Good Commit Practice

The following document is a fork of OpenStack's Git Commit Good Practice, rewritten to suit PayEx needs. It is based on experience doing code development, bug troubleshooting and code review across a number of projects using Git. Examination of other open source projects suggested they all follow a fairly common practice. It is motivated by a desire to improve the quality of the Git history in any repository. Quality is a hard term to define in computing; one person's "Thing of Beauty" is another's "Evil Hack". We can, however, come up with some general guidelines for what to do, or conversely what not to do, when publishing Git commits for merge with a project. This topic can be split into two areas of concern:

1. The structured set/split of the code changes
2. The information provided in the commit message

Executive Summary

The points and examples that will be raised in this document ought to clearly demonstrate the value in splitting up changes into a sequence of individual commits, and the importance in writing good commit messages to go along with them. If these guidelines were widely applied it would result in a significant improvement in the quality of the  Git history. Both a carrot and a stick will be required to effect changes. This document intends to be the carrot by alerting people to the benefits, while for anyone doing  code reviews, it can act as the stick.

In other words, when reviewing a change, do not simply look at the correctness of the code. Review the commit message itself and request improvements to its content. Look out for commits which are mixing multiple logical changes and require the submitter to split them into separate commits. Ensure whitespace changes are not mixed in with functional changes. Ensure no-op code refactoring is done separately from functional changes. And so on.

Software source code is "read mostly, write occassionally" and thus the most important criteria is to improve the long term maintainability by the large pool of developers in the community, and not to sacrifice too much for the sake of the single author who may never touch the code again.

And now the long detailed guidelines and examples of good and bad practice.

Structural split of changes

The cardinal rule for creating good commits is to ensure there is only one "logical change" per commit. There are many reasons why this is an important rule:

• The smaller the amount of code being changed, the quicker and easier it is to review and identify potential flaws.
• If a change is found to be flawed later, it may be necessary to revert the broken commit. This is much easier to do if there are not other unrelated code changes entangled with the original commit.
• When troubleshooting problems using Git's bisect capability, small well defined changes will aid in isolating exactly where the code problem was introduced.
• When browsing history using Git annotate/blame, small well defined changes also aid in isolating exactly where & why a piece of code came from.

Things to avoid when creating commits

With that in mind, there are some commonly encountered examples of bad things to avoid:

Mixing formatting changes with functional code changes

The formatting (code style, whitespace, etc.) changes will obscure the important functional changes, making it harder for a reviewer to correctly determine whether the change is correct.
Solution: Create 2 commits, one with the formatting changes and one with the functional changes. Typically the formatting change would be done first, but that need not be a hard rule.

Mixing two unrelated functional changes

Again the reviewer will find it harder to identify flaws if two unrelated changes are mixed together. If it becomes necessary to later revert a broken commit, the two unrelated changes will need to be untangled, with further risk of bug creation.

Sending large new features in a single giant commit

It may well be the case that the code for a new feature is only useful when all of it is present. This does not, however, imply that the entire feature should be provided in a single commit. To group related code changes that for the reasons explained here are split up into several separate commits, intuitively named branches should be used instead.

New features often entail refactoring existing code. It is highly desirable that any refactoring is done in commits which are separate from those implementing the new feature. This helps reviewers and test suites validate that the refactoring has no unintentional functional changes.

Even the newly written code can often be split up into multiple pieces that can be independently reviewed. For example, changes which add new internal APIs/classes, can be in self-contained commits. Again this leads to easier code review. It also allows other developers to cherry-pick small parts of the work, if the entire new feature is not immediately ready for merge.

Addition of new public HTTP APIs or RPC interfaces should be done in commits separate from the actual internal implementation. This will encourage the author & reviewers to think about the generic API/RPC design, and not simply pick a design that is easier for their currently chosen internal implementation. If patch impacts a public HTTP, use the APIImpact flag (see [including external references](#user-content-including-external-references)).

The basic rule to follow is:

 If a code change can be split into a sequence of patches/commits, then it
 should be split. Less is *not* more. More is more.

Examples of bad practice

Now for some illustrations from Nova history. NB, although commit hashes are quoted for reference, author names are removed, since no single person needs to be blamed/picked on. Almost everybody is guilty of violating these good practice rules at some time or another. In addition the people who reviewed and approved these commits are just as guilty as the person who wrote/submitted them.

There are at least two independent changes made in this commit.

1. The switch to use the new reset API for the hard_reboot method.
2. The adjustment to internal driver methods to not use hard_reboot.

What is the problem with this?

• First there is no compelling reason why these changes needed to be made at the same time. A first commit could have included the changes to stop calling hard_reboot in various places. A second commit could have re-written the hard_reboot impl.
• Second, as the switch to using the libvirt reset method was buried in the large code refactoring, reviewers missed the fact that this was introducing a dependency on a newer libvirt API version. This commit was identified as the culprit reasonably quickly, but a trivial revert is not possible, due to the wide variety of unrelated changes included.

This is introducing one major new feature, so on the surface it seems reasonable to use a single commit, but looking at the patch, it clearly has entangled a significant amount of code refactoring with the new LVM feature code. This makes it hard to identify likely regressions in support for QCow2/Raw images. This should have been split into at least four separate commits:

1. Replace the use_cow_images config FLAG with the new FLAG libvirt_local_images_type, with back-compat code for support of legacy use_cow_images FLAG 2. Creation of internal "Image" class and subclasses for Raw & QCow2 image type impls.
2. Refactor libvirt driver to replace raw/qcow2 image management code, with calls to the new Image class APIs.
3. Introduce the new "LVM" Image class implementation.

Examples of good practice

Together these two changes provide support for configuring the KVM guest timers. The introduction of the new APIs for creating libvirt XML configuration have been clearly separated from the change to the KVM guest creation policy, which uses the new APIs.

This sequence of commits refactored the entire RPC API layer inside nova to allow pluggable messaging implementations. With such a major change in a core piece of functionality, splitting up the work into a large sequence of commits was key to be able to do meaningful code review, and track / identify possible regressions at each step of the process.

Information in commit messages

As important as the content of the change, is the content of the commit message describing it. When writing a commit message there are some important things to remember:

Do not assume the reviewer understands what the original problem was

When reading bug reports, after a number of back & forth comments, it is often as clear as mud, what the root cause problem is. The commit message should have a clear statement as to what the original problem is. The bug is merely interesting historical background on *how* the problem was identified. It should be possible to review a proposed patch for correctness without needing to read the bug ticket.

Do not assume the reviewer has access to external web services/site

In 6 months time when someone is on a train/plane/coach/beach/pub troubleshooting a problem & browsing Git history, there is no guarantee they will have access to the online bug tracker, or online blueprint documents. The great step forward with distributed SCM is that you no longer need to be "online" to have access to all information about the code repository. The commit message should be totally self-contained, to maintain that benefit.

Do not assume the code is self-evident/self-documenting

What is self-evident to one person, might be clear as mud to another person. Always document what the original problem was and how it is being fixed, for any change except the most obvious typos, or whitespace only commits.

Describe *why* a change is being made

A common mistake is to just document how the code has been written, without describing *why* the developer chose to do it that way. By all means describe the overall code structure, particularly for large changes, but more importantly describe the intent/motivation behind the changes.

Read the commit message to see if it hints at improved code structure

Often when describing a large commit message, it becomes obvious that a commit should have in fact been split into 2 or more parts. Don't be afraid to go back and rebase the change to split it up into separate commits.

Ensure sufficient information to decide whether to review

When Gerrit sends out email alerts for new patch submissions there is minimal information included, principally the commit message and the list of files changes. Given the high volume of patches, it is not reasonable to expect all reviewers to examine the patches in detail. The commit message must thus contain sufficient information to alert the potential reviewers to the fact that this is a patch they need to look at.

The first commit line is the most important

In Git commits the first line of the commit message has special significance. It is used as email subject line, git annotate messages, gitk viewer annotations, merge commit messages and many more places where space is at a premium. As well as summarizing the change itself, it should take care to detail what part of the code is affected. eg if it affects the libvirt driver, mention 'libvirt' somewhere in the first line.

Describe any limitations of the current code

If the code being changed still has future scope for improvements, or any known limitations then mention these in the commit message. This demonstrates to the reviewer that the broader picture has been considered and what tradeoffs have been done in terms of short term goals vs. long term wishes.

Do not include patch set-specific comments

In other words, if you rebase your change please don't add "Patch set 2: rebased" to your commit message.  That isn't going to be relevant once your change has merged.  Please *do* make a note of that in Gerrit as a comment on your change, however.  It helps reviewers know what changed between patch sets.  This also applies to comments such as "Added unit tests", "Fixed localization problems", or any other such patch set to patch set changes that don't affect the overall intent of your commit.

The main rule to follow is:

The commit message must contain all the information required to fully understand & review the patch for correctness. Less is not more. More is more.

Including external references

The commit message is primarily targeted towards human interpretation, but there is always some metadata provided for machine use. For source code hosted on GitHub, a reference to an issue can be made by simply writin `#<issue-number>`.

All machine targeted metadata is of secondary consequence to humans and thus it should preferably be grouped together at the end of the commit message. For simple references like GitHub issues, this is not required.

Note: Although it is common practice across many open source projects using Git to include a "Signed-off-by" tag (generated by 'git commit -s'), this is not required. Prior to gaining the ability to submit code, it should rather be required that all contributors sign a CLA, which serves an equivalent purpose.

We encourage the use of Co-Authored-By: name <name@example.com> in commit messages to indicate people who worked on a particular patch. It's a convention for recognizing multiple authors, and our projects would encourage the stats tools to observe it when collecting statistics.

Summary of Git commit message structure

• Provide a brief description of the change in the first line.
• Insert a single blank line after the first line.
• Provide a detailed description of the change in the following lines, breaking paragraphs where needed.
• The first line should be limited to 50 characters and should not end with a period.
• Subsequent lines should be wrapped at 72 characters.
• vim (the default $EDITOR on most distros) can wrap automatically lines for you. In most cases you just need to copy the example vimrc file (which can be found somewhere like /usr/share/vim/vim74/vimrc_example.vim) to /.vimrc if you don't have one already.
• After editing a paragraph, you can re-wrap it by pressing escape, ensuring the cursor is within the paragraph and typing gqip.
• Put external references at the very end of the commit message.

For example:

Some examples of bad practice

Now for some illustrations from Nova history, again with authors names removed since no one person is to blame for these.

Problem: this does not mention what imports where missing and why they were needed. This info was actually in the bug tracker, and should have been copied into the commit message, so that it would provide a self-contained description. e.g.:

Problem: this does not mention what the current (broken) format is, nor what the new fixed format is. Again this info was available in the bug tracker and should have been included in the commit message. Furthermore, this bug was fixing a regression caused by an earlier change, but there is no mention of what the earlier change was. e.g.:

Problem: This commit message is merely documenting what was done, and not why it was done. It should have mentioned what earlier changeset introduced the new min libvirt version. It should also have mentioned what behaviour is when the check fails. e.g.:

Examples of good practice

Some things to note about this example commit message

- It describes what the original problem is (bad KVM defaults)
- It describes the functional change being made (the new PIT/RTC policies)
- It describes what the result of the change is (new the XML/QEMU args)
- It describes scope for future improvement (the possible per-OS type config)
- It uses the Closes-Bug notation

Some things to note about this example commit message

• It describes what the problem scenario is (mixed arch deployments)
• It describes the intent of the fix  (make the schedular filter on arch)
• It describes the rough architecture of the fix (how libvirt returns arch)
• It notes the limitations of the fix (work needed on Xen)