In software development, encountering bugs is a common occurrence. When faced with a bug, developers need to quickly identify the commit that introduced the issue to rectify it efficiently. Git bisect is a powerful tool provided by Git that automates this process by performing a binary search through the commit history to pinpoint the exact commit responsible for the bug. In this article, we’ll see more about the strategies for using Git bisect effectively to isolate bugs rapidly and streamline the debugging process.

What is Git Bisect?

Git bisect is a command-line tool provided by Git that automates the process of pinpointing the commit that introduced a bug. It works by performing a binary search through the commit history, and systematically decreasing the range of commits until the culprit is identified.

Efficient Git Bisect Strategies:

1. Start with Known Good and Bad Commits: Before initiating a bisect session, it’s important to identify a known good commit (a commit where the bug was not present) and a known bad commit (a commit where the bug is known to exist). This provides a starting point and endpoint for the binary search.
2. Automate the Bisect Process: Rather than manually checking out commits and testing for the presence of the bug, automate the bisect process by writing a script or test suite that can determine whether a commit is good or bad programmatically. This can significantly speed up the bisect process, especially for complex projects with extensive test suites.
3. Use Narrowing Down Techniques: As the bisect process progresses, use narrowing down techniques to further reduce the search space. For example, if you identify a commit where the bug is not present, mark it as good and skip testing other commits between that commit and the current known bad commit.
4. Leverage Git Bisect Skip: If you encounter commits that you know are not relevant to the bug being investigated, use the git bisect skip command to skip testing those commits. This can help expedite the bisect process by focusing on the most likely candidates.
5. Divide and Conquer: If the commit history is extensive, consider dividing the bisect process into smaller, manageable chunks by bisecting specific branches or subdirectories of the codebase. This can help isolate the bug more quickly and efficiently.
6. Revert Suspicious Commits: If you suspect that a specific commit introduced the bug but are unsure, temporarily revert the commit and perform additional testing to confirm whether the bug persists. This can help narrow down the search space and provide valuable insights into the root cause of the issue.

Example:

For example let’s say your commit history looks like this :

Assumed commit history

We know that that new feature is broken now, but we also don’t know when it was working perfectly, but we we do know when it was introduced in the source code which is at i , Pick the commit occurring before i which is c which is an good commit.

The bisecting process

Then the git bisect will perform a binary search on “d – e – f – g – h” until it finds a bad commit just after a good commit. Let’s consider that g is the starting point of the bad commit.

Considering g as a bad commit.

After we have labelled “g” as a bad commit, It will try the same on “e“ and let’s say that it is good, all the successive commits which were made after it will also be flagged as bad commit. If we assume d as a good commit, Git will then flag “e” as a bad commit. Then it will try with “f“ commit, Which let’s assume is bad.

“f” is the commit which introduced the bug.

This process will help in determining that the commit “f” is the one that broke the new feature.

FAQs

1. What will happen if merge commits are present in the commit history ?

If merge commit is present in a commit history, Git will automatically skip them during the binary search in bisect method, This is made so that only relevant commits are undergone tests.

2. How to use Git bisect along with software development workflow ?

Git bisect can be used by developers to communicate the results with different teams working on a project. It can also be integrated into testing frameworks like Jenkins and CI pipelines which can be used to terminate bugs in a software application.

3. How to identify changes made by third-party dependencies ?

It can be used to see changes, but only through one condition that these changes are present in the commit history of the application.

4. Any limitations of using Git Bisect ?

Git bisect focuses on determining the bug is there or not in the overall changes by the commit. Bisect do not discriminate between individual changes within a commit. If a specific part of the code in the commit contributed to the bug, Then the whole commit will be marked as bad commit instead of the part which introduced the bug. This will not tell which part of the code produced the error but will flag it completely.

5. Can we use Git Bisect to find performance decline ?

Certainly, it can be used to find performance decline, which can introduce decrease in speed of the software compared to its earlier version. The catch here is that performance decline will not be always flagged as explicit bugs. But by using bisect to check commit history and track changes in metrics of performance, we can flag that commit which introduced the decline of software performance.

Conclusion

In this article we learned that how bisect proves to be a powerful command, And by the use of binary search a large complex codebase of 10000 commits bisect can identify the bad commit which introduced the bug, in just 14 steps which reduces the time if linear search was to be used, If git bisect is integrated with testing frameworks like Jenkins and also in CI-pipelines it can exponential enhance the utility in software development workflow allowing less debugging period.