Challenge: Find the bug previous: NHibernate: Why do we need to specify the query type twice? next: A good refactoring session is… This piece of code caused a crashing bug on a QA server ( and we are lucky it didn’t go to production ). Can you spot the bug? I will give you a hint, it is the code that isn’t there. Comments 05/27/2009 12:25 AM by configurator Is a transaction or some other synchronization method missing here? That is, it seeks the values, then deletes them. The data could change during the deletions. 05/27/2009 05:17 AM by Gloubidou It's in the loop... ? 05/27/2009 06:16 AM by Vadi I think, you're missing a check before deleting it ... 05/27/2009 07:25 AM by Dave Where does 'data' comes from? (The ApplyToKeyAndActiveVersions line). 05/27/2009 08:04 AM by Rafal Ayende, where's your sweet spot in the Esent database? Do you miss good ol' days of WINAPI programming, or is Esent exceptionally performant/stable/powerful/whatever? Maybe there was a reason Microsoft has hidden it? 05/27/2009 09:43 AM by Ayende Rahien No, that is handled elsewhere 05/27/2009 09:48 AM by Ayende Rahien Dave, Another table that we join against. 05/27/2009 09:50 AM by Ayende Rahien Gloubidou, You are close... Vadi, No, that is not it. Rafal, I use it when I need a local DB. It is really easy to use when you get how it is working. It helps that it is the only local DB with true threading support. 05/27/2009 09:53 AM by Alexander maybe issue is related with closure of the session and data variables here: v=>Api.JetDelete(session, data) 05/27/2009 09:55 AM by Ayende Rahien Alexander, No, that is not it 05/27/2009 10:13 AM by meo Maybe "v => Api.JetDelete(session, data)" can't be run thru delegate for some reason? 05/27/2009 10:52 AM by Ayende Rahien meo, No, that is not it. It works just fine. 05/27/2009 11:01 AM by Stephen I take it the crash is a stack overflow from an infinite loop and that you are either missing a breaking condition or not doing something that would should effect the 'position'.. Basically no idea, is this something that will probably only make sense when illustrated and explained? 05/27/2009 11:17 AM by Gerke What happens to the table cursor after Api.JetDelete()? And does Api.JetDelete() immediately remove the table row or only mark it as deleted? Otherwise scenario could be: Api.JetDelete moves cursor forward Api.TryMovePrevious moves cursor backwards (to same row) => infinite loop 05/27/2009 11:48 AM by configurator Stephen, That would hang, not crash... 05/27/2009 12:25 PM by Stephen Ah, true ;) (btw ayende, having problems submitting comments from ie7/8- not sure if its just me, hitting submit doesn't do anything it just sits there). 05/27/2009 12:41 PM by Ayende Rahien Stephen, No stack overflow, no, and not an infinite loop, but you should concentrate on the loop 05/27/2009 12:42 PM by Ayende Rahien Gerke, It is marking that for deletion in the end of the current transaction, TryMovePrevious is not the issue. The loop will terminate at the expected time. 05/27/2009 12:58 PM by gloubidou I guess that you are trying to clean expired values (stuff we don't care about anymore).. thus having a big result set would pose a problem... am I right? 05/27/2009 01:23 PM by aaron do we need to know the contract of the jet api to find the problem? 05/27/2009 02:23 PM by DuvallBuck If the Api.RetrieveColumnAsString gets the first one then the Api.TryMovePrevious would never go to the rest but stop when the first one is deleted. 05/27/2009 03:22 PM by Ayende Rahien gloubidou, Tada, you are very close. aaron, No, you don't. 05/27/2009 03:24 PM by Ayende Rahien DuvallBuck, No, that is not the behavior we have 05/27/2009 04:38 PM by Itamar How often do values expire? 05/27/2009 04:53 PM by Michael Morton You're calling the function on a fixed timer and it's being executed again, before the previous execution has finished. Considering it results in a crash, it's probably stacking a good number of times. 05/27/2009 05:04 PM by Ayende Rahien That is a user defined value. In the system that we are talking abut, 24 hours 05/27/2009 05:05 PM by Ayende Rahien Michael, Huh? What timer? There isn't any external code involved that affect the bug in this function 05/27/2009 06:33 PM by Sander Rijken The call to Api.JetDelete should probably be called with session and key, not session and keyS (deleting everything)? 05/27/2009 06:59 PM by Ayende Rahien Sander, keys is the variable holding the table name, not a collection. It is instruction to delete the current row. 05/27/2009 08:14 PM by Paulo Köch Two Api.JetDelete calls? 05/27/2009 08:18 PM by Ayende Rahien Everyone, Notice what I said, what is the code that IS NOT THERE?! 05/27/2009 08:47 PM by Neal Blomfield You get the version but do not check it has expired? TBH I am guessing based on what I would expect to see rather than understanding the code / api that this is using but it seems like the most logical thing. 05/27/2009 09:07 PM by Ayende Rahien Neal, No, that is not that. And you don't need to understand anything about the API to see the problem. The problem is that until you see the problem, you don't know what it is. I am actually encouraged that no one managed to find it. It means that I am not that stupid 05/27/2009 09:11 PM by gloubidou Commit size will become huge in this case... Since you say that it is about code that isn't there, all I am able to come with is fixing a maximum of expired values to be processed in the loop and process the remaining later. That's similar to ORA-01555 snapshot too old! 05/27/2009 09:13 PM by Ayende Rahien gloubidou, TADA! You got it! When you hit it exactly right, you may get a LOT of expired values. Those expired values can be big enough to hit the commit limit, and cause an error! So I was stupid, I thought so! :-) 05/27/2009 10:51 PM by firefly Wow... gloubidou I am impress. Oren, how did you uncover the bug? If the application throw an exception somewhere then it's probably not that hard to trace down the culprit. Did you catch it through stress testing? 05/28/2009 12:07 AM by configurator Silly question: why is there a commit limit? 05/28/2009 12:28 AM by Ayende Rahien As I said, we run into this error in QA. Once I had the stack trace, and the error, it was really obvious what was wrong Comments have been closed on this topic.