RFC 80: Rename master branch to main in all repositories

rfcs/main_branch.md

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+# RFC #80: Rename `master` branch to `main` in all repositories
+
+## Summary
+
+Rename `master` branch to `main` in all repositories in the [web-platform-tests org](https://github.com/web-platform-tests).
+
+## Details
+
+### wpt.fyi
+
+[https://github.com/web-platform-tests/wpt.fyi](https://github.com/web-platform-tests/wpt.fyi)
+
+#### Run labelling
+
+**Example**: [https://wpt.fyi/api/runs?label=master](https://wpt.fyi/api/runs?label=master)
+
+Today we use the term 'master' to label runs that come from either pushes to the master branch in
+WPT (for Chrome Dev and Firefox Nightly) or from the various epoch/\* branches for other browsers
+and channels. These runs should be complete runs of the full test suite (as opposed to partial runs
+done for pull-requests), so are considered the most useful for displaying on wpt.fyi.
+
+There are two considerations to how this label is used:
+
+1.  Internally to the service (the variable/method names 'shared.MasterLabel' or 'IsMasterBranch',
+    values stored in the datastore, etc) - these could be renamed without too much difficulty.
+
+    - For example, one could consider renaming the concept to 'complete' runs, such that we had
+      'shared.CompleteRunLabel'.
+
+    - Method names like IsMasterBranch are harder to change until WPT changes its branch naming
+      convention.
+
+2.  Externally accessible information, e.g. as used in the APIs. We allow querying runs based on
+    their label, and both the [wpt.fyi](http://wpt.fyi) frontend and other callers may
+    rely on \`label=master\` today.
+
+    - This would be a slow process of introducing an alternative label (again say 'complete'),
+      labelling runs as both in the datastore (data fixups can be done with [existing
+      utilities](https://github.com/web-platform-tests/data-migration)), and then slowly
+      switching over consumers to the new label name.
+
+There are approximately 200 references to the string 'master' in the wpt.fyi codebase:
+
+- 106 in api/
+
+- 60 in webapp/
+
+- 28 in shared/
+
+- 4 in util/
+
+- 4 in webdriver/
+
+#### Primary branch name
+
+~~Today wpt.fyi has 'master' as its primary branch name in GitHub. Changing this to e.g. 'main'
+would require at least:~~
+
+- ~~Creating the new branch ('git branch -m master main; git push -u origin main'),~~
+
+- ~~Changing the primary branch in GitHub,~~
+
+- ~~Editing all open pull requests (currently only 5) to have 'main' as their target,~~
+
+- ~~Switching the branch protection rules to apply to 'main', and~~
+
+- ~~Updating the CI workflows (e.g. \`.github/workflows/ci.yml\`) to run on the new branch.~~
+
+- ~~Deleting the master branch~~
+
+~~There is also a reference to 'master' in the gcloud deployment steps in the Makefile.~~
+
+~~I am not aware of any downstream consumers of the branch name for wpt.fyi, so we could likely do
+the introduction of 'main' as a mostly-atomic move with no prerequisites.~~
+
+### WPT
+
+[https://github.com/web-platform-tests/wpt](https://github.com/web-platform-tests/wpt)
+
+#### Primary branch name
+
+Today wpt.fyi has 'master' as its primary branch name in GitHub. Changing this to e.g. 'main' would
+have similar steps [as for wpt.fyi](#wpt.fyi) (e.g. changing the primary GitHub
+branch, branch protection, etc), but with more complexity due to having more continuous integration
+(CI) systems and many downstream consumers.
+
+##### Outstanding Pull Requests
+
+WPT currently has 754 open pull requests, almost all targeting the 'master' branch. These would all
+need to be updated to target the 'main' branch instead.
+
+This can be done manually by maintainers (by editing the pull request via the GitHub API), but it
+appears that we could also use the [GitHub REST
+API](https://developer.github.com/v3/pulls/#update-a-pull-request) to do this via PATCH
+requests to open pull requests (changing the 'base' branch to e.g. 'main').
+
+#### Continuous Integration systems
+
+##### Taskcluster
+
+One of our two CI systems for running the test suite on browsers. Taskcluster has a few
+dependencies on the name 'master', but all could be done ahead of a switchover.
+
+- .taskcluster.yml would need 'main' added as a trigger branch
+
+- tools/ci/tc/tasks/test.yml would need 'main' added as a trigger branch
+
+- tools/ci/tc/download.py uses 'master' as the default branch for its '--ref' argument.
+
+  - This is the tc-download wpt command, but it is unclear if it is used in any automated setting -
+    perhaps this could be changed after the fact.
+
+- tools/docker/start.sh uses 'master' as a default ref, but it appears that the only caller
+  (tools/ci/tc/decision.py) passes an explicit ref anyway.
+
+  - This could possibly just be removed.
+
+##### Azure Pipelines
+
+Our other CI system for running the test suite on browsers. Azure Pipelines does not run on
+'master' today, so I believe it requires no changes in the case of a branch name switch.
+
+##### Pull-request previews (wptpr.live integration)
+
+This is a pair of GitHub Actions that support previewing pull requests on wptpr.live. For the
+actions themselves (see [Downstream Consumers of WPT branch
+names](#downstream-consumers-of-wpt-branch-names) for the wptpr.live side), there are two
+places that reference 'master':
+
+- .github/workflows/detect_pull_request_preview.yml deliberately checks out the 'master' branch
+  specifically.
+
+  - This would have to change to 'main', and would have to happen as part of the 'atomic'
+    switchover.
+
+- tools/ci/tests/test_pr_preview.py uses it in the testing logic as part of a set; this can be
+  changed at any point (the test creates a temporary git repository via git-init, so that would
+  have to change to not create a 'master' branch).
+
+##### Manifest generation
+
+This is a GitHub Action that publishes WPT MANIFEST.json files as [GitHub
+releases](https://github.com/web-platform-tests/wpt/releases). There are two places that
+reference 'master':
+
+- .github/workflows/manifest.yml triggers on pushes to master; this could be changed ahead of time
+  to also trigger on pushes to e.g. 'main'
+
+- tools/ci/manifest_build.py checks for 'master' to determine if this is a dry-run or not; another
+  branch name could be added to this check ahead of time.
+
+##### Epochs
+
+No obvious dependency. (Uses the default branch when checking out the code.)
+
+#### Documentation
+
+WPT has a sizeable amount of documentation (in docs/), which is generated by a GitHub action.
+Changes to both the generation process and the documentation itself would be necessary:
+
+**docs/ generation**:
+
+- .github/workflows/documentation.yml triggers on push to master; this could be changed ahead of
+  time to also trigger on pushes to e.g. 'main'
+
+- tools/ci/website_build.sh checks for 'master' to only publish when run via the GitHub Action;
+  this could be changed ahead of time to also publish for e.g. 'main'.
+
+**docs/ documentation**:
+
+- Three files have substantial content that mention 'master':
+
+  - docs/reviewing-tests/git.md
+
+  - docs/reviewing-tests/reverting.md
+
+  - docs/writing-tests/github-intro.md
+
+- Plus some minor links/etc in other documents.
+
+**README.md**
+
+- There are a few minor notes about 'master' here.
+
+#### WPT tools/ codebase
+
+There are a few places in the WPT infrastructure code that interact with 'master'.
+
+##### wpt branch-point
+
+- tools/wpt/testfiles.py checks for 'master' in 'branch_point()', would need changed as part of the
+  migration to use e.g. 'main'.
+
+- **Note**: it is unclear to me who uses this tool?
+
+##### wptrunner
+
+- tools/wptrunner/wptrunner.default.ini references 'master', but I'm unsure of the purpose of this
+  file.
+
+#### Uses of 'master' in test suites
+
+Some minor documentation changes (e.g. in css/README.md) would be required, but would be
+non-blocking.
+
+#### Downstream Consumers of WPT branch names
+
+This is one of the trickiest areas of changing the default branch name in WPT; there are multiple
+consumers of WPT that rely on the branch name today.
+
+The ones I am aware of are:
+
+- wpt.fyi:
+
+  - Would need to be updated to accept either 'master' or (e.g.) 'main' ahead of a switch of branch
+    name. See [wpt.fyi](#wpt.fyi) above.
+
+- wpt.live:
+
+  - src/supervisord.conf pulls from the 'master' branch specifically, this would need updated.
+
+  - Similarly, 'master' is passed to fetch-wpt.py as the branch to fetch from.
+
+  - These would be difficult to update ahead of time; they could possibly be removed entirely and
+    rely on the default branch.
+
+- w3c-test.org:
+
+  - This may also pull directly from master like wpt.live, unknown.
+
+- There is also the web-test runner, not sure if that has any dependencies on WPT branch names.
+
+- Various sync bots (might have “master” hard coded)
+
+  - autofoolip
+
+  - wpt-pr-bot
+
+- Downstream mirrors
+
+  - Chromium mirror:
+    [https://chromium.googlesource.com/external/github.com/web-platform-tests/wpt/](https://chromium.googlesource.com/external/github.com/web-platform-tests/wpt/)
+    (used by wpt-importer to avoid GitHub network issues)
+
+## Risk
+
+Calamity