`actions/checkout@v6` credential auth fails when `GITHUB_WORKSPACE` is a symlink created by this action

[srz-zumix]

Summary

Using setup-job-workspace-action (which replaces GITHUB_WORKSPACE with a symlink) followed by actions/checkout@v6 causes a credential authentication failure with:

fatal: could not read Username for 'https://github.com/': terminal prompts disabled

Root cause

actions/checkout@v6 changed how it stores credentials (compared to v5). Instead of writing http.https://github.com/.extraheader directly into .git/config, it now writes includeIf "gitdir:..." directives that reference the credential config file:

[includeIf "gitdir:/home/runner/work/repo/repo/.git"]
    path = /home/runner/work/_temp/git-credentials-<uuid>.config

The gitdir: path is derived from GITHUB_WORKSPACE, which is a symlink after setup-job-workspace-action runs.

Git resolves symlinks when evaluating gitdir: conditions, so the resolved .git path (the real path) never matches the symlink-based path in includeIf. As a result, the credential config file is never loaded and the fetch fails.

This is tracked upstream in actions/checkout#2393.

Steps to reproduce

jobs:
  build:
    runs-on: [self-hosted]
    steps:
      - uses: DeNA/setup-job-workspace-action@v4  # GITHUB_WORKSPACE becomes a symlink here
      - uses: actions/checkout@v6                  # writes includeIf with symlink path → auth fails

Expected behavior

actions/checkout@v6 should authenticate successfully regardless of whether GITHUB_WORKSPACE is a symlink.

Actual behavior

Setting up auth
  /usr/bin/git config --local includeIf.gitdir:/home/runner/work/repo/repo/.git.path \
    /home/runner/work/_temp/git-credentials-<uuid>.config
Fetching the repository
  Error: fatal: could not read Username for 'https://github.com/': terminal prompts disabled

The gitdir: condition uses the symlink path, but git evaluates it against the real resolved path — they never match.

Workarounds

1. Pin to actions/checkout@v5 — v5 writes credentials directly into .git/config as http.https://github.com/.extraheader, which is unaffected by symlinks.

Notes

• The fix for actions/checkout is proposed in actions/checkout#2394 (resolve symlinks before writing gitdir: paths), but has not been released yet.
• This action's own actions-test.yml avoids the issue by using persist-credentials: false in its integration tests, but the README examples do not mention this caveat.