git clone with --dissociate sometimes fails to check out target commit

[Rasmus Villemoes <ravi@prevas.dk>]

Hi

We have now seen this error a couple of times in our CI, and this time I
managed to grab a snapshot of the local mirror for which it fails. The
failing command is

  git clone --verbose --depth=20 --branch=whinlatter --reference-if-able=/yocto/meta-mirrors/core --dissociate https://git.openembedded.org/openembedded-core core
  Cloning into 'core'...
  POST git-upload-pack (388 bytes)
  POST git-upload-pack (986 bytes)
  POST git-upload-pack (gzip 1836 to 958 bytes)
  fatal: unable to parse commit 8751ec83421192fc0f8495fb95798f9eb7be77a0
  warning: Clone succeeded, but checkout failed.
  You can inspect what was checked out with 'git status'
  and retry with 'git restore --source=HEAD :/'

I wrapped up that local copy /yocto/meta-mirrors/core in a tarball, but
it's ~200M, and I don't know another way of reproducing. I also don't
have a better way of sharing such a file than [1], apologies.

Using that repository as both the remote url to clone and the local
reference, I can consistently reproduce the problem. That is:

  cd /tmp
  # fetch that core.tar.gz
  mkdir upstream-core local-core
  tar -xf core.tar.gz -C upstream-core/
  tar -xf core.tar.gz -C local-core/
  git clone --verbose --branch=whinlatter --reference-if-able=/tmp/local-core --dissociate --depth=20 file:///tmp/upstream-core core

fails in the same way, with both git 2.47.3 (Debian trixie) and 2.53.0
(Arch). Removing --depth=20 doesn't change anything, neither does
removing --branch=whinlatter (except of course for the commit it tries
to check out). But dropping --dissociate, the clone works as expected.

It doesn't happen very often, the last time was around January 30, where
it was for another repository
(https://github.com/openembedded/meta-openembedded.git), but exactly the
same symptoms, so about 100 nightly pipelines ago.

Are we using --dissociate wrongly, or are we perhaps not maintaining
those local mirror repos properly? They are essentially just created
with 'git clone --mirror', with 'git remote update' run periodically.

Naively, I'd expect the effects of --dissociate to only happen after
everything else the clone command does has been done, but it seems that
the ties to the reference repo are cut too soon.

Rasmus

[1] https://prevasonline-my.sharepoint.com/:u:/g/personal/rasmus_villemoes_prevas_dk/IQCRaxpwj5NfQYZNQJWc9PJTAY0C33XvXn8CnqPEdPAbpDA?e=zQAfg7