checkout v2 adr (#153)

adrs/0153-checkout-v2.md

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+# ADR 0153: Checkout v2
+
+**Date**: 2019-10-21
+
+**Status**: Accepted
+
+## Context
+
+This ADR details the behavior for `actions/checkout@v2`.
+
+The new action will be written in typescript. We are moving away from runner-plugin actions.
+
+We want to take this opportunity to make behavioral changes, from v1. This document is scoped to those differences.
+
+## Decision
+
+### Inputs
+
+- `repository`
+  - The qualified repository name (owner/repo)
+  - Defaults to the workflow repo
+- `ref`
+  - The ref to checkout
+  - For the workflow repo, defaults to the branch and sha from the workflow event payload
+  - Otherwise defaults to `master`
+- `token`
+  - Defaults to the job token
+- `persist-credentials`
+  - Indicates whether to embed the auth token into the git config. Allows users to script authenticated git commands.
+  - Defaults to `true`
+- `clean`
+  - Indicates whether to run `git clean -ffdx && git reset --hard`
+  - Defaults to `true`
+- `lfs`
+  - Indicates whether to download Git-LFS files
+  - Defaults to `false`
+- `path`
+  - Relative path under the `github.workspace` where the repository should be created
+  - Defaults to the `github.workspace`
+
+Note:
+- `persist-credentials` is new
+- `fetch-depth` was removed (refer [below](#fetch) for details)
+- `path` behavior is different (refer [below](#path) for details)
+- `submodules` was removed (error if specified; add later if needed)
+
+### Fallback to GitHub API
+
+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.
+
+Note:
+- LFS files are not included in the archive. Therefore fail if LFS is set to true.
+- Submodules are also not included in the archive. However submodules are not supported by checkout v2 anyway.
+
+### Persist credentials
+
+Persist the token in the git config (http.extraheader). This will allow users to script authenticated git commands, like `git fetch`.
+
+A post script will remove the credentials from the git config (cleanup for self-hosted).
+
+Users may opt-out by specifying `persist-credentials: false`
+
+Note:
+- 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.
+- The auth header (stored in the repo's git config), is scoped to all of github `http.https://github.com/.extraheader`
+  - Additional public remotes also just work.
+  - If users want to authenticate to an additional private remote, they should provide the `token` input.
+  - 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.
+  - Users opt out of persisted credentials (`persist-credentials: false`), or can script the removal themselves (`git config --unset-all http.https://github.com/.extraheader`).
+
+### Fetch
+
+Fetch only the SHA being built and set depth=1. This significantly reduces the fetch time for large repos.
+
+If a SHA isn't available (e.g. multi repo), then fetch only the specified ref with depth=1.
+
+Customers can run `git fetch --unshallow` to fetch all refs/commits. We will document this guidance.
+
+Note:
+- The v1 input `fetch-depth` no longer exists. We can add this back in the future if needed.
+- 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.
+- Git client version 2.18+ (released June 2018) is required for wire protocol version 2.
+
+### Checkout
+
+For CI, checkout will create a local ref with the upstream set. This allows users to script git as they normally would.
+
+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.
+
+Note:
+- Consider deleting all local refs during cleanup if that helps avoid collisions. More testing required.
+
+### Path
+
+For the mainline scenario, the disk-layout behavior remains the same.
+
+Remember, given the repo `johndoe/foo`, the mainline disk layout looks like:
+
+```
+GITHUB_WORKSPACE=/home/runner/work/foo/foo
+RUNNER_WORKSPACE=/home/runner/work/foo
+```
+
+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`.
+
+V2 no longer changes `github.workspace` to follow wherever the self repo is checked-out.
+
+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:
+
+- `/github/home`
+- `/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`.
+- `/github/workflow`
+
+Note:
+- The tracking config will not be updated to reflect the path of the workflow repo.
+- 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.
+- Actions that need to operate only against the root of the self repo, should expose a `path` input.
+
+#### Default value for `path` input
+
+The `path` input will default to `./` which is rooted against `github.workspace`.
+
+This default fits the mainline scenario well: single checkout
+
+For multi-checkout, users must specify the `path` input for at least one of the repositories.
+
+Note:
+- 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.
+
+#### Example - Nested layout
+
+The following example checks-out two repositories and creates a nested layout.
+
+```yaml
+# Self repo - Checkout to $GITHUB_WORKSPACE
+- uses: checkout@v2
+
+# Other repo - Checkout to $GITHUB_WORKSPACE/myscripts
+- uses: checkout@v2
+  with:
+    repository: myorg/myscripts
+    path: myscripts
+```
+
+#### Example - Side by side layout
+
+The following example checks-out two repositories and creates a side-by-side layout.
+
+```yaml
+# Self repo - Checkout to $GITHUB_WORKSPACE/foo
+- uses: checkout@v2
+  with:
+    path: foo
+
+# Other repo - Checkout to $GITHUB_WORKSPACE/myscripts
+- uses: checkout@v2
+  with:
+    repository: myorg/myscripts
+    path: myscripts
+```
+
+#### Path impact to problem matchers
+
+Problem matchers associate the source files with annotations.
+
+Today the runner verifies the source file is under the `github.workspace`. Otherwise the source file property is dropped.
+
+Multi-checkout complicates the matter. However even today submodules may cause this heuristic to be inaccurate.
+
+A better solution is:
+
+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.
+
+### Port to typescript
+
+The checkout action should be a typescript action on the GitHub graph, for the following reasons:
+- Enables customers to fork the checkout repo and modify
+- Serves as an example for customers
+- Demystifies the checkout action manifest
+- Simplifies the runner
+- Reduce the amount of runner code to port (if we ever do)
+
+Note:
+- This means job-container images will need git in the PATH, for checkout.
+
+### Branching strategy and release tags
+
+- Create a servicing branch for V1: `releases/v1`
+- Merge the changes into `master`
+- Release using a new tag `preview`
+- When stable, release using a new tag `v2`
+
+## Consequences
+
+- Update the checkout action and readme
+- Update samples to consume `actions/checkout@v2`
+- Job containers now require git in the PATH for checkout, otherwise fallback to REST API
+- Minimum git version 2.18
+- Update problem matcher logic regarding source file verification (runner)