[RFC 0085] NixOS Release Stabilization: ZHF on master, new timeline (#85)

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
Co-authored-by: Ryan Mulligan <ryan@ryantm.com>

rfcs/0085-nixos-release-stablization.md

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+---
+feature: nixos-release-stablization
+start-date: 2021-01-17
+author: Jonathan Ringer (@jonringer)
+co-authors:
+shepherd-team: @ryantm, @garbas, @Mic92
+shepherd-leader: @ryantm
+related-issues: [NixOS release schedule](https://github.com/NixOS/rfcs/pull/80)
+---
+
+# Summary
+[summary]: #summary
+
+To bring more certainty to the release cycle, add short periods where
+breaking changes are partially restricted. A list of Release Critical
+Packages is defined. Also, move most release stablization work from
+the `release` branch to the `master` branch to reduce backports.
+
+# Motivation
+[motivation]: #motivation
+
+We want to release on time without herculian efforts. Limited
+restrictions to merging breaking changes to `staging` should avoid the common
+practice of forcing many, largely-untested, commits into master before
+branch-off.
+
+This happened for the 20.09 release in which ~640 staging
+commits were pushed through in the days leading up to the branch off
+([#95492](https://github.com/NixOS/nixpkgs/pull/95492),
+[#96280](https://github.com/NixOS/nixpkgs/pull/96280),
+[#96437](https://github.com/NixOS/nixpkgs/pull/96437),
+[#97146](https://github.com/NixOS/nixpkgs/pull/97146)). The last PR included a
+release blocker fix (introduced in [one of the mentioned
+PRs](https://github.com/NixOS/nixpkgs/pull/96437)) which delayed the
+branch-off from 4 Sept 2020 to 7 Sept 2020, and ZHF had to be delayed
+another day to get a semi-accurate hydra build status. The systemd
+bump was also largely responsible delaying the release until after the
+scheduled release date where basic login features in plasma [weren't
+restored until Oct 5th, a week after the planned release
+date](https://github.com/NixOS/nixpkgs/pull/99629) and a backlog of
+other plasma related issues [weren't resolved until Oct 19th, almost a
+month after the planned release
+date](https://github.com/NixOS/nixpkgs/pull/101078).  This is not to
+discredit any individual, but demonstrate that certain packages have
+many dimensions of compatibility which need a longer time to stabilize
+on unstable before being included in a release.
+
+
+# Definitions
+[definitions]: #definition
+- Breaking change
+  - A change that is likely to break downstream.
+  - For projects following Semantic Versioning, major version bumps indicate breaking changes.
+- Restricted
+  - Breaking changes are disallowed.
+  - Breaking changes for security or critical bugs are allowed. (Patching is preferred over updates.)
+  - New packages and NixOS modules are welcome.
+- Unrestricted
+  - Breaking changes are allowed.
+  - New packages and NixOS modules are welcome.
+- ZHF: Zero Hydra Failures
+  - Period in which packages are stabilized in preparation of a release.
+
+# Changes
+
+## Release Critical Packages
+
+A list of Release Critical Packages will be maintained in the 
+[release wiki](https://nixos.github.io/release-wiki/).
+Release Critical Packages are important packages which have
+many dimensions of build or runtime behavior. Generally these will be
+packages which are a part of `stdenv`, or bootstrapping a system
+(e.g. systemd).
+
+Changes to the list will not require additional RFCs, and will occur
+through the release team. Likely through a release retrospective
+and/or pull request process to the release wiki.
+
+## Restriction Timeline
+
+This timeline represents when certain branches and packages will be
+restricted. It is not a complete timeline of all release activities.
+The actual timeline will still be determined by the Release Managers
+and may be adjusted as needed. The current position in the timeline
+will be communicated on Discourse.
+
+| Weeks from Release | Branches Affected | Events |
+| --- | --- | --- |
+| -8 Weeks | | Gnome and Plasma(YY.11) packaging begins |
+| -6 Weeks | `staging-next`, `staging` | Restrict breaking changes to Release Critical Packages |
+| -4 Weeks | `staging-next`, `staging` | Restrict all breaking changes: allow only non-breaking updates and Desktop Manager changes |
+| -3 Weeks | `master` | (Day before ZHF) merge `staging-next` into `master`, prep for ZHF |
+| -3 Weeks | `master` | Begin ZHF |
+| -3 Weeks | `master` | Focus on minimizing regressions in PRs targeting `master` |
+| -2 Weeks | `master` | Merge first `staging-next` fixes into `master`; begin second `staging-next` cycle |
+| -2 Weeks | `staging` | Unrestrict all breaking changes; new changes will not be present in `master` before branch-off |
+| -1 Weeks | `master` | Merge second `staging-next` fix cycle |
+| -1 Weeks | `staging-next` | Unrestrict all breaking changes; new changes will not be present in `master` before branch-off |
+| -1 Weeks | `master`, `release` | Perform Branch-off, create release channels, create new beta / unstable tags |
+| -1 Weeks | `master`, `release` | ZHF transitions to "backporting" workflow |
+| -1 Weeks | `release` | Prepare for release, finish remaining issues |
+| 0 Weeks | `release` | Release! |
+| 0 Weeks | | ZHF Ends |
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Breaking changes to Release Critical Packages will have to wait a
+maximum of 4 weeks to be merged into `staging`. Other breaking changes
+targeting `staging` will have to wait a maximum of 2 weeks to be merged.
+Staging-next iterations will follow a 1 week
+development cycle during the release timeline. Breaking changes to
+Release Critical Packages will not appear in `master` for around 7 weeks
+(4 weeks being disallowed to be merged to staging, 1 week for last
+`staging-next` during ZHF, ~2 weeks average in `staging-next`).
+Pull requests which are able to target master will not be interrupted during a release.
+
+# Alternatives
+[alternatives]: #alternatives
+
+## Maintain the status quo regarding release dates
+
+Continue with cramming staging changes right before branch-off,
+keeping a significant amount of stabilization work and risk before a
+release, and likely continuing the trend of delayed releases.
+
+# Future work
+[future]: #future-work
+
+- Update [release wiki to reflect changes](https://github.com/NixOS/release-wiki)
+- Inform community about changes (Discourse).