electron/docs/development/upgrading-chromium.md

# Upgrading Chromium

This document is meant to serve as an overview of what steps are needed
on each Chromium upgrade in Electron.

- Upgrade libcc to a new Chromium version
- Make Electron code compatible with the new libcc
- Update Electron dependencies (crashpad, NodeJS, etc.) if needed
- Make internal builds of libcc and electron
- Update Electron docs if necessary


## Upgrade `libcc` to a new Chromium

1. Get the code and initialize the project:
  - ```sh
  $ git clone git@github.com:electron/libchromiumcontent.git
  $ cd libchromiumcontent
  $ ./script/bootstrap -v```
2. Find the new beta/stable Chromium version from [OmahaProxy](https://omahaproxy.appspot.com/).
3. Put it into the `libchromiumcontent/VERSION` file, then run `$ ./script/update`
  - It will probably fail applying patches.
4. Fix `*.patch` files in the `/patches` and `/patches-mas` folders.
5. (Optional) Run a separate script to apply patches (`script/update` uses it internally):
  - `$ ./script/apply-patches`
  - There is also another script `/script/patch.py` that could be more useful
    - Check `--help` to learn how it works with `$ ./script/patch.py -h`
6. Run the build when all patches can be applied without errors
  - `$ ./script/build`
  - If some patches are no longer compatible with the Chromium code, fix compilation errors.
7. When build succeeds, create a `dist` for Electron
  - `$ ./script/create-dist  --no_zip`
    - It will create `dist/main` folder in the root of the libcc repo
      - You will need it to build Electron.
8. (Optional) Update script contents if there are errors resultant of some files being removed or renamed. (`--no_zip` prevents script from create `dist` archives, you don't need them.)


## Update Electron Code

1. Get the code:
  - ```sh
  $ git clone git@github.com:electron/electron.git
  $ cd electron
  ```
2. If you already have libcc built on you machine in its own repo, you need to tell Electron explicitly to use it:
  - ```sh
   $ ./script/bootstrap.py -v \
  	--libcc_source_path <libcc_folder>/src \
  	--libcc_shared_library_path <libcc_folder>/shared_library \
  	--libcc_static_library_path <libcc_folder>/static_library
   ```
- If you haven't yet built libcc but it's already supposed to be upgraded to a new Chromium, bootstrap Electron as usual
    `$ ./script/bootstrap.py -v`
    - Ensure that libcc submodule (`vendor/libchromiumcontent`) points to a right revision

3. Set CLANG_REVISION in` script/update-clang.sh` to match the version Chromium is using.
  - Located in `electron/libchromiumcontent/src/tools/clang/scripts/update.py`

4. Checkout Chromium if you haven't already:
  - https://chromium.googlesource.com/chromium/src.git/+/{VERSION}/tools/clang/scripts/update.py
    - (Replace the `{VERSION}` placeholder in the url above to the Chromium version libcc uses.)
5. Build Electron.
  - Try to build Debug version first: `$ ./script/build.py -c D`
  - You will need it to run tests
6. Fix compilation and linking errors
7. Ensure that Release build can be built too
  - `$ ./script/build.py -c R`
  - Often the Release build will have different linking errors that you'll need to fix.
  - Some compilation and linking errors are caused by missing source/object files in the libcc `dist`
8. Update `./script/create-dist` in the libcc repo, recreate a `dist`, and run Electron bootstrap script once again.

### Tips for fixing compilation errors
- Fix build config errors first
- Fix fatal errors first, like missing files and errors related to compiler flags or defines
- Try to identify complex errors as soon as possible.
  - Ask for help if you're not sure how to fix them
- Disable all Electron features, fix the build, then enable them one by one
- Add more build flags to disable features in build-time.

When a Debug build of Electron succeeds, run the tests:
`$ ./script/test.py`
Fix the failing tests.

Follow all the steps above to fix Electron code on all supported platforms.


## Updating Crashpad

If there are any compilation errors related to the Crashpad, it probably means you need to update the fork to a newer revision: see [Upgrading Crashpad](https://github.com/electron/electron/tree/master/docs/development/upgrading-crashpad.md) for instructions on how to do that.


## Updating NodeJS

Upgrade `vendor/node` to the Node release that corresponds to the v8 version being used in the new Chromium release. See the v8 versions in Node on

See [Upgrading Node](https://github.com/electron/electron/tree/master/docs/development/upgrading-node.md) for instructions on how to do this.

## Verify ffmpeg Support

Electron ships with a version of `ffmpeg` that includes proprietary codecs by default. A version without these codecs is built and distributed with each release as well. Each Chrome upgrade should verify that switching this version is still supported.

You can verify Electron's support for multiple `ffmpeg` builds by loading the following page. It should work with the default `ffmpeg` library distributed with Electron and not work with the `ffmpeg` library built without proprietary codecs.

```html
<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Proprietary Codec Check</title>
  </head>
  <body>
    <p>Checking if Electron is using proprietary codecs by loading video from http://www.quirksmode.org/html5/videos/big_buck_bunny.mp4</p>
    <p id="outcome"></p>
    <video style="display:none" src="http://www.quirksmode.org/html5/videos/big_buck_bunny.mp4" autoplay></video>
    <script>
      const video = document.querySelector('video')
      video.addEventListener('error', ({target}) => {
        if (target.error.code === target.error.MEDIA_ERR_SRC_NOT_SUPPORTED) {
          document.querySelector('#outcome').textContent = 'Not using proprietary codecs, video emitted source not supported error event.'
        } else {
          document.querySelector('#outcome').textContent = `Unexpected error: ${target.error.code}`
        }
      })
      video.addEventListener('playing', () => {
        document.querySelector('#outcome').textContent = 'Using proprietary codecs, video started playing.'
      })
    </script>
  </body>
</html>
```

## Useful Links

- [Chrome Release Schedule](https://www.chromium.org/developers/calendar)
- [OmahaProxy](http://omahaproxy.appspot.com)
- [Chromium Issue Tracker](https://bugs.chromium.org/p/chromium)