Sometimes the hash prefix returned by git-describe is 8 instead of 7 chars long

[agross]

I have this strange situation where git describe --always (as invoked by git-describe from npm) returns 8 instead of 7 characters.

The issue appears to be dependent on the machine, it's not consistent but happens sometimes:

Machine 1:

agross@gh-runner-1 $ git describe --always
384f5105

agross@gh-runner-1 $ git rev-list --count HEAD
1

Machines 2:

agross@gh-runner-2 $ git describe --always
384f510

agross@gh-runner-2 $ git rev-list --count HEAD
1

I can get the has down to 7 chars by passing --abbrev-7. It seems as if git-describe (from npm) supports that flag:

const gitInfo = gitDescribeSync({ customArguments: ['--abbrev=7'] });

It would be very beneficial for this situation to hard-code the hash to 7 characters.

[TimothyJones]

So, my understanding is that this is to do with ambiguity- if there is another hash with the same prefix in that repository, it will be extended to be long enough to disambiguate. I haven’t confirmed this, as I haven’t seen this situation myself.

Are the versions of git different in the two machines? Is there a difference in the way it is checked out? What happens if you do gif describe on a full local checkout that has all branches?

Is there a reason you’d like the hash to be a specific length?

[agross]

The issue could be that the hash is ambiguous, but I do not think this is the case. The commit number reachable from HEAD is just 1 (see output of rev-list). Of course, there could be dangling commits or other unreachable objects after a history rewrite causing the short hash to be extended to 8 chars.

My CI executes absolute-version on potentially multiple servers (each a GitHub runner) during a build. One such step would be to determine a docker image tag. Another to trigger the deployment for the current HEAD commit (referring to the docker image tag) if can-i-deploy succeeds.

If the image tag is determined as 12345678-main.12345678, but the deployment is triggered for 1234567-main.1234567 this will cause the deployment to fail because the image does not exist. So all involved machines would need to agree on the hash prefix length.

A workaround that seems to work is sudo git config --system core.abbrev 7. This config setting also applies to the git-describe called by the npm package.