Unraveling macOS Runner Permission Denials: A Key to Software Engineering Efficiency

Maintaining robust CI/CD pipelines is crucial for software engineering efficiency. When unexpected errors appear, especially after platform updates, they can significantly disrupt development cycles. A recent GitHub Community discussion highlighted just such a scenario, where a Haskell package's unit test began failing on the macos-15-arm64 GitHub Actions runner with a 'Permission denied' error, despite working perfectly on local machines and on the runner prior to a specific date.

The Mysterious macOS Runner Permission Denial

The core issue revolved around a withBinaryFileDurable unit test within a Haskell package named rio. This test, which involves file operations, consistently passed on the GitHub-hosted macos-15-arm64 runner until January 21, 2026. After that date, specifically with runner image version 20260123.484 (updated from 20260115.477), the test started failing with an IOException of type PermissionDenied at the path /Users/runner/work/_temp/rio5336/with_file_durable.

The initial confusion stemmed from a lack of clear documentation, leading to speculation about undocumented GitHub Actions filesystem restrictions. However, the community expert quickly clarified that such errors are rarely due to secret, undocumented rules, but rather standard POSIX permissions, ownership, or ACLs. This distinction is vital for effective troubleshooting and maintaining software engineering efficiency.

Diagnosing the Root Cause: Provisioner Regression or Workflow Contamination?

The expert's analysis pointed to two primary suspects for this sudden change in behavior:

• Runner Image Provisioner/Agent Regression: A change in the runner's underlying software (the provisioner or agent) might have inadvertently altered how permissions or Access Control Lists (ACLs) are handled within the temporary directories (like _temp).
• Permission Contamination: An earlier step in the workflow could be creating or modifying files/directories under $RUNNER_TEMP with incorrect permissions (e.g., using sudo), making them inaccessible to subsequent steps running as the standard runner user.

GitHub's documentation itself supports the idea that permission mismatches in the temporary directory are a valid failure mode, as it explicitly states that files won't be removed if the runner user lacks permission.

Probing for Permissions: A Diagnostic Toolkit

To pinpoint the exact cause and restore software engineering efficiency, the expert provided a crucial diagnostic step. Adding this snippet immediately before the failing test step on the macos-15-arm64 runner can reveal the permissions and ownership status of the temporary directories at the point of failure:

set -euxo pipefail
echo "whoami=$(whoami)"
id
umask
echo "RUNNER_TEMP=$RUNNER_TEMP"
echo "TMPDIR=${TMPDIR:-'(unset)'}"
# Show perms + ACLs on the temp area (macOS: -e shows ACLs)
ls -ldOe@ /Users/runner/work/_temp || true
ls -ldOe@ "$RUNNER_TEMP" || true
# If the failing directory exists, dump its perms too
ls -ldOe@ /Users/runner/work/_temp/rio* || true
# Repro: create + write in a fresh subdir
d="$(mktemp -d "$RUNNER_TEMP/rio-probe.XXXXXX")"
ls -ldOe@ "$d"
touch "$d/can_write"
ls -lOe@ "$d/can_write"

This script checks the current user, user ID, umask, and the permissions/ACLs of the relevant temporary directories, including attempting to create a new file to verify writability. Such systematic troubleshooting is invaluable for quickly resolving CI/CD hiccups and maintaining high software engineering efficiency.

Conclusion

When faced with 'Permission denied' errors in CI/CD, especially after runner image updates, it's essential to look beyond undocumented platform restrictions and focus on fundamental filesystem permissions. By leveraging diagnostic tools and understanding the runner environment, developers can quickly identify and rectify issues, ensuring their automated tests run smoothly and their software engineering efficiency remains uncompromised. This community insight underscores the importance of a methodical approach to debugging complex CI/CD failures.