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Kode Vicious

Divide and Conquer


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Dear KV,

Many of our newer developers—those who have worked only with git—seem to find bugs in their code only by using git's bisect command. This is troubling for a couple of reasons. The first is that often—once they find where the change occurred that caused the problem—they do not understand the cause, only that it happened between versions X and Y. The second is that they do not seem to understand the limits of debugging in this way, which, perhaps, is more a topic for you than for me to describe to you. Do you find this practice becoming more widespread and perhaps debilitating to good debugging?

Vivisected by Bisection

Dear Vivisected,

Nearly all new tools are both a blessing and a curse, as close readers of KV will know by now, and the ability to bisect a set of changes quickly is no different. It is quite definitely a blessing to have automation take over the tedious work of checking out a change, building the system, running a test, and seeing if the test fails, and then if it doesn't fail in the right way, doing this all over again until the change that introduced the bug is found. That kind of work is something you want automated, and, therefore, in that case it is a blessing—a limited one, but a blessing nonetheless. I mean, it is not manna from heaven, is it?

What you are asking me to rant about (you are asking for a rant, right?) is how such a tool can create lazy thinkers, and by extension, lazy engineers. Well, there are a few problems to talk about even before we get to whether having such automation leads to laziness.

Tools such as bisection are great if, and only if, you have a well-understood bug that occurs with 100% consistency so that the bisection can work. Bisection is of no use if you have a heisenbug, or something similarly subtle, that will fail only from time to time; and, while we do not want any bugs in our systems, we know these subtle bugs are the most difficult to fix and the ones that cause us—well, some of us—truly to think critically about what we are doing.

Timing bugs, bugs in distributed systems, and all the difficult problems we face in building increasingly complex software systems cannot yet be addressed by simple bisection. It is often the case that it would take longer to write a usable bisection test (the damnable thing you must write to get the bisection to tell you where the bad change was) for a complex problem than it would to analyze the problem while at the tip of the tree.

Another thing developers often fail to understand is the bug may not be related to any previous change; it might be right there in front of them, staring back, in orange on black. I have watched several developers who were absolutely convinced the bug was "somebody else's problem" run and rerun bisections only to realize that the actual problem was in their latest, uncommitted change. It is unfair to laugh at people in the middle of a debugging session, and, with KV, it is a risk to life and limb, but it is still damned tempting.

What bisection provides all developers is simply another tool to find bugs in their code. Sure, the bug must be easy to test for, likely cannot be in a distributed system, and cannot be a timing or a heisenbug, but it is still better than finding these simpler bugs by hand or writing your own script to do just what bisect is going to do.

Does this tool make us dumber? Probably not. What it does is perhaps help a less-seasoned developer to find bugs; however, if that developer wishes to learn, the tool does not prevent that, and that is why such a tool is a boon to some and a cushion to others.

KV

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Author

George V. Neville-Neil (kv@acm.org) is the proprietor of Neville-Neil Consulting and co-chair of the ACM Queue editorial board. He works on networking and operating systems code for fun and profit, teaches courses on various programming-related subjects, and encourages your comments, quips, and code snips pertaining to his Communications column.


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