fix: allow for empty files on hook check

[nzspambot]

Description

Empty hook will fail on agent on run

Context

We produce an empty checkout hook file :

checkout_override_file=$(mktemp "${rw_root_dir}/tmp/checkout-override.XXXXXXXX")

    # We need to have a custom override for the checkout hook. This copies an
    # empty checkout file so buildkite-agents can still run the pre-checkout and post-checkout
    # hooks. This is very much necessary as a several plugins seem to rely on these
    # hooks to run their own logic like secrets-manager
    # We're also copying the git repo structure as well. This preserves
    # the current file structure, mainly due to developers using
    # this exact path for their pipeline scripts. Without this file path structure
    # pipelines end up breaking.

This currently doesn't work as the file is empty as the hook check will fail (this is your extracted function):

bash-5.2$ go run /tmp/main.go -file /tmp/checkout-override.kFBBE4Ia 
Error: reading first four bytes of file "/tmp/checkout-override.kFBBE4Ia": EOF

Changes

Adds a check to see if the file has 4 bytes if not then return nil (this is your updated function):

bash-5.2$ go run /tmp/main.go -file /tmp/checkout-override.kFBBE4Ia 
The file /tmp/checkout-override.kFBBE4Ia is not a binary executable.

Testing

I would like to test this but see no tests around this? unless I am missing them, also it builds fine locally

[DrJosh9000]

Thanks @nzspambot! Looking good, I have one comment.

Re the lack of tests, I agree it would be good to have a couple of fixtures to test this function. I might add something. I don't see it as a blocker to this PR though.

[nzspambot]

thanks for the review @DrJosh9000

[DrJosh9000]

LGTM!

[nzspambot]

thanks @DrJosh9000 I think you'll need to merge as that status check is missing 👍🏻