[PATCH] t3910: show failure of core.precomposeunicode with decomposed filenames

If you have existing decomposed filenames in your git repository (e.g., that were created with older versions of git that did not precompose unicode), a modern git with core.precomposeunicode set does not handle them well.

The problem is that we normalize the paths coming from the disk into their precomposed form, and then compare them against the literal bytes in the index. This makes things better if you have the precomposed form in the index. It makes things worse if you actually have the decomposed form in the index.

As a result, paths with decomposed filenames may have their precomposed variants listed as untracked files (even though the precomposed variants do not exist on-disk at all).

This patch just adds a test to demonstrate the breakage. Some possible fixes are:

  1. Tell everyone that NFD in the git repo is wrong, and
     they should make a new commit to normalize all their
     in-repo files to be precomposed.

     This is probably not the right thing to do, because it
     still doesn't fix checkouts of old history. And it
     spreads the problem to people on byte-preserving
     filesystems (like ext4), because now they have to start
     precomposing their filenames as they are adde to git.

  2. Do all index filename comparisons using a UTF-8 aware
     comparison function when core.precomposeunicode is set.
     This would probably have bad performance, and somewhat
     defeats the point of converting the filenames at the
     readdir level in the first place.

  3. Convert index filenames to their precomposed form when
     we read the index from disk. This would be efficient,
     but we would have to be careful not to write the
     precomposed forms back out to disk.

  4. Introduce some infrastructure to efficiently match up
     the precomposed/decomposed forms. We already do
     something similar for case-insensitive files using
     name-hash.c. We might be able to adapt that strategy
     here.

Signed-off-by: Jeff King <peff@peff.net>
---
 t/t3910-mac-os-precompose.sh | 10 ++++++++++
 1 file changed, 10 insertions(+)