CONTRIBUTING: review and merge conventions (#438686)

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CONTRIBUTING.md
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6. [Create a pull request](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request#creating-the-pull-request) from the new branch in your Nixpkgs fork to the upstream Nixpkgs repository.
Use the branch from step 1 as the PR's base branch.
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Go through the [pull request template](#pull-request-template).
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Go through the [pull request template][pr-template].
7. Respond to review comments and potentially to CI failures and merge conflicts by updating the PR.
Always keep it in a mergeable state.
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Simple package version updates need to include the attribute name, old and new versions, as well as a reference to the release notes or changelog.
Package upgrades with more extensive changes require more verbose commit messages.
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Pull requests should not be squash-merged, as this discards information including detail from commit messages, GPG signatures, and authorship.
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Many pull requests don't make sense as a single commit anyway.
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## Review and Merge conventions
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Comments on Pull Requests are considered non-blocking by default.
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Every blocking comment must be explicitly marked as such by using GitHub's "Request Changes" review type.
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A reviewer who submits a blocking review should be available for discussion and re-review.
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An abandoned review may be dismissed after reasonable time was given at the discretion of the merger.
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All suggestions for change, blocking or not, should be acknowledged before merge.
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This can happen implicitly by applying the suggestion, or explicitly by rejecting it.
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To make changes on commit structure and commit messages or apply simple suggestions, committers are encouraged to [checkout the PR](https://cli.github.com/manual/gh_pr_checkout) and push directly to the contributor's branch before merging.
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Committers will carefully weigh the cost of another review cycle against the feelings of the contributor when pushing to their branch.
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They should also transparently communicate which changes they made.
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If a contributor does not want committers to push to their branch, they must uncheck the "Allow edits and access to secrets by maintainers" box explicitly.
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> [!WARNING]
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> Committers: Branches created via `gh pr checkout` can't be pushed with `--force-with-lease`, so do a sanity check before pushing.
## Code conventions
[code-conventions]: #code-conventions