How to contribute to CRIU

CRIU project is (almost) the never-ending story, because we have to always keep up with the Linux kernel supporting checkpoint and restore for all the features it provides. Thus we're looking for contributors of all kinds -- feedback, bug reports, testing, coding, writing, etc. Here are some useful hints to get involved.

• We have both -- very simple and more sophisticated coding tasks;
• CRIU does need extensive testing;
• Documentation is always hard, we have some information that is to be extracted from people's heads into wiki pages as well as some texts that all need to be converted into useful articles;
• Feedback is expected on the GitHub issues page and on the mailing list;
• We accept GitHub pull requests and this is the preferred way to contribute to CRIU. If you prefer to send patches by email, you are welcome to send them to CRIU development mailing list. Below we describe in more detail recommend practices for CRIU development.
• Spread the word about CRIU in social networks;
• If you're giving a talk about CRIU -- let us know, we'll mention it on the wiki main page;

Setting up the development environment

Although criu could be run as non-root (see Security), development is better to be done as root. For example, some tests require root. So, it would be a good idea to set up some recent Linux distro on a virtual machine.

Get the source code

The CRIU sources are tracked by Git. Official CRIU repo is at https://github.com/checkpoint-restore/criu.

The repository may contain multiple branches. Development happens in the criu-dev branch.

To clone CRIU repo and switch to the proper branch, run:

        git clone https://github.com/checkpoint-restore/criu criu
        cd criu
        git checkout criu-dev

Compile

First, you need to install compile-time dependencies. Check Installation dependencies for more info.

To compile CRIU, run:

        make

This should create the ./criu/criu executable.

Edit the source code

If you use ctags, you can generate the ctags file by running

        make tags

When you change the source code, please keep in mind the following code conventions:

• we prefer tabs and indentations to be 8 characters width
• CRIU mostly follows Linux kernel coding style, but we are less strict than the kernel community.

Other conventions can be learned from the source code itself. In short, make sure your new code looks similar to what is already there.

The following command can be used to automatically run a code linter for Python files (flake8), Shell scripts (shellcheck), text spelling (codespell), and a number of CRIU-specific checks (usage of print macros and EOL whitespace for C files).

         make lint

In addition, we have adopted a clang-format configuration file based on the kernel source tree. However, compliance with the clang-format autoformat rules is optional. If the automatic code formatting results in decreased readability, we may choose to ignore these errors.

Run the following command to check if your changes are compliant with the clang-format rules:

         make indent

This command is built upon the git-clang-format tool and supports two options BASE and OPTS. The BASE option allows you to specify a range of commits to check for coding style issues. By default, it is set to HEAD~1, so that only the last commit is checked. If you are developing on top of the criu-dev branch and want to check all your commits for compliance with the clang-format rules, you can use BASE=origin/criu-dev. The OPTS option can be used to pass additional options to git-clang-format. For example, if you want to check the last N commits for formatting errors, without applying the changes to the codebase you can use the following command.

         make indent OPTS=--diff BASE=HEAD~N

Note that for pull requests, the "Run code linter" workflow runs these checks for all commits. If a clang-format error is detected we need to review the suggested changes and decide if they should be fixed before merging.

Test your changes

CRIU comes with an extensive test suite. To check whether your changes introduce any regressions, run

         make test

The command runs ZDTM Test Suite. Check for any error messages produced by it.

In case you'd rather have someone else run the tests, you can use travis-ci for your own GitHub fork of CRIU. It will check the compilation for various supported platforms, as well as run most of the tests from the suite. See https://travis-ci.org/checkpoint-restore/criu for more details.

Describe your changes

Describe your problem. Whether your change is a one-line bug fix or 5000 lines of a new feature, there must be an underlying problem that motivated you to do this work. Convince the reviewer that there is a problem worth fixing and that it makes sense for them to read past the first paragraph.

Once the problem is established, describe what you are actually doing about it in technical detail. It's important to describe the change in plain English for the reviewer to verify that the code is behaving as you intend it to.

Solve only one problem per commit. If your description starts to get long, that's a sign that you probably need to split up your commit. See Separate your changes.

Describe your changes in imperative mood, e.g. "make xyzzy do frotz" instead of "[This commit] makes xyzzy do frotz" or "[I] changed xyzzy to do frotz", as if you are giving orders to the codebase to change its behaviour.

If your change fixes a bug in a specific commit, e.g. you found an issue using git bisect, please use the Fixes: tag with the abbreviation of the SHA-1 ID, and the one line summary. For example:

	Fixes: 9433b7b9db3e ("make: use cflags/ldflags for config.h detection mechanism")

The following git config settings can be used to add a pretty format for outputting the above style in the git log or git show commands:

	[pretty]
		fixes = Fixes: %h (\"%s\")

If your change address an issue listed in GitHub, please use Fixes: tag with the number of the issue. For instance:

	Fixes: #339

The Fixes: tags should be put at the end of the detailed description.

Please add a prefix to your commit subject line describing the part of the project your change is related to. This can be either the name of the file or directory you changed, or just a general word. If your patch is touching multiple components you may separate prefixes with "/"-es. Here are some good examples of subject lines from git log:

criu-ns: Convert to python3 style print() syntax
compel: Calculate sh_addr if not provided by linker
style: Enforce kernel style -Wstrict-prototypes
rpc/libcriu: Add lsm-profile option

You may refer to How to Write a Git Commit Message article for recommendations for good commit message.

Separate your changes

Separate each logical change into a separate commit.

For example, if your changes include both bug fixes and performance enhancements for a single driver, separate those changes into two or more commits. If your changes include an API update, and a new driver which uses that new API, separate those into two commits.

On the other hand, if you make a single change to numerous files, group those changes into a single commit. Thus a single logical change is contained within a single commit.

The point to remember is that each commit should make an easily understood change that can be verified by reviewers. Each commit should be justifiable on its own merits.

When dividing your change into a series of commits, take special care to ensure that CRIU builds and runs properly after each commit in the series. Developers using git bisect to track down a problem can end up splitting your patch series at any point; they will not thank you if you introduce bugs in the middle.

Sign your work

To improve tracking of who did what, we ask you to sign off the commits in your fork of CRIU or the patches that are to be emailed.

The sign-off is a simple line at the end of the explanation for the patch, which certifies that you wrote it or otherwise have the right to pass it on as an open-source patch. The rules are pretty simple: if you can certify the below:

Developer's Certificate of Origin 1.1

By making a contribution to this project, I certify that:

(a) The contribution was created in whole or in part by me and I
    have the right to submit it under the open source license
    indicated in the file; or

(b) The contribution is based upon previous work that, to the best
    of my knowledge, is covered under an appropriate open source
    license and I have the right under that license to submit that
    work with modifications, whether created in whole or in part
    by me, under the same open source license (unless I am
    permitted to submit under a different license), as indicated
    in the file; or

(c) The contribution was provided directly to me by some other
    person who certified (a), (b) or (c) and I have not modified
    it.

(d) I understand and agree that this project and the contribution
    are public and that a record of the contribution (including all
    personal information I submit with it, including my sign-off) is
    maintained indefinitely and may be redistributed consistent with
    this project or the open source license(s) involved.

then you just add a line saying

        Signed-off-by: Random J Developer <random at developer.example.org>

using your real name (please, no pseudonyms or anonymous contributions if it possible).

Hint: you can use git commit -s to add Signed-off-by line to your commit message. To append such line to a commit you already made, use git commit --amend -s.

 From: Random J Developer <random at developer.example.org>
 Subject: [PATCH] component: Short patch description

 Long patch description (could be skipped if patch
 is trivial enough)

 Signed-off-by: Random J Developer <random at developer.example.org>
 ---
 Patch body here

Submit your work upstream

We accept GitHub pull requests and this is the preferred way to contribute to CRIU. For that you should push your work to your fork of CRIU at GitHub and create a pull request

Pull request guidelines

Pull request comment should contain description of the problem your changes solve and a brief outline of the changes included in the pull request.

Please avoid pushing fixup commits to an existent pull request. Each commit should be self contained and there should not be fixup commits in a patch series. Pull requests that contain one commit which breaks something and another commit which fixes it, will be rejected.

Please merge the fixup commits into the commits that has introduced the problem before creating a pull request.

It may happen that the reviewers were not completely happy with your changes and requested changes to your patches. After you updated your changes please close the old pull request and create a new one that contains the following:

• Description of the problem your changes solve and a brief outline of the changes
• Link to the previous version of the pull request
• Brief description of the changes between old and new versions of the pull request. If there were more than one previous pull request, all the revisions should be listed. For example:

	v3: rebase on the current criu-dev
	v2: add commit to foo() and update bar() coding style

If there are only minor updates to the commits in a pull request, it is possible to force-push them into an existing pull request. This only applies to small changes and should be used with care. If you update an existing pull request, remember to add the description of the changes from the previous version.

Mailing list submission

Historically, CRIU worked with mailing lists and patches so if you still prefer this way continue reading till the end of this section.

Make a patch

To create a patch, run

    git format-patch --signoff origin/criu-dev

You might need to read GIT documentation on how to prepare patches for mail submission. Take a look at http://book.git-scm.com/ and/or http://git-scm.com/documentation for details. It should not be hard at all.

We recommend to post patches using git send-email

  git send-email --cover-letter --no-chain-reply-to --annotate \
                 --confirm=always [email protected] criu-dev

Note that the git send-email subcommand may not be in the main git package and using it may require installation of a separate package, for example the "git-email" package in Fedora and Debian.

If this is your first time using git send-email, you might need to configure it to point it to your SMTP server with something like:

    git config --global sendemail.smtpServer stmp.example.net

If you get tired of typing [email protected] all the time, you can configure that to be automatically handled as well:

    git config sendemail.to [email protected]

If a developer is sending another version of the patch (e.g. to address review comments), they are advised to note differences to previous versions after the --- line in the patch so that it helps reviewers but doesn't become part of git history. Moreover, such patch needs to be prefixed correctly with --subject-prefix=PATCHv2 appended to git send-email (substitute v2 with the correct version if needed though).

Mail patches

The patches should be sent to CRIU development mailing list, criu AT openvz.org. Note that you need to be subscribed first in order to post. The list web interface is available at https://openvz.org/mailman/listinfo/criu; you can also use standard mailman aliases to work with it.

Please make sure the email client you're using doesn't screw your patch (line wrapping and so on).

Note: When sending a patch set that consists of more than one patch, please, push your changes in your local repo and provide the URL of the branch in the cover-letter

Wait for response

Be patient. Most CRIU developers are pretty busy people so if there is no immediate response on your patch — don't be surprised, sometimes a patch may fly around a week before it gets reviewed.

Continuous integration

Wiki article: Continuous integration

CRIU tests are run for each series sent to the mailing list. If you get a message from our patchwork that patches failed to pass the tests, you have to investigate what is wrong.

We also recommend you to enable Travis CI for your repo to check patches in your git branch, before sending them to the mailing list.