Git Bisect: A Developer’s Guide to Debugging

Enhance your debugging skills with Git Bisect, a powerful tool for pinpointing bug-introducing commits using binary search. Learn how to leverage this feature effectively, especially when tagging releases.

As developers, we often encounter bugs that seem to appear out of nowhere. Recently, while working on a project to migrate our views to SwiftUI, our team faced an issue with an unresponsive button. To track down the root cause, I turned to a powerful tool in Git’s arsenal: git bisect. This tool proved invaluable in identifying the problematic commit, and I’d like to share how it works and why it’s essential, especially when used in conjunction with tagging releases.

What is Git Bisect

git bisect is a command that helps you find the commit that introduced a bug by using a binary search algorithm. It effectively narrows down the range of commits to identify the exact one where the issue was introduced. This can save you a lot of time compared to manually checking each commit.

How Does Git Bisect Work

The process of using git bisect involves marking commits as either “good” or “bad.” Git then checks out the midpoint between these commits, allowing you to test and further narrow down the range. Here’s a step-by-step guide to using git bisect:

Start the Bisect Session: Begin by telling Git that you want to start a bisect session.
```
 git bisect start
```
Mark the Bad Commit: Identify the commit where the bug is present.
```
 git bisect bad [commit_hash_of_buggy_version]
```
Mark the Good Commit: Identify a commit where the bug is not present.
```
 git bisect good [commit_hash_of_working_version]
```
Test Each Bisected Commit: Git will check out the midpoint commit. Test your project to see if the bug is present. Mark the commit as good or bad accordingly.
```
 git bisect good  # if the bug is not present
 git bisect bad   # if the bug is present
```
Repeat Until Found: Git will continue to narrow down the commits until it identifies the first bad commit.
End the Bisect Session: Once the problematic commit is found, end the bisect session.
```
 git bisect reset
```

The Binary Search Algorithm in Git Bisect

The power of git bisect lies in its use of the binary search algorithm. Here’s a deeper look at how it works:

Initial Setup: You start by telling Git which commits are good and bad. This provides the starting and ending points for the search.
Divide and Conquer: Git then checks out the commit that is roughly halfway between the good and bad commits. This midpoint commit is where you begin testing.
Test and Mark: You test the midpoint commit to see if the bug is present. Based on the result, you mark the commit as either good or bad:
1. If the commit is good, the bug must have been introduced after this commit. Git now narrows the search to the range between this midpoint and the bad commit.
2. If the commit is bad, the bug must have been introduced before this commit. Git narrows the search to the range between the good commit and this midpoint.
Iterative Process: This process repeats, each time halving the number of commits to be checked. Because it’s a binary search, the number of commits to check is reduced logarithmically. For example, if you have 1,024 commits to check, a binary search would require at most 10 tests.
Identify the Culprit: The search continues until it narrows down to a single commit—the one where the bug was introduced.

Example: Finding the Unresponsive Button Bug

In our project, we suspected the bug with the unresponsive button might have been introduced during our migration to SwiftUI. Here’s how I used git bisect to track it down:

Starting Bisect: I started the bisect session.
```
 git bisect start
```
Identifying Commits: I knew the button was working in the previous production release, so I marked that commit as good.
```
 git bisect good v1.0.0
```
The current commit was marked as bad.
```
 git bisect bad HEAD
```
Testing Commits: Git checked out a midpoint commit. I ran the project, tested the button, and found it responsive.
```
 git bisect good
```
Git then moved to the next midpoint. This process repeated until I found the commit where the button stopped responding.
Ending Bisect: After identifying the bad commit, I reset the bisect session.
```
 git bisect reset
```

The Importance of Tagging Releases

One of the key factors that made using git bisect effective in our case was the practice of tagging our releases. Tags in Git are references to specific points in your history, typically used to mark release points (e.g., 1.0.0). This practice provides clear reference points to mark as “good” commits, making it easier to start the bisect process from known stable versions.

Conclusion

Using git bisect can significantly streamline the debugging process by quickly pinpointing the commit that introduced a bug. It’s a powerful tool that every developer should have in their toolkit. Moreover, tagging releases is an essential practice that enhances the effectiveness of tools like git bisect, providing clear milestones in your project history.

By integrating these practices into your workflow, you can tackle bugs with confidence and precision, ensuring a smoother development process and more reliable codebase.

Help yourself and even others sharing this post to improve their skills sharing this article, #HappyTesting and #HappyCoding👨‍💻.

References

Pro Git by Scott Chacon and Ben Straub (book)
Git Bisect Documentation (official documentation)
Mastering Git by James Bell (book)
Binary Search Algorithm by GeeksforGeeks (article)
Introduction to Git Tags by GitHub (documentation)

Git Bisect: A Developer's Guide to Debugging