Attention: Here be dragons
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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.
Danger
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.
Note
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.)
Note that compiling Godot can take a while on slow hardware (up an hour for each full rebuild on a slow dual-core CPU). This means the full process can take up to several hours. If your hardware is too slow, you may want to stop there and report the results of your "pre-bisecting" on the GitHub issue so another contributor can continue bisecting from there.
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>
Compile Godot. This assumes you've set up a build environment:
scons
Run the engine
Run the binary located in the bin/ folder and try to reproduce the bug.
Note
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.
If the build still exhibits the bug, run the following command:
git bisect bad
If the build does not exhibit the bug, run the following command:
git bisect good
After entering one of the commands above, Git will switch to a different commit.
You should now build Godot again, try to reproduce the bug, then enter git
bisect good or git bisect bad depending on the result. You'll have to
repeat this several times. The longer the commit range, the more steps will be
required. 5 to 10 steps are usually sufficient to find most regressions; Git
will remind you of the number of steps remaining (in the worst case scenario).
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 :)
See also
You can read the full documentation on git bisect
here.