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checkout v2 adr (#153)
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eric sciple
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adrs/0153-checkout-v2.md
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# ADR 0153: Checkout v2
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**Date**: 2019-10-21
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**Status**: Accepted
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## Context
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This ADR details the behavior for `actions/checkout@v2`.
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The new action will be written in typescript. We are moving away from runner-plugin actions.
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We want to take this opportunity to make behavioral changes, from v1. This document is scoped to those differences.
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## Decision
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### Inputs
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- `repository`
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- The qualified repository name (owner/repo)
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- Defaults to the workflow repo
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- `ref`
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- The ref to checkout
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- For the workflow repo, defaults to the branch and sha from the workflow event payload
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- Otherwise defaults to `master`
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- `token`
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- Defaults to the job token
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- `persist-credentials`
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- Indicates whether to embed the auth token into the git config. Allows users to script authenticated git commands.
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- Defaults to `true`
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- `clean`
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- Indicates whether to run `git clean -ffdx && git reset --hard`
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- Defaults to `true`
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- `lfs`
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- Indicates whether to download Git-LFS files
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- Defaults to `false`
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- `path`
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- Relative path under the `github.workspace` where the repository should be created
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- Defaults to the `github.workspace`
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Note:
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- `persist-credentials` is new
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- `fetch-depth` was removed (refer [below](#fetch) for details)
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- `path` behavior is different (refer [below](#path) for details)
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- `submodules` was removed (error if specified; add later if needed)
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### Fallback to GitHub API
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When a sufficient version of git is not in the PATH, fallback to the [web API](https://developer.github.com/v3/repos/contents/#get-archive-link) to download a tarball/zipball.
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Note:
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- LFS files are not included in the archive. Therefore fail if LFS is set to true.
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- Submodules are also not included in the archive. However submodules are not supported by checkout v2 anyway.
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### Persist credentials
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Persist the token in the git config (http.extraheader). This will allow users to script authenticated git commands, like `git fetch`.
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A post script will remove the credentials from the git config (cleanup for self-hosted).
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Users may opt-out by specifying `persist-credentials: false`
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Note:
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- Users scripting `git commit` may need to set the username and email. The service does not provide any reasonable default value. Users can add `git config user.name <NAME>` and `git config user.email <EMAIL>`. We will document this guidance.
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- The auth header (stored in the repo's git config), is scoped to all of github `http.https://github.com/.extraheader`
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- Additional public remotes also just work.
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- If users want to authenticate to an additional private remote, they should provide the `token` input.
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- Lines up if we add submodule support in the future. Don't need to worry about calculating relative URLs. Just works, although needs to be persisted in each submodule git config.
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- Users opt out of persisted credentials (`persist-credentials: false`), or can script the removal themselves (`git config --unset-all http.https://github.com/.extraheader`).
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### Fetch
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Fetch only the SHA being built and set depth=1. This significantly reduces the fetch time for large repos.
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If a SHA isn't available (e.g. multi repo), then fetch only the specified ref with depth=1.
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Customers can run `git fetch --unshallow` to fetch all refs/commits. We will document this guidance.
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Note:
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- The v1 input `fetch-depth` no longer exists. We can add this back in the future if needed.
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- Fetching a single commit is supported by Git wire protocol version 2. The git client uses protocol version 0 by default. The desired protocol version can be overridden in the git config or on the fetch command line invocation (`-c protocol.version=2`). We will override on the fetch command line, for transparency.
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- Git client version 2.18+ (released June 2018) is required for wire protocol version 2.
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### Checkout
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For CI, checkout will create a local ref with the upstream set. This allows users to script git as they normally would.
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For PR, continue to checkout detached head. The PR branch is special - the branch and merge commit are created by the server. It doesn't match a users' local workflow.
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Note:
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- Consider deleting all local refs during cleanup if that helps avoid collisions. More testing required.
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### Path
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For the mainline scenario, the disk-layout behavior remains the same.
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Remember, given the repo `johndoe/foo`, the mainline disk layout looks like:
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```
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GITHUB_WORKSPACE=/home/runner/work/foo/foo
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RUNNER_WORKSPACE=/home/runner/work/foo
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```
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V2 introduces a new contraint on the checkout path. The location must now be under `github.workspace`. Whereas the checkout@v1 constraint was one level up, under `runner.workspace`.
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V2 no longer changes `github.workspace` to follow wherever the self repo is checked-out.
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These behavioral changes align better with container actions. The [documented filesystem contract](https://help.github.com/en/actions/automating-your-workflow-with-github-actions/virtual-environments-for-github-hosted-runners#docker-container-filesystem) is:
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- `/github/home`
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- `/github/workspace` - Note: GitHub Actions must be run by the default Docker user (root). Ensure your Dockerfile does not set the USER instruction, otherwise you will not be able to access `GITHUB_WORKSPACE`.
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- `/github/workflow`
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Note:
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- The tracking config will not be updated to reflect the path of the workflow repo.
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- Any existing workflow repo will not be moved when the checkout path changes. In fact some customers want to checkout the workflow repo twice, side by side against different branches.
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- Actions that need to operate only against the root of the self repo, should expose a `path` input.
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#### Default value for `path` input
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The `path` input will default to `./` which is rooted against `github.workspace`.
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This default fits the mainline scenario well: single checkout
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For multi-checkout, users must specify the `path` input for at least one of the repositories.
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Note:
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- An alternative is for the self repo to default to `./` and other repos default to `<REPO_NAME>`. However nested layout is an atypical git layout and therefore is not a good default. Users should supply the path info.
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#### Example - Nested layout
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The following example checks-out two repositories and creates a nested layout.
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```yaml
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# Self repo - Checkout to $GITHUB_WORKSPACE
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- uses: checkout@v2
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# Other repo - Checkout to $GITHUB_WORKSPACE/myscripts
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- uses: checkout@v2
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with:
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repository: myorg/myscripts
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path: myscripts
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```
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#### Example - Side by side layout
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The following example checks-out two repositories and creates a side-by-side layout.
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```yaml
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# Self repo - Checkout to $GITHUB_WORKSPACE/foo
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- uses: checkout@v2
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with:
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path: foo
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# Other repo - Checkout to $GITHUB_WORKSPACE/myscripts
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- uses: checkout@v2
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with:
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repository: myorg/myscripts
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path: myscripts
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```
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#### Path impact to problem matchers
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Problem matchers associate the source files with annotations.
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Today the runner verifies the source file is under the `github.workspace`. Otherwise the source file property is dropped.
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Multi-checkout complicates the matter. However even today submodules may cause this heuristic to be inaccurate.
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A better solution is:
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Given a source file path, walk up the directories until the first `.git/config` is found. Check if it matches the self repo (`url = https://github.com/OWNER/REPO`). If not, drop the source file path.
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### Port to typescript
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The checkout action should be a typescript action on the GitHub graph, for the following reasons:
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- Enables customers to fork the checkout repo and modify
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- Serves as an example for customers
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- Demystifies the checkout action manifest
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- Simplifies the runner
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- Reduce the amount of runner code to port (if we ever do)
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Note:
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- This means job-container images will need git in the PATH, for checkout.
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### Branching strategy and release tags
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- Create a servicing branch for V1: `releases/v1`
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- Merge the changes into `master`
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- Release using a new tag `preview`
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- When stable, release using a new tag `v2`
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## Consequences
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- Update the checkout action and readme
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- Update samples to consume `actions/checkout@v2`
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- Job containers now require git in the PATH for checkout, otherwise fallback to REST API
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- Minimum git version 2.18
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- Update problem matcher logic regarding source file verification (runner)
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