Re: [PATCH 4/9] repository: add repo reference to index_state

[Elijah Newren <newren@gmail.com>]

On Wed, Jan 20, 2021 at 8:54 AM Derrick Stolee via GitGitGadget
<gitgitgadget@gmail.com> wrote:
>
> From: Derrick Stolee <dstolee@microsoft.com>
>
> It will be helpful to add behavior to index opertations that might

s/opertations/operations/

> trigger an object lookup. Since each index belongs to a specific
> repository, add a 'repo' pointer to struct index_state that allows
> access to this repository.
>
> This will prevent future changes from needing to pass an additional
> 'struct repository *repo' parameter and instead rely only on the 'struct
> index_state *istate' parameter.
>
> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
> ---
>  cache.h      | 1 +
>  repository.c | 4 ++++
>  2 files changed, 5 insertions(+)
>
> diff --git a/cache.h b/cache.h
> index 71097657489..f9c7a603841 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -328,6 +328,7 @@ struct index_state {
>         struct ewah_bitmap *fsmonitor_dirty;
>         struct mem_pool *ce_mem_pool;
>         struct progress *progress;
> +       struct repository *repo;
>  };
>
>  /* Name hashing */
> diff --git a/repository.c b/repository.c
> index a4174ddb062..67a4c1da2d9 100644
> --- a/repository.c
> +++ b/repository.c
> @@ -264,6 +264,10 @@ int repo_read_index(struct repository *repo)
>         if (!repo->index)
>                 repo->index = xcalloc(1, sizeof(*repo->index));
>
> +       /* Complete the double-reference */
> +       if (!repo->index->repo)
> +               repo->index->repo = repo;
> +
>         return read_index_from(repo->index, repo->index_file, repo->gitdir);
>  }
>
> --
> gitgitgadget

Since we have repo->index and we have index->repo, which are intended
to be circular...what if they aren't?  Do we want or need to add
assertions anywhere that repo == repo->index->repo or that index ==
index->repo->index ?

My initial implementations of --remerge-diff[1] played around with
creating a second repo, with a different primary object store but
everything else the same.  The index for the two repository objects
was thus the same, and thus clearly would have violated this assumed
invariant for one of the two repos.  I discarded that initial
implementation (which I didn't quite have working) because I
discovered tmp-objdir.h and was able to add some
tmp_objdir_make_primary() and tmp_objdir_remove_as_primary() functions
that merely altered the existing repo's primary object store, but I'm
curious if there might be other cases of folks doing stuff that might
have weird failures with this new invariant.

It's entirely possible that --remerge-diff was just so different, and
I was so unfamiliar with repo objects (and still kind of am) that I
was just doing weird stuff no one has done before, so perhaps no
additional checks are needed -- I'm just throwing my gut question out
there as food for thought.



[1] I have not yet submitted `--remerge-diff` to the list; you haven't
missed anything.  I'm waiting for merge-ort to be submitted, reviewed,
and merged first.  It's the remerge-diff branch in my fork on GitHub
if anyone is curious, though.