Sparse Index: fix a checkout bug with deep sparse-checkout patterns

This week, we rolled out the sparse index to a large internal monorepo. We got two very similar bug reports that dealt with a strange error that involved the same set of paths. One was during git pull (pull was a red herring) and the other was git checkout. The git checkout case gave enough of a reproduction to debug deep into unpack-trees.c and find the problem.

This bug dates back to 523506d (unpack-trees: unpack sparse directory entries, 2021-07-14). The reason we didn't hit this before is because it requires the following:

1. The sparse-checkout definition needs to have recursive inclusion of deep folders (depth 3 or more).
2. Adjacent to those deep folders, we need a deep sparse directory entry that receives changes.
3. In this particular repo, deep directories are only added to the sparse-checkout in rare occasions and those adjacent folders are rarely updated. They happened to update this week and hit our sparse index dogfooders in surprising ways.

The first patch adds a test that fails without the fix. It requires modifying our test data to make adjacent, deep sparse directory entries possible. It's a rather simple test after we have that data change.

The second patch includes the actual fix. It's really just an error of not understanding the difference between the name and traverse_path members of the struct traverse_info structure. name only stores a single tree entry while traverse_path actually includes the full name from root. The method we are editing also has an additional struct name_entry that fills in the tree entry on top of the traverse_path, which explains how this worked to depth two, but not depth three.

Update in v2
============

* Fixed the comment describing the sparse_dir_matches_path() method.

Thanks, -Stolee

Derrick Stolee (2):
t1092: add deeper changes during a checkout
unpack-trees: use traverse_path instead of name

 t/t1092-sparse-checkout-compatibility.sh | 16 +++++++++++++++-
 unpack-trees.c                           | 14 ++++++++------
 2 files changed, 23 insertions(+), 7 deletions(-)

base-commit: cd3e606211bb1cf8bc57f7d76bab98cc17a150bc
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1092%2Fderrickstolee%2Fsparse-index%2Fcheckout-bug-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1092/derrickstolee/sparse-index/checkout-bug-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1092

Range-diff vs v1:

 1:  ba05d7d4149 = 1:  ba05d7d4149 t1092: add deeper changes during a checkout
 2:  c9142199656 ! 2:  aa37168dcb4 unpack-trees: use traverse_path instead of name
     @@ t/t1092-sparse-checkout-compatibility.sh: test_expect_success 'add, commit, chec
       	test_sparse_match git sparse-checkout set deep/deeper1/deepest &&
      
       ## unpack-trees.c ##
     +@@ unpack-trees.c: static int find_cache_pos(struct traverse_info *info,
     + 
     + /*
     +  * Given a sparse directory entry 'ce', compare ce->name to
     +- * info->name + '/' + p->path + '/' if info->name is non-empty.
     ++ * info->traverse_path + p->path + '/' if info->traverse_path
     ++ * is non-empty.
     ++ *
     +  * Compare ce->name to p->path + '/' otherwise. Note that
     +  * ce->name must end in a trailing '/' because it is a sparse
     +  * directory entry.
      @@ unpack-trees.c: static int sparse_dir_matches_path(const struct cache_entry *ce,
       	assert(S_ISSPARSEDIR(ce->ce_mode));
       	assert(ce->name[ce->ce_namelen - 1] == '/');

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