Occasionally, one of our tests hangs. Everything seems to be honky dory, but it just freezes and does not complete. This is a new piece of code, and thus is it suspicious unless proven otherwise, but an exhaustive review of it looked fine. It took over two days of effort to narrow it down, but eventually we managed to point the finger directly at this line of code:
In certain cases, this line would simply not read anything on the server. Even though the client most definitely sent the data. Now, given that TCP is being used, dropped packets might be expected. But we are actually testing on the loopback device, which I expect to be reliable.
We spent a lot of time investigating this, ending up with a very high degree of certainty that the problem was in the TCP stack somewhere. Somehow, on the loopback device, we were losing packets. Not always, and not consistently, but we were absolutely losing packets, which led the server to wait indefinitely for the client to send the message it already did.
Now, I’m as arrogant as the next developer, but even I don’t think I found that big a bug in TCP. I’m pretty sure that if it was this broken, I would have known about it. Beside, TCP is supposed to retransmit lost packets, so even if there were lost packets on the loopback device, we should have recovered from that.
Trying to figure out what was going on there sucked. It is hard to watch packets on the loopback device in WireShark, and tracing just told me that a message is sent from the client to the server, but the server never got it.
But we continued, and we ended up with a small reproduction of the issue. Here is the code, and my comments are below:
This code is pretty simple. It starts a TCP server, and listens to it, and then it reads and writes to the client. Nothing much here, I think you’ll agree.
If you run it, however, it will mostly work, except that sometimes (anywhere between 10 runs and 500 runs on my machine), it will just hang. I’ll save you some time and let you know that there are no dropped packets, TCP is working properly in this case. But the code just doesn’t. What is frustrating is that it is mostly working, it takes a lot of work to actually get it to fail.
Can you spot the bug? I’ll continue discussion of this in my next post.
More posts in "Challenge" series:
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- (05 May 2022) Spot the optimization
- (06 Apr 2022) Why is this code broken?
- (16 Dec 2021) Find the slow down–answer
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- (03 Nov 2021) The code review bug that gives me nightmares–The fix
- (02 Nov 2021) The code review bug that gives me nightmares–the issue
- (01 Nov 2021) The code review bug that gives me nightmares
- (16 Jun 2021) Detecting livelihood in a distributed cluster
- (21 Apr 2020) Generate matching shard id–answer
- (20 Apr 2020) Generate matching shard id
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- (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
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- (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?
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- (02 Sep 2009) The lazy loaded inheritance many to one association OR/M conundrum
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- (26 Jul 2008) Find the bug fix
- (05 Jul 2008) Find the deadlock
- (03 Jul 2008) Find the bug
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- (27 May 2008) Striving for better syntax
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- (21 Jan 2008) Strongly typing weakly typed code
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