[PATCH] unpack-trees: fix sparse directory recursion check

From: Victoria Dye <vdye@github.com>

Ensure 'is_sparse_directory_entry()' receives a valid 'name_entry *' if one exists in the list of tree(s) being unpacked in 'unpack_callback()'.

Currently, 'is_sparse_directory_entry()' is called with the first 'name_entry' in the 'names' list of entries on 'unpack_callback()'. However, this entry may be empty even when other elements of 'names' are not (such as when switching from an orphan branch back to a "normal" branch). As a result, 'is_sparse_directory_entry()' could incorrectly indicate that a sparse directory is *not* actually sparse because the name of the index entry does not match the (empty) 'name_entry' path.

Fix the issue by using the existing 'name_entry *p' value in 'unpack_callback()', which points to the first non-empty entry in 'names'. Because 'p' is 'const', also update 'is_sparse_directory_entry()'s 'name_entry *' argument to be 'const'.

Finally, add a regression test case.

Reported-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Victoria Dye <vdye@github.com>
---
    unpack-trees: fix sparse directory recursion check
    
    This issue was found when the updates from v2.37.3 introduced a test
    failure in a downstream test suite.
    
    The issue stems from the fact that, before v2.37.3, 'unpack_callback()'
    could previously "assume" that 'names[0]' was non-empty if a cache entry
    was unpacked as a sparse index. When b15207b8cf (unpack-trees: unpack
    new trees as sparse directories, 2022-08-08)) was introduced, it
    invalidated that assumption by allowing sparse directories to be
    unpacked based on the contents of other 'names' entries, rather than
    unnecessarily recursing into them and unpacking files individually. As a
    result, certain scenarios could cause a sparse directory to be unpacked
    then also recursively unpacked via 'traverse_trees_recursive()',
    creating duplicate index entries.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1344%2Fvdye%2Fbugfix%2Fsparse-index-orphan-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1344/vdye/bugfix/sparse-index-orphan-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1344

 t/t1092-sparse-checkout-compatibility.sh | 9 +++++++++
 unpack-trees.c                           | 4 ++--
 2 files changed, 11 insertions(+), 2 deletions(-)