HN Reader

I hardly think binary search is an unknown algorithm even by beginner standards for someone from a completely different field

Git bisect was an extremely powerful tool when I worked in a big-ball-of-mud codebase that had no test coverage and terrible abstractions which made it impossible to write meaningful tests in the first place. In that codebase it was far easier to find a bug by finding the commit it was introduced in - simply because it was impossible to reason through the codebase otherwise.

In any high quality codebase I’ve worked in, git bisect has been totally unnecessary. It doesn’t matter which commit the bug was introduced in when it’s simple to test the components of your code in isolation and you have useful observability to instruct you on where to look and what use inputs to test with.

This has been my experience working on backend web services - YMMV wildly in different domains.

git bisect gets interesting when API signatures change over a history - when this does happen, I find myself writing version-checking facades to invoke the "same" code in whatever way is legal

I've used bisect a few times in my life. Most of the time, I already know which files or functions might have introduced a bug.

Looking at the history of specific files or functions usually gives a quick idea. In modern Git, you can search the history of a specific function.

    >> git log -L :func_name:path/to/a/file.c

You need to have a proper .gitattributes file, though.

One place bisect shines is when a flaky test snuck in due to some race condition but you can’t figure out what. If you have to run a test 100000 times to be convinced the bug isn’t present, this can be pretty slow. Bisecting makes it practical to narrow in on the faulty commit, and with the right script you can just leave it running in the background for an hour.

Binary searching your commit history and using version control software to automate the process just seems so...obvious?

I get that author learned a new-to-him technique and is excited to share with the world. But to this dev, with a rapidly greying beard, the article has the vibe of "Hey bro! You're not gonna believe this. But I just learned the Pope is catholic."

Honestly, after 20 years in the field: optimising the workflow for when you can already reliably reproduce the bug seems misapplied because that's the part that already takes the least amount of time and effort for most projects.

When I learned about git bisect I thought it was a little uppity. I thought it was something I would never use in a practical scenario. Working on large code bases. However, sometimes a bug pops up and we don't know when it started. We use git bisect not place blame on a person, but to try to figure out when the bug was no longer there so we know what code introduced it. Yes, clean code helps. Sometimes git bisect is really nice to have.

Make sure you know about exit code 125 to your test script. You can use it in those terrible cases where the test can't tell, one way or another, whether the failure you seek happened, for example when there is an unrelated build problem.

I wrote a short post on this:

https://speechcode.com/blog/git-bisect

Wow and here I was doing this manually all these years.

I’ve used bisect a couple of times but really it’s a workaround for having a poor process. Automatic unit tests, CI/CD, should have caught it first.

It’s still very satisfying to watch run though, especially if you write a script that it can run automatically (based on the existing code) to determine if it’s a good or bad commit.

> People rant about having to learn algorithmic questions for interviews. I get it — interview system is broken, but you ought to learn binary search at least.

Well, the example of git bisect tells you that you should know of the concept of binary search, but it's not a good argument for having to learn how to implement binary search.

Also just about any language worth using has binary search in the standard library (or as a third party library) these days. That's saner than writing your own, because getting all the corner cases right is tricky (and writing tests so they stay right, even when people make small changes to the code over time).

Git has some really good tools for searching code and debugging. A few years ago I wrote a blog post abot them, including bisect, log -L, log -S and blame. You can see it and the discussion here: https://news.ycombinator.com/item?id=39877637

Surely everyone has a CI pipeline that wont allow merges with failing tests?

`git-bisect` is legit if you have to do the history archaeological digging. Though, there is the open question of how git commit history is maintained, the squash-and-merge vs. just retain all history. With squash-and-merge you're looking at the merged pull-request versus with full history you can find the true code-level inflection point.

I used git bisect in anger for the first time recently and it felt like magic.

Background: We had two functions in the codebase with identical names and nearly identical implementations, the latter having a subtle bug. Somehow both were imported into a particular python script, but the correct one had always overshadowed the incorrect one - that is, until an unrelated effort to apply code formatting standards to the codebase “fixed” the shadowing problem by removing the import of the correct function. Not exactly mind bending - but, we had looked at the change a few times over in GitHub while debugging and couldn’t find a problem with it - not until we knew for sure that was the commit causing the problem did we find the bug.

> the OG tool `git`

This phrase immediately turned the rest of my hair gray. I'm old enough to still think of Git as the "new" version control system, having survived CVS and Subversion before it.

I agree with the post.

I also think that typically if you have to resort to bisect you are probably in a wrong place. You should have found the bug earlier so if do not even know when the bug came from

- your test coverage isn't good sufficient

- your tests are probably not actually testing what you believe they do

- your architecture is complex, too complex for you

To be clear though I do include myself in this abstract "you".

'git bisect run' is probably one of the most important software tools ever.

I recently used git bisect to help find the root cause of a bug in a fun little jam of mine (a music player/recorder written in Svelte - https://lets-make-sweet-music.com).

My scenario with the project was:

- no unit/E2E tests - no error occurring, either from Sentry tracking or in the developer tools console. - Many git commits to check through as GitHub's dependabot alerts had been busy in the meantime.

I would say git bisect was a lifesaver - I managed to trace the error to my attempt to replace a file I had with the library I extracted for what it did (http://github.com/anephenix/event-emitter).

It turns out that the file had implemented a feature that I hadn't ported to the library (to be able to attach multiple event names to call the same function).

I think the other thing that helps is to keep git commits small, so that when you do discover the commit that breaks the app, you can easily find the root cause among the small number of files/code that changed.

Where it becomes more complex is when the root cause of the error requires evaluating not just one component that can change (in my case a frontend SPA), but also other components like the backend API, as well as the data in the database.