focus type events - which aren’t timer driven - at least in our codebase.

“Opening Dataset: SELECT ... FROM ... ”
“Resource Status: ...”
“Opened Dataset”```

or

“Opening Dataset: SELECT ... FROM ... ”
“TextItem.OnLostFocus Item1 ...”
“TextItem.Validate ...”
“TextItem.OnGotFocus Item2 ...
“Opened Dataset”

So my initial question is, in what scenario can what should be a synchronous operation be interrupted and appear to act asynchronously.

For example, and we aren’t doing this, I could imagine writing some unsafe code whereby by using a multimedia timer (on another thread) and supplying an AddressOf parameter to the address of a function on one of our modules, that that timer initiates execution of our code, separate to the correct control flow. Other than something like that, I just can’t see how synchronous vb6 code could be interrupted in this way.

I’d be really grateful of any thoughts, suggestions or advice. I’m really sorry if this is soo vague. It perhaps reflects how I’m struggling to get my head round this problem.

vb6 thread-safety
2 Answers

Just to say, we tracked this down to Windows 10 plus an old (out of support) socket component we are using. It looks like it is pumping the message queue "at the wrong time" and hence we are seeing UI events appear in the middle of a synchronous process. We don't see this behaviour on earlier Windows versions.

I don't know what may have changed in Win10 which would result in this, but we obviously need to upgrade.

user_41218690

In our case we had a few long running timers to pull status/changes from the DB which caused this. We are using ADO with SQL Native Client and MARS, which worked great up until Windows 10 where intermittent lock ups occurred. Logging and Windbg confirmed this was happening when 2 requests where hitting the ADO connection at the same time. The error from ADO was "Unable to open a logical session" error number -2147467259, and actually caused SQL Server 2014 (running on another machine) to block all other client queries from multiple different applications and machines until the locked up app was killed. I could not replicate this in the IDE as apparently that forces timers to work the way they always did. The fix was to async our ADO implementation and put a connection manager on top of the SQL connections to force requestors to wait their turn (basically taking the Win10 async'd timer feature back out). My only performance impact was the additional few milliseconds of delaying the timer fired SQL query when it collided with a another query.

user_99231020
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