Contributing to GStreamer

This document provides instructions and guidelines for submitting issues, feature requests and patches to GStreamer. The following applies to all these operations:

• Please use freedesktop.org GitLab to perform any of the aforementioned operations. You will need to create a freedesktop.org GitLab account if you don't have one yet (yep, that's just how it is. Sorry for the inconvenience) If you don't want to create a new account you should also be able to sign in with a Google, GitHub or Twitter account.

• Due to an influx of spam user accounts on GitLab are restricted by default and can't fork projects, which is needed to create a Merge Request.

  Please file an issue and fill in the User Verification template in order to get full access. Sorry for the hassle, and thanks for your understanding.

How to File Issues and Request for Enhancements

Where to File Issues and Feature Requests

• If your issue is security related, please see GStreamer Security Center before continuing.

• Create a new issue if there is no existing report for this problem yet. The GStreamer bugs page also has shortcuts for the major components and simple search functionality if you'd like to browse or search for existing issues, or use the GitLab search bar for the GStreamer project at the top of the page.

• If you are filing a feature request (i.e. anything that is not supposed to work already, that is anything not an issue), please add the Enhancement label. Feel free to add any other appropriate already existing labels. Please don't create new labels just for your issue. This won't affect the way we prioritise the issue, but it will make triaging easier for us. In particular, do not add the Blocker label to an issue just because an issue is important to you. This label should be added only by GStreamer maintainers.

• If your issue is about a specific plugin, element or utility library, please prefix the issue summary with element-name:, plugin-name: or lib: and keep the rest of the description as short and precise as possible.

  Examples:

  • id3demux: fails to extract composer tags
  • tsdemux: does not detect audio stream
  • Internal flow error when playing matroska file

  This makes sure developers looking through the list of open issues or issue notification mails can quickly identify what your issue is about. If your text is too long and only contains fill words at the beginning, the important information will be cut off and not show up in the list view or mail client.

• If you don't know which component to file the issue against, just pick the one that seems the most likely to you, or file it against the gstreamer-project component. If in doubt just pop into our Matrix Discussion channel. In any case, if it's not the right component someone will move the issue once they have a better idea what the problem is and where it belongs.

• Please mention:

  • what version of GStreamer you are using
  • what operating system you are using (Windows, macOS, Linux)
  • if you're on Linux, please mention your distro and distro version
  • if this is on an embedded device please provide details

• Try to describe how the issue can be reproduced. If it is triggered by any specific file, try to make the file available somewhere for download and put the link into the issue. The easier it is for us to reproduce the issue, the easier it is to fix it.

• If you experience a crash (that is: the application shuts down unexpectedly, usually with a segfault or bus error or memory access violation or such), please try to obtain a stack trace. If there are criticals or warnings printed right before the crash, run with the environment variable G_DEBUG=fatal_warnings set, then it will abort on the first warning, which should hopefully give an indication to where the problem is. You can then obtain a stack trace from where it aborts.

• If the application errors out, please provide a gst debug log. You can get one by setting the GST_DEBUG=*:6 environment variable, combined with GST_DEBUG_FILE=/tmp/dbg.log. The resulting file might end up being very large, so it's advisable to compress it with xz -9 /tmp/dbg.log before sharing. You may also be asked to provide debug logs for specific debug categories rather than everything (*:6).

How to Submit Patches

Where to Submit Patches

Patches need to be submitted through GitLab in form of a "Merge Request" (MR), which is the same as a "Pull Request" (PR) in GitHub, and uses the same workflow.

In a nutshell, you will:

1. Clone the gstreamer repository on your development machine
2. Create a fork of it in your gitlab namespace (you may have to obtain forking permission first)
3. Add that fork as a git "remote" to the clone on your machine
4. Setup a branch with the commits you want to submit
5. Push that branch to the fork in your gitlab namespace
6. Open request to merge that branch into the main repository, which is called a merge request

Pre-requisites

In order to submit a merge request you must first:

1. Sign into or create an account on the Freedesktop GitLab website
2. Setup SSH keys in the GitLab user settings
3. Clone the gstreamer repository on your development machine

Forking the gstreamer repository

Then you need to create a personal fork of gstreamer. To fork gstreamer go to the repository and hit the "Fork" button:

Select the namespace (your username), ensure that the visibility is set to "Public", and hit the "Fork project" button:

Please note that new gitlab accounts do not have permission to create forks by default, so you will have to first file an issue and fill in the user verification template as mentioned above in order to get forking rights. This should be a fairly quick process.

A new repository will be created in your user namespace (https://gitlab.freedesktop.org/$GITLAB_USERNAME/gstreamer). You will be redirected there automatically once the forking process is finished. The forking might take a few minutes.

Once this is done you can add your personal fork as new remote to the gstreamer repository clone you made in step 3 above with the following command (replace $GITLAB_USERNAME with your actual gitlab username):

git remote add $GITLAB_USERNAME git@gitlab.freedesktop.org:$GITLAB_USERNAME/gstreamer.git

Check with

git fetch $GITLAB_USERNAME

that it is accessible and working.

Common mistakes at this point are:

a) Not setting up an SSH key

b) Having a private fork. You can change your fork to public in the settings: https://gitlab.freedesktop.org/$GITLAB_USERNAME/gstreamer/edit (replace $GITLAB_USERNAME with your actual gitlab username)

Setting up your branch

Next, you make a git branch with one or more commits you want to submit for review and merging. For that you will first need a local branch which you can create with e.g.

git checkout -b fix-xyz

Then you can make your modifications and create a local commit with e.g.

git commit path/to/file1.[ch]

This will pop up an editor where you can create your commit message. It should look something like:

exampledemux: fix seeking without index in push mode

Without an index we would refuse to seek in push mode. Make
seeking without an index work by estimating the position
to seek to. It might not be 100% accurate, but better than
nothing.

Then exit the editor, and you should have a commit.

Please make sure your commits are as terse and precise as possible. Do not include 'clean-ups' or non-functional changes, since they distract from the real changes and make things harder to review, and also lower the chances that the patch will still apply cleanly to the latest version in git. If you feel there are things to clean up, please submit the clean-ups as a separate patch that does not contain any functional changes. See Writing Good Commit Messages for more information.

It's best to run git add or git commit on specific directories or files instead of using git commit -a, as it's too easy to accidentally contaminate a patch with changes that belong into it with git commit -a, in particular changes to the common submodule.

You can check your commit(s) with git show or git log -p or using a GUI such as gitg or gitk.

Make sure the author is correctly set to your full name and e-mail address.

If you haven't used git before, it would be a good idea to tell it who you are:

git config --global user.name "George S. Treamer"
git config --global user.email "george.s.treamer@example.com"

You can make changes to the last commit using:

• git commit --amend to fix up the commit message

• git commit --amend --author='John You <john@you.com>' to fix up the author

• git add path/to/file1.[ch]; git commit --amend to incorporate fixes made to the files since the last commit (i.e. what shows up in git diff). If you just want to add some of the changes, but not all of them you can use git add -p file.c, then it will ask you for each individual change whether you want to add it or leave it.

Once everything looks fine, you can push the branch to your personal fork git repository with:

git push -u $GITLAB_USERNAME

You can use

git push -u --dry-run $GITLAB_USERNAME

for testing to see what would happen without actually doing anything yet. As before, replace $GITLAB_USERNAME with your actual gitlab username.

After you have pushed the branch to your personal fork you will see a link on the terminal with which you can create a merge request for the upstream repository. It will look something like this:

$ git push -u $GITLAB_USERNAME
Enumerating objects: 4436, done.
Counting objects: 100% (4436/4436), done.
Delta compression using up to 8 threads
Compressing objects: 100% (1161/1161), done.
Writing objects: 100% (3616/3616), 1.47 MiB | 8.12 MiB/s, done.
Total 3616 (delta 2973), reused 3022 (delta 2443), pack-reused 0
remote: Resolving deltas: 100% (2973/2973), completed with 704 local objects.
remote:
remote: To create a merge request for fix-xyz, visit:
remote:   https://gitlab.freedesktop.org/$GITLAB_USERNAME/gstreamer/-/merge_requests/new?merge_request%5Bsource_branch%5D=fix-xyz
remote:
To gitlab.freedesktop.org:$GITLAB_USERNAME/gstreamer.git
 * [new branch]            fix-xyz -> fix-xyz
Branch 'fix-xyz' set up to track remote branch 'fix-xyz' from '$GITLAB_USERNAME' by rebasing.

Clicking on that link will take you to a page where you can enter a description of the changes you are submitting. If your branch contains only one commit, the title and description will be auto-filled using the commit message.

On the same page, ensure that these checkboxes are selected and create the merge request:

If you don't see the link or want to do this later, you can go to the branch list of your personal repository at https://gitlab.freedesktop.org/$GITLAB_USERNAME/gstreamer/branches and hit the 'Merge Request' button when ready. This will open a new page where you can select the source project $GITLAB_USERNAME/gstreamer + source branch fix-xyz and the target project gstreamer/gstreamer and target branch main. Then you will be able to enter a description of the changes you are submitting.

Revising your merge request

You will often need to revise the commits in your merge request based on review by maintainers of the project. You do not need to open a new merge request to do that. You need to edit the commits in your local branch and force-push it to sync the branch in your namespace. That will automatically update the merge request.

The most common case is when you have a single commit in your branch that you want to edit. To do this, first make any changes you want to do, then check the branch status:

$ git status
On branch fix-xyz
Your branch is up to date with '$GITLAB_USERNAME/fix-xyz'.

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
	modified:   subprojects/gst-docs/markdown/contribute/index.md

no changes added to commit (use "git add" and/or "git commit -a")

Next, add the changes and amend your commit:

$ git add subprojects/gst-docs/markdown/contribute/index.md
$ git commit --amend

This will open your editor and allow you to edit the commit message if necessary. Closing the editor will save the new (updated) commit.

Next, push the changes to your remote branch:

$ git push --force-with-lease

This will overwrite the old commit in your remote branch with the new commit that you just created. If this command completes successfully, that's it! The merge request has been automatically updated.

If the command fails, that means that there were changes in the remote branch that you hadn't pulled into your branch before amending your commit.

This can happen if, for example, someone uses the "Rebase" button on your merge request to rebase the commit(s) on top of the latest changes in the main branch.

In that case you need to resync the two branches. This usually requires you to understand git rebase, which is beyond the scope of this document. There's tutorials you can find online, such as git-rebase.io

In the simplest case, you might be able to get away with just doing a git pull --rebase.

Additional points

• If you are submitting a Merge Request for an issue (or multiple issues) that already exist, please add 'Fixes #123' to the commit message of one of your commits, so that there is a cross-reference in GitLab and the issue will be closed automatically when your Merge Request is merged.

• You do not have to file an issue to go with each Merge Request, it's fine to just submit a Merge Request on its own.

• If your proposed changes are proposed for review but not ready to be merged yet, please prefix the Merge Request title with WIP: for Work-in-Progress. That will prevent us from inadvertently merging it and make clear its status.

• Please make sure the 'Author' field in your commit messages has your full and proper name and e-mail address. You can check with e.g. git log or gitk.

• If your change is for an enhancement (anything that is not supposed to work already, i.e. anything not a bug) or adds new API, please add the Enhancement label. This won't affect the way we prioritise your issue, but it does make triaging easier for us.

• If your Merge Request is against a specific plugin or element or utility library, please prefix the Merge Request summary with element-name:, plugin-name: or lib: and keep the rest of the description as short and precise as possible.

  Examples:

  • id3demux: add support for WCOP frame
  • riff: add RGB16 support
  • playbin: detect if video-sink supports deinterlacing
  • tests: rtprtx unit test is racy

  This makes sure developers looking through the list of open merge requests or notification mails can quickly identify what your change is about. If your text is too long and only contains fill words at the beginning, the important information will be cut off and not show up in the list view or mail client.

• Make liberal use of the reference syntax available to help cross-linking different issues and merge requests. e.g. #100 references issue 100 in the current project, and !100 references merge request 100 in the current project. A complete list is available from gitlab's documentation.

• Please create separate merge requests for separate issues. There is no golden rule when something counts as a separate issue, please just use your best judgment. If there is an issue that requires related fixes in multiple elements or libraries, please also feel free to put everything into one issue. If you just happen to have multiple patches for us but they are not really related, please put them in separate issues and merge requests. The main question is if it makes sense to discuss and review these patches together or if they could just as well be handled completely separately.

• Please do not send patches to the gstreamer-devel mailing list. Patches submitted on the mailing list are most likely going to be ignored, overlooked, or you will get a brief reply asking you to put them into gitlab. We do not use the mailing list for patch review.

• Please do not send pull requests to our github mirror. They will be closed automatically.

• Please do not attach patches to issues, except in the special case of a fix for a security issue.

• Please prepare any merge request against a current git checkout of the GStreamer monorepo (gstreamer module), against the tip of the main branch. If a merge request is prepared against an old commit or older branch and can't be easily rebased you may be asked to rebase and update the branch on top of the main branch.

• If you have created a new plugin, please submit a merge request that adds it to subprojects/gst-plugins-bad, including any required meson.build modifications, new files and documentation.

Coding Style

Try to stick to the GStreamer indentation and coding style. There is a script called gst-indent which you can run over your .c or .cpp files if you want your code auto-indented before making the patch. The script requires GNU indent to be installed already. Please do not run gst-indent on header files, our header file indentation is free-form. If you build GStreamer from git, a local commit hook will be installed that checks if your commit conforms to the required style (also using GNU indent).

Different versions of GNU indent may occasionally yield slightly different indentations. If that happens, please ignore any indentation changes in sections of code that your patch does not touch. You can do that by staging changes selectively via git add -p. You can bypass the local indentation check hook by using git commit -n, but it will still be checked again later when you submit your changes through GitLab for merging.

We are working on making this less hasslesome.

Compiler requirements:

• we are targetting the C99 compiler and preprocesser feature subset supported by gcc 5.4, clang and vs2017. Newer features can be used conditionally as long as there is a suitable fallback. This is similar to GLib's compiler requirements.

Other style guidelines:

• do not use tabs for indentation in new code, not even in freeform header files
• do not use packed structures for parsing external data, instead use helper APIs such as GstByteReader and GstBitReader for data parsing
• from GStreamer 1.20 onwards you may in particular also
  • use c++-style // comments
  • declare variables inline (as opposed to only at the beginning of a block)
  • use advanced/nicer struct initialisers

Writing Good Commit Messages

Please take the time to write good and concise commit messages.

The first line of each commit message should be a short and concise summary of the commit. If the commit applies to a specific subsystem, library, plugin or element, prefix the message with the name of the component, for example:

oggdemux: fix granulepos query for the old theora bitstream

or

docs: add new stream API

or

tests: video: add unit test for converting RGB to XYZ colorspace

This should be a summary of the change and not a description of the change. Meaning: don't say how you did something but what you fixed, improved or changed, what the most important practical effect of the change is. Example:

qtdemux: fix crash when doing reverse playback in push mode (good)

instead of

qtdemux: use signed integer to avoid counter underrun (bad)

The second line of the commit message should be empty.

The third and following lines should contain an extensive description and rationale of the change made: what was changed, what was broken, how did it get fixed, what bugs or issues does this fix? And most importantly: why was something changed.

Trivial commits do not require a description, e.g. if you fix a memory leak it's usually enough to just say that you fixed a leak. Maybe mention what was leaked and perhaps also if it was an important leak or only happens in some corner case error code path, but in any case there's no need to write a long explanation why leaks are bad or why this needed fixing.

The important part is really what the reasoning behind the change is, since that's what people want to know if they try to figure out twelve months later why a line of code does what it does.

If the commit is related to any particular issues in gitlab, please add a reference to the issue (e.g. See #123 or Fixes #123 if it fixes it the issue). For issues in other repositories (gst-plugins-{base,good,ugly,bad} etc.) please add the full issue URL to the commit message instead (or ask for the issue to be moved to the monorepo gstreamer repository), e.g.

https://gitlab.freedesktop.org/gstreamer/gst-plugins-foo/issues/123

We do not use Signed-off by: lines in GStreamer, please create commits without such lines.

Please do not add references to private company-internal bug trackers or code repositories in commit messages.

After Submitting your Merge Request

Whenever you submit a new Merge Request, add a comment to an existing issue or Merge Request, GitLab will send a notification e-mail to GStreamer developers. This means that there is usually no need to advertise the fact that you have done so in other forums such as on Matrix or Discourse, unless you have been asked to file an issue there, in which case it's nice to follow up with the link to the issue.

Most of all, please be patient.

We try to review patches as quickly as possible, but there is such a high volume of issues, merge requests and feature requests that it is not always possible to tend to them all as quickly as we'd like. This is especially true for completely new plugins or new features.

If you haven't received any response at all for a while (say two weeks or so), do feel free to ping developers by posting a quick follow-up comment on the issue or merge request.

If you do not get a response, this is usually not a sign of people ignoring the issue, but usually just means that it's fallen through the cracks or people have been busy with other things.

Most GStreamer developers have a review workflow that's driven by e-mail or GitLab notifications, so posting a follow-up comment is the best way to draw attention to an issue or merge request.

Updating Your Merge Request and Addressing Review Comments

When someone reviews your changes, they may leave review comments for particular sections of code or in general. These will usually each start a new "Discussion" which is basically a thread for each comment.

When you believe that you have addressed the issue raised in a discussion, either by updating the code or answering the questions raised, you should "Resolve the Discussion" using the button, ideally also leaving a comment saying so (e.g. "done", "fixed", "updated", "no longer needed" or such). The comment makes sure a notification e-mail is generated, which makes it easier for GStreamer developers to keep track of what's happening.

At the top of each merge request in GitLab is a tracker with the number of unresolved discussions. This way it's easy for maintainers (and yourself) to see what's left to do and if there are any open questions/issues.

Whenever you have made changes to your patches locally you can just git push -f your-personal-gitlab-fork your-branch to your personal fork. GitLab will then pick up the changes automatically. You do not need to submit a new Merge Request whenever you make changes to an already-submitted patchset, and in fact you shouldn't do that because it means all the previous discussion context is lost and it's also not easy for reviewers to see what changed. Just force-update your existing branch.

You do not need to add individial "fixup commits" to your branch when you address issues raised. Instead just fix up the original commit(s) directly using git rebase etc. GitLab is able to track and show changes made between different revisions of a merge request branch, so just keep the branch always to the "latest clean version". See the Rewriting History section of the Pro Git book for more details.

GStreamer maintainers will typically receive e-mail notifications when you add a comment and when all oustanding discussions have been resolved. They may or may not receive e-mail notifications when you update the commits in your branch.

Workflows for GStreamer developers

Merging merge requests

Merge requests should be merged either by assigning the merge request to Marge, our merge bot, or by adding the Merge in X hours or Merge in X days labels.

Marge merge bot

Marge will rebase the merge request on top of the target branch and will also automatically add Part-of: <url> footers to each commit that point back to the merge request in gitlab.

When Marge rebases, the continuous integration (CI) pipeline will start automatically and the merge request will get merged once it passes. Since the CI pipeline is triggered by Marge in this case, it will also have the right permissions to trigger the Cerbero sub-pipeline, which won't run for monorepo pipelines triggered by external contributors.

Sidenote: Our CI pipelines don't start automatically usually, but are gated by a manual trigger in order not to waste CI resources. However, pipelines will start running automatically if Marge triggers them which will happen when Marge rebases commits. This means that when pushing updates to a merge request branch (especially if Marge touched it once already) it's usually a good idea to either remove the Part-of footers from the top commit message, or to not be entirely on top of the target branch, so that Marge is forced to rebase again when being re-assigned the merge request. Otherwise Marge will not rebase and wait for the pipeline to complete, but the pipeline won't actually run because it's not been triggered.

If Marge is assigned a merge request but nothing happens, it usually means she's busy with another merge request in the same project and target branch. Check GitLab for a list of merge requests assigned to Marge. If no other merge requests are being processed, you may be encountering the above pipeline trigger problem and may have to trigger the merge request pipeline manually.

"Merge in X days" and "Merge in X hours" labels

We have a range of Merge in X hours or Merge in X days labels available.

When these labels are added to a merge request, Marge will assign the merge request to herself after the specified amount of time has passed (roughly).

They can be useful for multiple purposes:

• Schedule merge requests for later in the day or night, when the CI farm is less busy

• Provide a heads-up to other developers that you intend to merge something, whilst still allowing some time for others to review or comment.

It's possible to subscribe to specific labels in GitLab and receive notifications when they are added to a merge request.

When Marge picks up the label and assigns merge requests to itself, a notification will be generated. When merging fails, a notification will also be generated, which helps make sure things don't fall through the cracks once they've been scheduled for merging.

It's of course fine for another developer to assign a merge request with a Merge in X label to Marge immediately if they think it's fine to go in now and don't expect further comments or review being needed by others.

Backporting to a stable branch

Before backporting any changes to a stable branch, they should first be applied to the main branch, and should obviously not have caused any known outstanding regressions. The only exception here are changes that do not apply to the main branch anymore.

You do not need to create backport merge requests against stable branches. Backport merge requests for stable branches will be created automatically based on labels on main branch merge requests.

Existing merge request against the main branch, including merged ones, that should be considered for backporting in the future should be labeled with the Needs backport label and the label for the target stable branch (e.g. 1.18). All merge requests with the Needs backport label will be regularly considered for backporting by GStreamer developers.

Changes that add API or change existing API will usually not be considered for backporting.

In case patches don't apply cleanly to the target stable branch, draft WIP merge requests will be created by the above-mentioned scripts. If that happens developers will have to backport the desired commits and resolve any cherry-pick conflicts manually. The result should then be pushed into the existing branch for the WIP backport merge request. Any GStreamer developer should be able to do this, and help in this area is always welcome. Please resolve the WIP/Draft status once all the relevant commits have been backported and assign the merge request to the merge bot for merging.

There is also a Maybe backport label which can be added if you're not sure if commits should be backported or not, or you want to revisit it later once the changes in question have seen some testing in the development branch. If you tag a merge request with Maybe backport it's usually useful to add a comment about why it's being tagged only as a maybe. That will make it easier for others to evaluate whether it should be backported or not later.