While working on sparse index integration for 'git rm' [1], Shaoxuan found
that removed sparse directories, when reset, would no longer be sparse. This
was due to how 'unpack_trees()' determined whether a traversed directory was
a sparse directory or not; it would only unpack an entry as a sparse
directory if it existed in the index. However, if the sparse directory was
removed, it would be treated like a non-sparse directory and its contents
would be individually unpacked.

To avoid this unnecessary traversal and keep the results of 'reset' as
sparse as possible, the decision logic for whether a directory is sparse is
changed to:

 * If the directory is a sparse directory in the index, unpack it.
 * If not, is the directory inside the sparse cone? If so, do not unpack it.
 * If the directory is outside the sparse cone, does it have any child
   entries in the index? If so, do not unpack it.
 * Otherwise, unpack the entry as a sparse directory.

In the process of updating 'reset', a separate issue was found in 'checkout'
where collapsed sparse directories did not have modified contents reported
file-by-file. A similar bug was found with 'status' in 2c521b0e49 (status:
fix nested sparse directory diff in sparse index, 2022-03-01), and
'checkout' was corrected the same way (setting the diff flag 'recursive' to
1).

Changes since V2
================

 * Adjusted 'reset hard with removed sparse dir' test in
   't1092-sparse-checkout-compatibility.sh' to avoid 'git rm' log message
   conflicts with [1]

Changes since V1
================

 * Reverted the removal of 'index_entry_exists()' to avoid breaking other
   in-flight series.
 * Renamed 'is_missing_sparse_dir()' to 'is_new_sparse_dir()'; revised
   comments and commit messages to clarify what that function is doing and
   why.
 * Handled "unexpected" inputs to 'is_new_sparse_dir()' more gently,
   returning 0 if 'p' is not a directory or the directory already exists in
   the index (rather than exiting with 'BUG()'). This is intended to make
   'is_new_sparse_dir()' less reliant on information about the index
   established by 'unpack_callback()' & 'unpack_single_entry()', resulting
   in easier-to-read and more reusable code.

Thanks!

 * Victoria

[1]
https://lore.kernel.org/git/20220803045118.1243087-1-shaoxuan.yuan02@gmail.com/

Victoria Dye (4):
  checkout: fix nested sparse directory diff in sparse index
  oneway_diff: handle removed sparse directories
  cache.h: create 'index_name_pos_sparse()'
  unpack-trees: unpack new trees as sparse directories

 builtin/checkout.c                       |   1 +
 cache.h                                  |   9 ++
 diff-lib.c                               |   5 ++
 read-cache.c                             |   5 ++
 t/t1092-sparse-checkout-compatibility.sh |  25 ++++++
 unpack-trees.c                           | 106 ++++++++++++++++++++---
 6 files changed, 141 insertions(+), 10 deletions(-)

base-commit: 4af7188bc97f70277d0f10d56d5373022b1fa385

Submitted-As: https://lore.kernel.org/git/pull.1312.v3.git.1659985672.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.1312.git.1659645967.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.1312.v2.git.1659841030.gitgitgadget@gmail.com