The other day I had made several commits to a feature branch. While I coded, I only ran the test files associated with the code I wrote.
After around 10 commits, I found out another test was suddenly failing. I was pretty sure it was not introduced by the last commit, but I had no idea which of the 10 commits made the test fail.
Git bisect to the rescue!
git bisect is basically a feature where git does binary search in your
commits. You tell it which commit is after a bug is introduced and which commit
is before. Typically these commits will be HEAD and master.
I only knew about the manual way to run git bisect which I will explain first:
Start git bisect, then tell it which commit is bad and which is good:
git bisect start
git bisect bad # Current commit is "bad" because it has a broken test
git bisect good master # master is "good", because no test fails in master
After this series of commands, git has now checked out a commit that is approximately in the middle between the good and the bad commit.
You can now do your thing to find out if this commit is good or bad. In my case I would run my tests.
Then if the tests fail I will mark it as bad:
git bisect bad
and if the tests pass I will mark it as good:
git bisect good
Either way, git will now take me to the next commit I should check, just like when doing a binary search.
I will then repeat the process of finding out if the current commit is good or bad until git tells me it has found the first bad commit.
This is all good, but a lot of manual typing. There must be a better way. Spoiler: There is! :)
After the first commands where we tell git which commits are known to be good and bad, we can simply tell git bisect to run a certain script that will tell if the current commit is good or bad.
The script should exit with code 0 if the commit is good and any other code if it's bad (some exit codes have special meaning in git bisect, but I'll leave that out here).
In my case I would simply run my tests. Normally I run them with mix test so
I just prepend git bisect run to that:
git bisect run mix test
Then git bisect will do a binary search along several commits and run my command for each one. When it's done it will tell me which commit was the first bad one. Nice.
Note: When running a command that is more than a simple script. E.g. multiple
scripts concatinated with &&, you should either put it in an executable
script or run it as an inline bash script like so:
git bisect run bash -c "mix test && mix credo"
I recommend reading the man page for git bisect.
man git bisect