The bug from yesterday would only show when a particular query is being run concurrently, and not always then.
Here is the code that is responsible for the bug:
It is quite hard to see, because it is so subtle. The code here create a cached lambda that is global for the process. The lambda takes the current engine, the object to transform return the converted object.
So far, so good, right?
Except that in this case, the lambda is capturing the engine parameter that is passed to the function. The engine is single threaded, and must not be used concurrently. The problem is that the code already handles this situation, and the current engine instance is passed to the lamda, where it is never used. The original engine instance is being used concurrently, violating its invariants and causing errors down the line.
The fix was to simply use the current engine instance that was passed to us, but this was really hard to figure out.
More posts in "Challenge" series:
- (28 Sep 2018) The loop that leaks–Answer
- (27 Sep 2018) The loop that leaks
- (03 Apr 2018) The invisible concurrency bug–Answer
- (02 Apr 2018) The invisible concurrency bug
- (31 Jan 2018) Find the bug in the fix–answer
- (30 Jan 2018) Find the bug in the fix
- (19 Jan 2017) What does this code do?
- (26 Jul 2016) The race condition in the TCP stack, answer
- (25 Jul 2016) The race condition in the TCP stack
- (28 Apr 2015) What is the meaning of this change?
- (26 Sep 2013) Spot the bug
- (27 May 2013) The problem of locking down tasks…
- (17 Oct 2011) Minimum number of round trips
- (23 Aug 2011) Recent Comments with Future Posts
- (02 Aug 2011) Modifying execution approaches
- (29 Apr 2011) Stop the leaks
- (23 Dec 2010) This code should never hit production
- (17 Dec 2010) Your own ThreadLocal
- (03 Dec 2010) Querying relative information with RavenDB
- (29 Jun 2010) Find the bug
- (23 Jun 2010) Dynamically dynamic
- (28 Apr 2010) What killed the application?
- (19 Mar 2010) What does this code do?
- (04 Mar 2010) Robust enumeration over external code
- (16 Feb 2010) Premature optimization, and all of that…
- (12 Feb 2010) Efficient querying
- (10 Feb 2010) Find the resource leak
- (21 Oct 2009) Can you spot the bug?
- (18 Oct 2009) Why is this wrong?
- (17 Oct 2009) Write the check in comment
- (15 Sep 2009) NH Prof Exporting Reports
- (02 Sep 2009) The lazy loaded inheritance many to one association OR/M conundrum
- (01 Sep 2009) Why isn’t select broken?
- (06 Aug 2009) Find the bug fixes
- (26 May 2009) Find the bug
- (14 May 2009) multi threaded test failure
- (11 May 2009) The regex that doesn’t match
- (24 Mar 2009) probability based selection
- (13 Mar 2009) C# Rewriting
- (18 Feb 2009) write a self extracting program
- (04 Sep 2008) Don't stop with the first DSL abstraction
- (02 Aug 2008) What is the problem?
- (28 Jul 2008) What does this code do?
- (26 Jul 2008) Find the bug fix
- (05 Jul 2008) Find the deadlock
- (03 Jul 2008) Find the bug
- (02 Jul 2008) What is wrong with this code
- (05 Jun 2008) why did the tests fail?
- (27 May 2008) Striving for better syntax
- (13 Apr 2008) calling generics without the generic type
- (12 Apr 2008) The directory tree
- (24 Mar 2008) Find the version
- (21 Jan 2008) Strongly typing weakly typed code
- (28 Jun 2007) Windsor Null Object Dependency Facility