Re: [PATCH 2/6] diff: ignore sparse paths in diffstat

[Elijah Newren <newren@gmail.com>]

On Tue, Aug 17, 2021 at 10:08 AM Derrick Stolee via GitGitGadget
<gitgitgadget@gmail.com> wrote:
>
> From: Derrick Stolee <dstolee@microsoft.com>
>
> The diff_populate_filespec() method is used to describe the diff after a
> merge operation is complete, especially when a conflict appears. In
> order to avoid expanding a sparse index, the reuse_worktree_file() needs
> to be adapted to ignore files that are outside of the sparse-checkout
> cone. The file names and OIDs used for this check come from the merged
> tree in the case of the ORT strategy, not the index, hence the ability
> to look into these paths without having already expanded the index.

I'm confused; I thought the diffstat was only shown if the merge was
successful, in which case there would be no conflicts appearing.

Also, I'm not that familiar with the general diff machinery (just the
rename detection parts), but...if the diffstat only shows when the
merge is successful, wouldn't we be comparing two REVS (ORIG_HEAD to
HEAD)?  Why would we make use of the working tree at all in such a
case?  And, wouldn't using the working tree be dangerous...what if
there was a merge performed with a dirty working tree?

On a bit of a tangent, I know diffcore-rename.c calls into
diff_populate_filespec() as well, and I have some code doing merges in
a bare repository (where there obviously is no index).  It seemed to
be working, but given this commit message, now I'm wondering if I've
missed something fundamental either in that implementation or there's
something amiss in this patch.  Or both.  Maybe I need to dig into
diff_populate_filespec() more, but it seems you already have.  Any
pointers to orient me on why your changes are right here (and, if you
know, why diffcore-rename.c should or shouldn't be using
diff_populate_filespec() in a bare repo)?


> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
> ---
>  diff.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/diff.c b/diff.c
> index a8113f17070..c457cfa0e59 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -26,6 +26,7 @@
>  #include "parse-options.h"
>  #include "help.h"
>  #include "promisor-remote.h"
> +#include "dir.h"
>
>  #ifdef NO_FAST_WORKING_DIRECTORY
>  #define FAST_WORKING_DIRECTORY 0
> @@ -3900,6 +3901,13 @@ static int reuse_worktree_file(struct index_state *istate,
>         if (!FAST_WORKING_DIRECTORY && !want_file && has_object_pack(oid))
>                 return 0;
>
> +       /*
> +        * If this path does not match our sparse-checkout definition,
> +        * then the file will not be in the working directory.
> +        */
> +       if (!path_in_sparse_checkout(name, istate))
> +               return 0;
> +
>         /*
>          * Similarly, if we'd have to convert the file contents anyway, that
>          * makes the optimization not worthwhile.
> --
> gitgitgadget
>