## 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](https://github.com/checkpoint-restore/criu/issues?q=is%3Aissue+is%3Aopen+label%3Aenhancement) and [more sophisticated](https://github.com/checkpoint-restore/criu/issues?q=is%3Aissue+is%3Aopen+label%3A%22new+feature%22) coding tasks;
* CRIU does need [extensive testing](https://github.com/checkpoint-restore/criu/issues?q=is%3Aissue+is%3Aopen+label%3Atesting);
* Documentation is always hard, we have [some information](https://criu.org/Category:Empty_articles) that is to be extracted from people's heads into wiki pages as well as [some texts](https://criu.org/Category:Editor_help_needed) that all need to be converted into useful articles;
* Feedback is expected on the GitHub issues page and on the [mailing list](https://lore.kernel.org/criu);
* 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](https://lore.kernel.org/criu).
Below we describe in more detail recommend practices for CRIU development.
* Spread the word about CRIU in [social networks](http://criu.org/Contacts);
* If you're giving a talk about CRIU -- let us know, we'll mention it on the [wiki main page](https://criu.org/News/events);

### Setting up the development environment

Although `criu` could be run as non-root (see [Security](https://criu.org/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
```

### Building from source

Follow these steps to compile CRIU from source code.

#### Installing build dependencies

First, you need to install the required build dependencies. We provide scripts to simplify this process for several Linux distributions in [contrib/dependencies](contrib/dependencies). For a complete list of dependencies, please refer to the [installation guide](https://criu.org/Installation).

##### On Ubuntu/Debian-based systems:

```
./contrib/dependencies/apt-packages.sh
```

##### On Fedora/CentOS-based systems:

```
./contrib/dependencies/dnf-packages.sh
```

##### Using Nix:

```
nix develop
```

#### Compiling CRIU

Once the dependencies are installed, you can compile CRIU by running the `make` command from the root of the source directory:

```
make
```

This should create the `./criu/criu` executable.

## Edit the source code

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

* code is written to be read, so the code readability is the most important thing you need to have in mind when preparing patches
* we prefer tabs and indentations to be 8 characters width
* we prefer line length of 80 characters or less, more is allowed if it helps with code readability
* CRIU mostly follows [Linux kernel coding style](https://www.kernel.org/doc/Documentation/process/coding-style.rst), 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.

## Automatic tools to fix coding-style

Important: These tools are there to advise you, but should not be considered as a "source of truth", as tools also make nasty mistakes from time to time which can completely break code readability.

The following command can be used to automatically run a code linter for Python files (ruff), 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](https://www.kernel.org/doc/Documentation/process/clang-format.rst)
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.

Here are some bad examples of clang-format-ing:

* if clang-format tries to force 120 characters and breaks readability - it is wrong:

```
@@ -58,8 +59,7 @@ static int register_membarriers(void)
         }

         if (!all_ok) {
-                fail("can't register membarrier()s - tried %#x, kernel %#x",
-                     barriers_registered, barriers_supported);
+                fail("can't register membarrier()s - tried %#x, kernel %#x", barriers_registered, barriers_supported);
                 return -1;
         }
```

* if clang-format breaks your beautiful readability friendly alignment in structures, comments or defines - it is wrong:

```
--- a/test/zdtm/static/membarrier.c
+++ b/test/zdtm/static/membarrier.c
@@ -27,9 +27,10 @@ static const struct {
        int register_cmd;
        int execute_cmd;
 } membarrier_cmds[] = {
-       { "",           MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED,           MEMBARRIER_CMD_PRIVATE_EXPEDITED },
-       { "_SYNC_CORE", MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE, MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE },
-       { "_RSEQ",      MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ,      MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ },
+       { "", MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED, MEMBARRIER_CMD_PRIVATE_EXPEDITED },
+       { "_SYNC_CORE", MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE,
+         MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE },
+       { "_RSEQ", MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ, MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ },
 };
```

## 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](https://criu.org/ZDTM_Test_Suite). Check for any error messages produced by it.

## 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](#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](https://chris.beams.io/posts/git-commit/) 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](https://github.com) and create a [pull request](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/about-pull-requests)

### 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 --to=criu@lists.linux.dev 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 `--to=criu@lists.linux.dev` all the time,
you can configure that to be automatically handled as well:

```
git config sendemail.to criu@lists.linux.dev
```

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 lists.linux.dev`. Note that you need to be subscribed first in order to post. The list web interface is available at https://lore.kernel.org/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](https://criu.org/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.