Victoria Dye [off-list ref] writes:

For example, in addition to what you mentioned here w.r.t. '.hash_algo',
there are also differences in how 'repository_format_partial_clone' is
assigned: it's deep-copied in 'check_repository_format', but shallow-copied
(then subsequently NULL'd in the 'struct repository_format' to avoid freeing
the pointer when the struct is disposed of) in 'discover_git_directory()' &
'setup_git_directory_gently()'. 

Thanks for the analysis and explanation. It's quite a pain that the
various sites are similar but subtly different.

If we were to settle on a single "copy repository format settings" function,
it's not obvious what the "right" approach is. We could change
'check_repository_format()' to the shallow-copy-then-null like the others:
its two callers (in 'init-db.c' and 'path.c') don't use the value of
'repository_format_partial_clone' in 'struct repository_format' after
calling 'check_repository_format()'. But, if we did that, it'd introduce a
side effect to the input 'struct repository_format', which IMO would be
surprising behavior for a function called 'check_<something>()'. Conversely,
unifying on a deep copy or adding a flag to toggle deep vs. shallow copy
feels like unnecessary complexity if we don't actually need a deep copy.

Beyond the smaller subtleties, there's the larger question (that you sort of
get at with the questions around 'discover_git_directory()') as to whether
we should more heavily refactor or consolidate these setup functions. The
similar code implies "yes", but such a refactor feels firmly out-of-scope
for this series. A smaller change (e.g. just moving the assignments into
their own function) could be less of a diversion, but any benefit seems like
it'd be outweighed by the added churn/complexity of a new function.

I don't agree that this refactor is out of scope. I think we agree that
the refactor is desirable, but if we apply the same heuristics in the
future, the next author to copy a member from 'repository_format' to
'repository' could do the same and we'd never end up with the refactor
we wanted. I strongly feel that if we don't put in a concerted effort
into such refactors along the way, we end up creating more of the churn
that made our lives harder in the first place.

I sympathize with the 'out-of-scope' sentiment, though, and I find it
frustrating when a simple change starts growing in scope because a
reviewer suggests fixing oddities in the codebase that I didn't think
were in scope. In that vein, I think the helper function can simplify
the in-scope things even if we punt on the difficult-to-reason-about
parts.

E.g. we could support both deep and shallow copying, like:

  /*
   * Copy members from a repository_format to repository.
   *
   * If 'src' will no longer be read after copying (e.g. it will be
   * cleared soon), pass a nonzero value so that pointer members will be
   * moved to 'dest' (NULL-ed and shallow copied) instead of being deep
   * copied.
   */
  void copy_repository_format(struct repository *dest,
                              struct repository_format *src,
                              int take_ownership);

And in discover_git_directory(), where we don't copy .hash_algo, we
could leave the code as-is and put a FIXME to figure out if we should
use the helper function or drop the copying entirely.

(I'm somewhat convinced that we can just do shallow copying, though.
Inspecting check_repository_format() shows that it calls
clear_repository_format() right afterwards, so we really don't need the
deep copy there. Using shallow copying seems to work just fine here [1].
I'll ping Jonathan Tan to see if there was a good reason to deep copy.)

[1] https://github.com/chooglen/git/actions/runs/5246795137/jobs/9476098535

In any case, sorry for the long-winded response. I'd initially tried to
implement your feedback, but every time I did I'd get stopped up on the
things I mentioned above. So, rather than continue to put off responding to
this thread, I tried to capture what kept stopping me from moving forward -
hopefully it makes (at least a little bit of) sense!

Thanks for being receptive to the feedback in the first round. I really
appreciate the response, and I agree that discussing this was a better
way forward than being stuck.