Bisect for finding bug-introducing commits in Git - Deep Dive

Overview - Bisect for finding bug-introducing commits

What is it?

Git bisect is a tool that helps you find the exact commit where a bug was introduced in your code. It works by automatically checking different points in your project's history to narrow down the commit that caused the problem. This saves time compared to manually checking each commit one by one. It is especially useful in large projects with many changes.

Why it matters

Without git bisect, finding the commit that introduced a bug can be like searching for a needle in a haystack, taking hours or days. Bisect makes this process fast and systematic, reducing frustration and speeding up bug fixes. This means developers can deliver more reliable software faster, improving user experience and trust.

Where it fits

Before learning git bisect, you should understand basic git commands like commit, checkout, and log. After mastering bisect, you can explore automated testing and continuous integration to further improve code quality and debugging speed.

Mental Model

Core Idea

Git bisect uses a smart search method to quickly find the first bad commit by testing commits between a known good and a known bad point.

Think of it like...

Imagine you have a long book and suddenly notice a typo on page 200. Instead of reading every page, you open the book in the middle, check if the typo is before or after, and keep narrowing down until you find the exact page with the typo.

Start with two points:
┌───────────────┐       ┌───────────────┐
│ Known Good    │       │ Known Bad     │
│ Commit A      │──────▶│ Commit Z      │
└───────────────┘       └───────────────┘
          ▲                      ▲
          │                      │
          └───── Bisect tests commits in between ─────▶

Each test halves the search space until the bad commit is found.

Build-Up - 7 Steps

1

Foundation

Understanding Git Commit History

Concept: Learn what commits are and how git tracks changes over time.

A commit in git is like a snapshot of your project at a moment in time. Each commit has a unique ID and points to the previous commit, forming a chain called history. You can view this history using 'git log' to see all changes made.

Result

You can see a list of commits with their IDs, messages, and authors.

Knowing that commits form a timeline helps you understand how bisect navigates through this history to find problems.

2

Foundation

Identifying Good and Bad Commits

Concept: Learn to mark commits as good or bad based on whether they have the bug.

A 'good' commit is one where the bug does not exist. A 'bad' commit is one where the bug is present. You need to know at least one good and one bad commit to start bisecting. You can test your code at different commits by checking them out.

Result

You can tell git which commits are good or bad to guide the bisect process.

Understanding the difference between good and bad commits is key to narrowing down the bug's origin.

3

Intermediate

Starting Git Bisect Session

Concept: Learn how to initiate a bisect session and mark known commits.

Use 'git bisect start' to begin. Then mark the bad commit with 'git bisect bad' (usually the current commit with the bug) and the good commit with 'git bisect good '. Git will then checkout a commit halfway between these points for testing.

Result

Git checks out a commit between good and bad points for you to test.

Starting bisect sets up the search boundaries and automates the process of checking commits.

4

Intermediate

Marking Commits During Bisect

🤔Before reading on: do you think you must test every commit manually during bisect, or does git help reduce the number of tests? Commit to your answer.

Concept: Learn how to mark each tested commit as good or bad to guide git bisect.

After testing the checked-out commit, you tell git if it is good or bad using 'git bisect good' or 'git bisect bad'. Git then automatically selects the next commit to test, halving the search space each time.

Result

The number of commits to test reduces quickly, usually in a few steps.

Knowing that bisect uses a binary search means you only test a small fraction of commits, saving time.

5

Intermediate

Ending Bisect and Resetting State

Concept: Learn how to finish bisect and return to your original branch.

Once git finds the first bad commit, it shows the commit ID and message. Use 'git bisect reset' to stop bisect and return to your previous branch or commit.

Result

You know exactly which commit introduced the bug and your workspace is back to normal.

Resetting bisect cleans up your workspace, preventing confusion or accidental work on bisect commits.

6

Advanced

Automating Bisect with Scripts

🤔Before reading on: do you think bisect can run tests automatically, or must you always test manually? Commit to your answer.

Concept: Learn how to automate testing during bisect using scripts.

You can use 'git bisect run