[talkingtab]

Hunting good bugs is something every good software developer should experience. A good interview question is "tell me your favorite bug". Bugs are about reasoning, not intelligence. And I will take someone who can tell me what is wrong over what is correct any day. It requires a focus on getting things actually work.

I have two favorite bug stoies. The first is from a printout from the run of an IBM 360 assembly language program when I was just learning. Someone asked em why their program failed to run. I glanced quickly at the front page of the printout and it said "Too Long". So I told the person that was the problem. Something was too long. He looked at me very strangely, so I looked back at the page a little more closely, only to notice "Too Long" was in the name field of the person running the program. He was Vietnamese and his name was Too Long - literally. There is a powerful lesson (at least one) there.

The other happened when I was implementing some AppleTalk protocols - NBP to be exact. (Don't ask). I would capture the working packets then compare all the checksums, headers, constants, length fields in the packet my code generated and fix any problems. I was stuck on one failure. I just could not see any difference as I went through byte by byte, time after time. It was late and time to go home so I decided to print off each packet on paper and compare them later - certain I was missing something. The problem was instantly obvious. One printout took a page, the two pages. I had been appending junk data in the packet. Sigh

[win311fwg]

> A good interview question is "tell me your favorite bug".

I wouldn't have a clue how to recall any details about the bugs I've seen. I don't put much emphasis on past events. Looking forward is what I find to be a far more valuable use of my mental energy. I have vague recollections of debugging some doozies, but that is where the recall ends. It is clearly something you are passionate about, which no doubt keeps it something front of mind for you, but for many of it is just part of the job; like asking someone at McDonald's how their favourite burger flip landed.

You could say that I'm not the one of the for the job, which is a fair take, but if we reason through this some more, would we not conclude that there is no such thing as a good canned interview question? Given that no two people are the same, good interview questions can only be established in the context of who is being interviewed.

[toast0]

I mean, you do you, but I'm not sure how to take this. Maybe I'm a master debugger, but a lot of the problems I run into and debug tend to reoccur, sometimes in different circumstances, sometimes in the same circumstances (which can be amazingly frustrating, but...).

Remembering and being able to tell the narrative about how I figured out why something that people like to do is a really bad idea is very helpful to convince people not to repeat the mistakes of the past when they aren't receptive to "trust me, this is a bad idea and we shouldn't do it" or "if you do that, let me know when you undo it, otherwise don't call me"

Personally, I don't have any skill at giving this kind of story time interview question, so I don't. But it does seem concerning to me if someone has 5-10 years of software experience and can't articulate any debugging stories. How were you working where you never ran into a problem that took you/your team 2 weeks of pain to figure out?

[win311fwg]

> but a lot of the problems I run into and debug tend to reoccur

Whereas with directing my attention to looking forward to the future I don't see the same issues turn up ever again. I learn from my mistakes. Now that I am multiple decades into my career, I see almost no bugs turn up at all. That doesn't mean I will never encounter a bug again. It is inevitable that I will. But they will be novel when I do. Novel still doesn't mean particularly interesting, however. The burger flip landing a new way never seen before still isn't likely to register.

> it does seem concerning to me if someone has 5-10 years of software experience

Is that where you are at in your software career? I can attest that I saw a lot more bugs when in I was in that stage. One is still quite green and learning a tremendous amount even 5-10 years in. However, that was a long time ago. I've forgotten the details by now. This tune might change for you as well as you progress further.

Or maybe not. Everyone is different. You do seem passionate about bugs. I, on the other hand, hate them, so I lean heavily into processes to avoid them to the greatest extent possible.

[toast0]

I'm 22 years into real professional software development (I've semi-retired, but the world still needs debugging). Plus a few years of junior level IT/sysadmin stuff. Of course, my code is perfect by now. But my code runs on other people's code. And nobody else writes perfect code.

So I have to debug other people's libraries and operating systems. And other people's networks. Turns out other people often make similar mistakes. Some people say 'select isn't broken', but lots of things are[1]. Most of my debugging stories would tend to be centered around a problem that my team found/uncovered, not one that we created... although certainly I did make some bugs in my youth (definitely none lately!).

I put 5-10 years there because someone under 5 years of experience could maybe not have ever run into a troublesome issue, or they always had a senior to do the hard stuff. Between 5 and 10 years, maybe they find their first tricky bug. After 10+ years, you've got to have run into something.

[1] Here's some war stories:

I fixed an interop issue between OpenSSL and Microsoft schannel where rsa dhe would fail if the generated public key had leading zeros; OpenSSL would encode it in fewer bytes and schannel would return 'out of memory'. The RFC was vague. People had observed the failures for years, but I had to fix it. At the time, it was considered a reasonable optimization to generate a dhe keypair and reuse it for the lifetime of the server process... If we generated a problematic keypair on a given server, windows clients couldn't connect at all. Now, if I run into an issue and a working trace has structures of nice power of 2 lengths and a broken trace has one a little smaller, that's where I dig.

I found (but didn't make a patch) a bug in Firefox where POSTs to an http/2 server with tls 1.3 early data enabled would stall for about a minute when there was no connection to reuse. Fixing it was out of my league, but I was able to get it fixed by giving a clear bug report. This one was fairly new when I saw it, but there was a much less clear bug open against Thunderbird caused by the underlying issue. Not sure what I learned here really other than if you're expecting data sent to the network and it doesn't happen, it's usually an application problem... and clear bugs with clear logs help get things fixed.

I fixed an issue with FreeBSD where it would send the whole sendq when it received an icmp needs frag message, even when the maximum mtu sent in the icmp was the same as or greater than the current path mtu. This was happening when a Linux router was using large receive offload to aggregate inbound packets on a flow and then they were too large to forward; that bug was fixed long before I experienced it, but the router in question never got updated. I could not get ahold of the operator for them to fix the broken machine, but I was able to get a patch into FreeBSD so that the broken router(s) only impacted our customers that were behind it. ... this is another indication that PathMTU is hard, but also it helped me tune methods of sampling packets from production. PS, pathmtu issues are their own repetitive problem space.

That one time FreeBSD broke syncookies, so connections got reformed after close, and the tcp state was unsynchronizable between peers so they kept sending challenge acks... and IIRC, they broke it a second time, too. But maybe it was just we ran into it in a different context.

I've recently found some issues leading to out of order packet delivery with FreeBSD's dummynet traffic shaping; again, other people already experienced it, but nobody wrote a good bug report or submitted a patch, so I guess I'll have to do it, if it's still broken when I have time for it. This one is probably not going to be a repeating bug... not a lot of traffic shapers, but maybe there will be something learned about scheduling i/o

What processes could I use to avoid bugs like these? Hoping things magically get fixed in an update does sometimes work, and sometimes the bug becomes less relevant as the industry moves on (ecdhe has almost completely replaced rsadhe, but it hadn't at the time that my customers ran into the bug).

[win311fwg]

> So I have to debug other people's libraries

Been there, but then I soon learned to reduce dependencies to essentially none. Those that do make the cut need to be of high quality such that the authors of those libraries are also as perfect as you are. There is absolutely no need to depend on code written by those with <=10 years under their belt. The world is full of developers with 20+ years of experience.

> After 10+ years, you've got to have run into something.

Sure. And after 10+ years of flipping burgers, there will have been some pretty sweet lands. Who is going to remember, though? It is fine if you do. Everyone has their thing. But I'd say it is not exactly among the most memorable of events. It is not like time spent with your child, or something that actually has some kind of meaning. You even say you are semi-retired, so you must agree that things on the job don't really matter. If it did, why not dedicate every possible moment to it?