Philipp Hancke c8f3c68618 docs: how to do stacked diffs

documenting some arcane but highly useful knowledge.

Bug: None
No-Try: true
Change-Id: I866f5d5bd2464400d4a10afcbbe550fc4d9ef9c7
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/394005
Reviewed-by: Elad Alon <eladalon@webrtc.org>
Commit-Queue: Philipp Hancke <phancke@meta.com>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#44842}

2025-06-05 07:58:31 -07:00

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Contributing to the WebRTC project

License Agreement

WebRTC welcomes patches for features and bug fixes!

For contributors external to Google, follow the instructions given in the Google Individual Contributor License Agreement. In all cases, contributors must sign a contributor license agreement before a contribution can be accepted. Please complete the agreement for an individual or a corporation as appropriate.

Instructions

Contributing your First Patch

You must do some preparation in order to upload your first CL:

Check out and build the code
Fill in the Contributor agreement (see above)
If you’ve never submitted code before, you must add your (or your organization’s in the case the contributor agreement is signed by your organization) name and contact info to the AUTHORS file
Go to https://webrtc.googlesource.com/new-password and login with your email account. This should be the same account as returned by git config user.email
Then, run: git cl creds-check. If you get any errors, ask for help on discuss-webrtc

You will not have to repeat the above. After all that, you’re ready to upload:

Uploading your First Patch

Now that you have your account set up, you can do the actual upload:

Do this:
- Assuming you're on the main branch:
  - git checkout -b my-work-branch
- Make changes, build locally, run tests locally
  - git commit -am "Changed x, and it is working"
  - git cl upload
  This will open a text editor showing all local commit messages, allowing you to modify it before it becomes the CL description.
  
  Fill out the bug entry properly. Please specify the issue tracker prefix and the issue number, separated by a colon, e.g. webrtc:123 or chromium:12345. If you do not have an issue tracker prefix and an issue number just add None.
  
  Save and close the file to proceed with the upload to the WebRTC code review server.
  
  The command will print a link like https://webrtc-review.googlesource.com/c/src/+/53121. if everything goes well.
Click this CL Link
If you’re not signed in, click the Sign In button in the top right and sign in with your email
Click Start Review and add a reviewer. You can find reviewers in OWNERS files around the repository (take the one closest to your changes)
Address any reviewer feedback:
- Make changes, build locally, run tests locally
  - git commit -am "Fixed X and Y"
  - git cl upload
Once the reviewer LGTMs (approves) the patch, ask them to put it into the commit queue

NOTICE: On Windows, you’ll need to run the above in a Git bash shell in order for gclient to find the .gitcookies file.

Stacked Diffs

Stacked diffs are an advanced feature to "stack" multiple gerrit changes on top of each other. They show up as separate changes in gerrit codereview but the relationship is retained. To create a stacked diff:

Assuming you are on the main branch:
- git checkout -b my-first-work-branch
Make changes, build locally, run tests locally
- git commit -am "Changed x, and it is working"
- git cl upload
Checkout the second branch from the current branch:
- git checkout -b my-second-work-branch
Set the upstream branch to the first branch:
- git cl upstream my-first-work-branch
Make changes, build locally, run tests locally
- git commit -am "Changed y, and it is working too"
- git cl upload
You can now run tests or ask for review of my-second-work-branch independently of my-first-work-branch.
You can go back to the first branch, update, rebase and reupload the second branch afterwards:
- git rebase-update --no-fetch my-second-work-branch
- git cl upload

See https://newsletter.pragmaticengineer.com/p/stacked-diffs for an in-depth explanation of stacked diffs.

Trybots

If you're working a lot in WebRTC, you can apply for try rights. This means you can run the trybots, which run all the tests on all platforms. To do this, file a bug using this template and the WebRTC EngProd team will review your request.

To run a tryjob, upload a CL as described above and click either CQ dry run or Choose Trybots in the Gerrit UI. You need to have try rights for this. Otherwise, ask your reviewer to kick off the bots for you.

If you encounter any issues with the bots (flakiness, failing unrelated to your change etc), please file a bug using this template.

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