git clone fails when using --dissociate together with a reference repository that contains a commit-graph

[Daniel Mach <daniel.mach@suse.com>]

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Hi,

I've stumbled upon a bug that the following command failed:

$ git clone <url> <dir> --reference <old-dir> --dissociate
fatal: unable to parse commit <SHA>
warning: Clone succeeded, but checkout failed.
You can inspect what was checked out with 'git status'
and retry with 'git restore --source=HEAD :/'

Omitting --dissociate fixed the error, but it wasn't clear to me what 
might be the root cause.

$ git --version
git version 2.54.0

With the help of AI I was able to create a reproducer (see the attached 
script).
I have verified that the reproducer works and also simplified it.

AI generated report (take it with a grain of salt):
* The Bug: During dissociation, Git correctly repacks objects and 
removes the objects/info/alternates file. However, the git clone process 
has already initialized its object store including the alternate's 
commit-graph.
   After dissociation, it fails to "forget" or reload the object store, 
leading to a crash when it tries to use the commit-graph (which refers 
to the now-unlinked alternate) to perform the initial checkout.
* Proof of state: As shown in the script output, a manual git checkout 
immediately after the failure succeeds, proving the repository is 
structurally sound but the clone process itself was in an inconsistent 
state.
* Workaround: Passing -c core.commitGraph=false to the clone command 
prevents the crash.

regards,
Daniel


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