VCS bisection steps should always be reversible

November 18, 2015

So this happened:

@thatcks: I think I just ruined my bisect run with one errant 'hg bisect --bad', because I can't see a way to recover from it in the Mercurial docs.
This is my extremely angry face. Why the hell won't Mercurial give me a list of the bisect operations I did? Then I could fix things.
Instead I appear to have just lost hours of grinding recompilation to a UI mistake. And Mercurial is supposed to be the friendly VCS.

VCS bisection is in general a great thing, but it's also a quite mechanical, repetitive process. Any time you have a repetitive process that's done by people, you introduce the very real possibility of error; when you do the same thing five times in a row, it's very easy to accidentally do it the sixth time. Or to just know that you want the same command as the time before and simply recall it out of your shell's command history except that nope, your reflexes were a bit fast off the mark there.

(It's great when bisection can be fully automated but there are plenty of times when it can't because one or more of the steps requires human intervention to run a test, decide if the result is correct, or the like. Then you have a human performing a series of steps over and over again but they're supposed to do different things at the end step. We should all know how that one goes by now.)

So inevitably, sooner or later people are going to make a mistake during the bisection process. They're going to reflexively mark the point under testing as good when it's actually bad, or mark it as bad when they just intended to skip it, or all of the other variants. It follows directly that a good bisection system that's designed for real people should provide ways to recover from this, to say 'whoops, no, I was wrong, undo that and go back a step' (ideally many steps, all the way back to the start). Bisection systems should also provide a log, so that you can see both what you did and the specific versions you marked in various ways. And they should document this clearly, of course, because stressed out people who have just flubbed a multi-hour bisection are not very good at carefully reading through three or four different sections of your manual and reasoning out what bits they need to combine, if it's even possible.

Of course, this sort of thing is not strictly speaking necessary. Bisection works just fine without it, provided that people don't make mistakes, and if people make mistakes they can just redo their bisection run again from the start. A bisection system with no log and no undo has a pleasantly mathematical sort of minimalism. It's just not humane, as in 'something that is intended to be used by actual humans and thus to cope with their foibles and mistakes'.

Overall, I suppose I shouldn't be surprised. Most version control systems are heavily into mathematical perfection and 'people should just do it right' in general.

(This is a terrible misunderstanding but that's another entry.)

Written on 18 November 2015.
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Last modified: Wed Nov 18 23:12:25 2015
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