Re: [PATCH v2 4/8] repository: add repo reference to index_state

"Derrick Stolee via GitGitGadget" [off-list ref] writes:

From: Derrick Stolee <redacted>

It will be helpful to add behavior to index operations that might
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 <redacted>
---

I think this makes sense, but shouldn't we insist on these
bidirectional links to point at each other?  Otherwise we cannot
simplify the function signatures safely later.  That is ...

+	/* Complete the double-reference */
+	if (!repo->index->repo)
+		repo->index->repo = repo;
+

	else if (repo->index->repo != repo)
		BUG("the repo->index instance does not belong to the repo???");

... a check like this?

 	return read_index_from(repo->index, repo->index_file, repo->gitdir);
 }