checkout doesn't report anything helpful when the requested reference is missing.

[jsoref]

I've run across people / repositories where they have workflows that for varying reasons end up asking the checkout action (typically actions/checkout@v4) for a branch/tag/reference that doesn't exist:

If one uses git directly:

Cloning into 'checkout-missing-reference-0'...
warning: Could not find remote branch hello-world to clone.
fatal: Remote branch hello-world not found in upstream origin
Error: Process completed with exit code 128

If one uses actions/checkout@v4, one sees exit code 1 but this isn't particularly helpful:

Fetching the repository
/usr/bin/git -c protocol.version=2 fetch --no-tags --prune --no-recurse-submodules --depth=1 origin +refs/heads/hello-world*:refs/remotes/origin/hello-world* +refs/tags/hello-world*:refs/tags/hello-world*
The process '/usr/bin/git' failed with exit code 1
Waiting 15 seconds before trying again
/usr/bin/git -c protocol.version=2 fetch --no-tags --prune --no-recurse-submodules --depth=1 origin +refs/heads/hello-world*:refs/remotes/origin/hello-world* +refs/tags/hello-world*:refs/tags/hello-world*
The process '/usr/bin/git' failed with exit code 1
Waiting 20 seconds before trying again
/usr/bin/git -c protocol.version=2 fetch --no-tags --prune --no-recurse-submodules --depth=1 origin +refs/heads/hello-world*:refs/remotes/origin/hello-world* +refs/tags/hello-world*:refs/tags/hello-world*
Error: The process '/usr/bin/git' failed with exit code 1

Note that users are using actions/checkout@v4, they aren't using git ... fetch and there's no reason to expect end users to understand implementation details of git ... fetch -- that's why they're using actions/checkout@....

It is possible to improve the user experience -- consider these changes:

/usr/bin/git -c protocol.version=2 fetch --no-tags --prune --no-recurse-submodules --depth=1 origin +refs/heads/hello-world*:refs/remotes/origin/hello-world* +refs/tags/hello-world*:refs/tags/hello-world*
The process '/usr/bin/git' failed with exit code 1
Waiting 10 seconds before trying again
/usr/bin/git -c protocol.version=2 fetch --no-tags --prune --no-recurse-submodules --depth=1 origin +refs/heads/hello-world*:refs/remotes/origin/hello-world* +refs/tags/hello-world*:refs/tags/hello-world*
The process '/usr/bin/git' failed with exit code 1
Waiting 14 seconds before trying again
/usr/bin/git -c protocol.version=2 fetch --no-tags --prune --no-recurse-submodules --depth=1 origin +refs/heads/hello-world*:refs/remotes/origin/hello-world* +refs/tags/hello-world*:refs/tags/hello-world*
/usr/bin/git -c protocol.version=2 ls-remote origin +refs/heads/hello-world*:refs/remotes/origin/hello-world* +refs/tags/hello-world*:refs/tags/hello-world*
Warning: No objects found matching refSpec +refs/heads/hello-world*:refs/remotes/origin/hello-world*,+refs/tags/hello-world*:refs/tags/hello-world* -- this is why checkout failed
Error: The process '/usr/bin/git' failed with exit code 1

https://github.com/check-spelling-sandbox/checkout-missing-reference-0/actions/runs/8694496977

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Note: the error handling and reporting is much better when a repository doesn't exist.