Understanding the "fatal: reference is not a tree" Error in Git

Introduction

While working with Git, you may encounter an error message that reads fatal: reference is not a tree. This error typically occurs when Git is unable to check out a specific commit because it cannot find the commit SHA (Secure Hash Algorithm) reference in the tree. This article will delve into the possible reasons for this error and provide solutions to resolve it.

Possible Causes of the Error

1. The Git SHA is No Longer Available in the Referenced Branch

One of the most common reasons for this error is that the Git SHA, which is a unique identifier for each commit, is no longer available in the branch that is being referenced. This can happen if the commit has been deleted or if there has been a force push that overwrote the history of the branch. When the checkout operation runs, it is unable to locate the reference, leading to the "fatal: reference is not a tree" error.

2. The Current Deploy Key is Not Allowed to Checkout the Branch or the Git Commit

Another possible cause of this error is related to permissions. The current deploy key being used may not have the necessary permissions to checkout the branch or the Git commit. If the deploy key is restricted or has been changed, Git may not be able to access the required reference, resulting in the same error.

3. CircleCI Starts a Job Too Fast for Your VCS to Create the Commit SHA

In some cases, especially when using Continuous Integration (CI) systems like CircleCI, the error might occur if a job starts too quickly before your Version Control System (VCS) has had the chance to create the commit SHA. The CI system attempts to checkout a commit that doesn't yet exist in the VCS, which leads to the "fatal: reference is not a tree" error.

Solutions to the Error

Cause 1: The Git SHA is No Longer Available in the Referenced Branch

If the Git SHA is no longer available in the branch, you might need to recreate the commit. This can be done by checking out to the branch where the commit was initially made, making the changes again, and then committing them. Ensure to keep a copy of the changes before performing any force push operation.

Cause 2: The Current Deploy Key is Not Allowed to Checkout the Branch or the Git Commit

If the deploy key is the issue, you may need to update the deploy key with the appropriate permissions to allow it to checkout the branch or commit. Check your Git settings or consult with your system administrator to ensure the deploy key has the correct permissions.

Cause 3: CircleCI Starts a Job Too Fast for Your VCS to Create the Commit SHA

If you're using a CI system like CircleCI, consider adding a delay or a wait command in your CI configuration before the job starts. This gives your VCS enough time to create the commit SHA before the CI system attempts to checkout the commit.

Conclusion

The "fatal: reference is not a tree" error in Git can be a hurdle, but understanding its causes can help in finding the appropriate solution. Whether it's a missing Git SHA, a permissions issue with the deploy key, or a timing issue with your CI system, identifying the cause is the first step to resolving the error.