Thursday 26 May 2005 — This is 18 years old. Be careful.
Just to show that this blog is fair and balanced, after yesterday’s story of bad hardware, here’s a story with the opposite moral.
We were getting a failed assertion on our server when it performed a certain task (post-processing completed action items with attachments). The assertion was about a null smart pointer, and the assertion message simply pointed to the code in the template in the C++ header, meaning that we didn’t know what source line contained the bad pointer. (I know: we really should get stack traces for these things! Hi Nate!) But the type of the pointer was included in the assert, and it was a fairly unusual type, so there were only a few places in the code where they were used.
We examined the processing code, and found a use of the unusual pointer type. The code was using a database query to get the latest version of the attachment, and when it got the result set back, it assumed that it contained a row. We surmised that the assert was happening because it did not contain a row. So that was one bug: I changed the assert to be a more informative exception. But how could it happen in any case? How could we not find a row?
The latest version of the attachment is found by following a link on a bookkeeping record. The bookkeeping record is found from a link on any version of the attachment. We were processing an incoming transaction which included an update to the bookkeeping record, and a new version of the attachment.
As it happens, when processing that transaction, the code starts processing the completed action item on another thread. That’s another bug: there’s a race condition between committing the completed action item and the other thread doing the subsequent processing. The other thread might see old data if it wins the race, or might see the new data if it loses the race.
But regardless how that race turns out, the database should contain a consistent set of records. The attachment should point to a bookkeeping record, which should point to the latest version, which should exist. So again: how could that result set be empty?
The QE engineer looking into the problem reported that it wasn’t consistently reproducable. There seemed to be a random element, bolstering our finger-pointing at the race condition. He thought that putting more than one attachment on an action item made the problem worse. Maybe loading up the transaction with more attachments affected the timing of the race condition.
But nagging at us was the question of how the result set could be empty. It seemed that the new bookkeeping record was present, but the attachment it referenced was not. Somehow, it looked like half of the action item transaction was being committed, or only half of it was visible to the other thread. This was bad news: it meant that the database or our layer on top of the database, was not delivering ACID transaction semantics to us.
As it happens, a co-worker was also working through a thorny debug session, and was talking about strange behavior that could be attributed to bogus transactions.
I wasn’t relishing the prospect of figuring out whether the transaction isolation was wrong, and determining how to fix it, but we didn’t have another explanation for the strange behavior we were seeing. I started to look into how to test transaction semantics. Searching the web, I found the Transaction Processing Performance Council’s TPC-B, a detailed test of transaction throughput which includes verifying that transaction semantics are correct. I liked this part of the description: “First, let’s look at what TPC-B is not. It is not TPC-A.” Thanks.
It seems to be a well thought-out test, and I was mulling over adapting it to test our database supprt, when the QE engineer showed me the results of running the new code with the exception instead of the assert.
It turned out the whole transaction semantic issue was a paranoid fantasy. The exception showed that in fact, the problem was that we were using a bogus record id for the attachment to look up. When the action item had two attachments, the two ids were concatenated into one, and no attachment could be found, so no bookkeeping record could be found, so no latest version could be found. I had assumed that the record set was empty because the latest version was missing, when in fact the link was missing because an input to the method was totally bogus. The exception I added included the “id” being looked up, and once we saw the exception being thrown, the entire situation was obvious.
Because of a few assumptions (the attachment id must be OK), I ended up going down a path that ended with a monster at the end of it (our database doesn’t work!), when really it turned out to be a very simple and understandable bug.
- Sometimes, it really is your software’s fault.
- Challenge all of your assumptions.
- Find a simpler explanation.
- Put helpful information in all error messages.
Glad to hear it was something simple to fix and not something like the whole transaction layer was borked.
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