Bisecting regressions

Bisecting is a way to find regressions in software. After reporting a bug on the Godot repository on GitHub, you may be asked by a contributor to bisect the issue. Bisecting makes it possible for contributors to fix bugs faster, as they can know in advance which commit caused the regression. Your effort will be widely appreciated :)

The guide below explains how to find a regression by bisecting.

What is bisecting?

Godot developers use the Git version control system. In the context of Git, bisecting is the process of performing a manual binary search to determine when a regression appeared. While it's typically used for bugs, it can also be used to find other kinds of unexpected changes such as performance regressions.

Using official builds to speed up bisecting

Before using Git's bisect command, we strongly recommend trying to reproduce the bug with an older (or newer) official release. This greatly reduces the range of commits that potentially need to be built from source and tested. You can find binaries of official releases, as well as alphas, betas, and release candidates here.

危険

Project files may be incompatible between Godot versions. Make a backup of your project before starting the bisection process.

Going from the oldest to the newest build generally reduces the risk of the project not being able to successfully open in the editor, thanks to backwards compatibility. Try to reduce your project to the smallest repeatable example too. The more minimal the project is, the more likely you'll be able to open it without compatibility issues in newer engine versions.

The Git bisect command

If you've found a build that didn't exhibit the bug in the above testing process, you can now start bisecting the regression. The Git version control system offers a built-in command for this: git bisect. This makes the process semi-automated as you only have to build the engine, run it and try to reproduce the bug.

注釈

Before bisecting a regression, you need to set up a build environment to compile Godot from source. To do so, read the Compiling page for your target platform. (Compiling Godot from source doesn't require C++ programming knowledge.)

遅いハードウェアでは、Godotのコンパイルに時間がかかることに注意してください (遅いデュアルコアCPUでは、フルリビルドするたびに1時間かかります)。つまり、フルプロセスには数時間かかる可能性があります。ハードウェアの速度が遅すぎる場合は、ここで止めて GitHub issue で "pre-bisecting" の結果を報告すると、別の貢献者がそこからバイセクティングを続けられるようになります。

Determine the commit hashes

To start bisecting, you must first determine the commit hashes (identifiers) of the "bad" and "good" build. "bad" refers to the build that exhibits the bug, whereas "good" refers to the version that doesn't exhibit the bug.

You can use either a commit hash (like 06acfccf8), the tag of a stable release (like 4.2.1-stable), or a branch like master.

If you're using a pre-release build as the "good" or "bad" build, you can find the commit hash in the Project Manager in the lower-right corner, or in in the Help > About Godot dialog in the Godot editor. The version information will look something like v4.4.beta3.official [06acfccf8], and the commit hash is within the brackets, in this case 06acfccf8. You can click on the version information to copy it, including the commit hash.

Alternately, you can browse the interactive changelog to find commits for all releases, including development builds. The commits will be listed as a range, like commits: a013481b0...06acfccf8, and the second commit is the one you should use for bisecting. You can also browse the Godot Archive, and find the commit hash within the release page linked from the News button.

If you're using a stable release as the "good" or "bad" build, you can use the tag of that release directly, such as 4.2-stable or 4.2.1-stable. A full list of release tags is available on GitHub, and you can also find the actual commit hash that corresponds to a stable release there.

To refer to the latest state of the master branch, you can use master instead of a commit hash. Note that unlike tagged releases or snapshot commit hashes, master is a perpetually moving target.

Build the engine

Get Godot's source code using Git. Once this is done, in the terminal window, use cd to reach the Godot repository folder and enter the following command:

# <good commit hash> is hash of the build that works as expected.
# <bad commit hash> is hash of the build exhibiting the bug.
git bisect start
git bisect good <good commit hash>
git bisect bad <bad commit hash>

Godotをコンパイルします。これは、ビルド環境を設定したことを前提としています。

scons

Run the engine

bin/ フォルダにあるバイナリを実行して、バグを再現してみてください。

注釈

Double-check the output file name in bin/ to make sure you're actually running the binary you've just compiled. Different Godot versions will output binaries with different names.

それでもそのビルドにバグが現れる場合は、以下のコマンドを実行してください:

git bisect bad

ビルドしてもバグが発生しない場合は、以下のコマンドを実行してください:

git bisect good

上記のいずれかのコマンドを入力すると、Git は別のコミットに切り替わります。もう一度 Godot をビルドしてバグを再現してみて、結果に応じて git bisect good または git bisect bad を入力してください。これを何度か繰り返す必要があります。コミット範囲が長くなればなるほど、より多くのステップが必要になります。ほとんどのリグレッションを見つけるには、通常 5 から 10 のステップで十分です。Gitは、(最悪の場合における)残りのステップ数を教えてくれます。

Once you've completed enough steps, Git will display the commit hash where the regression appeared. Write this commit hash as a comment to the GitHub issue you've bisected. This will help in solving the issue. Thanks again for contributing to Godot :)

参考

You can read the full documentation on git bisect here.