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Re: [PATCH] diff-tree: read the index so attribute checks work in bare repositories

On Tue, Dec 5, 2017 at 2:13 PM, Brandon Williams <bmwill@xxxxxxxxxx> wrote:
> A regression was introduced in 557a5998d (submodule: remove
> gitmodules_config, 2017-08-03) to how attribute processing was handled
> in bare repositories when running the diff-tree command.
> By default the attribute system will first try to read ".gitattribute"
> files from the working tree and then falls back to reading them from the
> index if there isn't a copy checked out in the worktree.  Prior to
> 557a5998d the index was read as a side effect of the call to
> 'gitmodules_config()' which ensured that the index was already populated
> before entering the attribute subsystem.
> Since the call to 'gitmodules_config()' was removed the index is no
> longer being read so when the attribute system tries to read from the
> in-memory index it doesn't find any ".gitattribute" entries effectively
> ignoring any configured attributes.
> Fix this by explicitly reading the index during the setup of diff-tree.
> Reported-by: Ben Boeckel <ben.boeckel@xxxxxxxxxxx>
> Signed-off-by: Brandon Williams <bmwill@xxxxxxxxxx>
> ---
> This patch should fix the regression.  Let me know if it doesn't solve the
> issue and I'll investigate some more.

Thanks for fixing this bug! The commit message is helpful
to understand how this bug could slip in!

> diff --git a/builtin/diff-tree.c b/builtin/diff-tree.c
> index d66499909..cfe7d0281 100644
> --- a/builtin/diff-tree.c
> +++ b/builtin/diff-tree.c
> @@ -110,6 +110,7 @@ int cmd_diff_tree(int argc, const char **argv, const char *prefix)
>         git_config(git_diff_basic_config, NULL); /* no "diff" UI options */
>         init_revisions(opt, prefix);
> +       read_cache();

Although we do have very few unchecked calls to read_cache, I'd suggest
to avoid spreading them. Most of the read_cache calls are guarded via:

    if (read_cache() < 0)
        die(_("index file corrupt"));

I wonder if this hints at a bad API, and we'd rather have read_cache
die() on errors, and the few callers that try to get out of trouble might
need to use read_cache_gently() instead.
(While this potentially large refactoring may be deferred, I'd ask for
an if at least)