[PATCH v2 0/7] Sparse index: integrate with 'read-tree'

["Victoria Dye via GitGitGadget" <gitgitgadget@gmail.com>]

Like previous integrations [1] [2], this series allows 'git read-tree' to
operate using a sparse index.

The first two patches are bugfixes for issues found while implementing the
'read-tree' integration:

 * The first (patch 1/7) fixes an edge case in which a repo with no in-cone
   files or directories would have its root collapsed into a sparse
   directory; the fix ensures the root directory is never collapsed into a
   sparse directory.
 * The second (patch 2/7) corrects the 'git status' reporting of changes
   nested inside the subdirectory of a sparse directory, ensuring that the
   modified file (not the subdirectory) is correctly reported as having
   changes.

The remainder of the series focuses on utilizing the sparse index in 'git
read-tree'. After some baseline behavior-establishing tests (patch 3/7),
sparse index usage is trivially enabled (patch 4/7) for 'read-tree' except:

 * usage with '--prefix'
 * two- and three-way merge

These cases require additional changes in order to work as expected (i.e.,
outwardly matching non-sparse index sparse-checkout). For the former, the
sparse index can be enabled as long as the index is expanded when the prefix
is a directory outside the sparse cone (patch 5/7). For the latter, sparse
directories that cannot be trivially merged must have their contents merged
file-by-file, done by recursively traversing the trees represented by the
sparse directories (patches 6/7 & 7/7).


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

 * switched an empty string check from '!strlen(path)' to the
   slightly-less-expensive '!*path'

Thanks!

 * Victoria

[1]
https://lore.kernel.org/git/pull.1109.v2.git.1641924306.gitgitgadget@gmail.com/
[2]
https://lore.kernel.org/git/pull.1048.v6.git.1638201164.gitgitgadget@gmail.com/

Victoria Dye (7):
  sparse-index: prevent repo root from becoming sparse
  status: fix nested sparse directory diff in sparse index
  read-tree: expand sparse checkout test coverage
  read-tree: integrate with sparse index
  read-tree: narrow scope of index expansion for '--prefix'
  read-tree: make two-way merge sparse-aware
  read-tree: make three-way merge sparse-aware

 builtin/read-tree.c                      |  10 +-
 dir.c                                    |   7 +-
 t/perf/p2000-sparse-operations.sh        |   1 +
 t/t1092-sparse-checkout-compatibility.sh | 129 +++++++++++++++++++++++
 unpack-trees.c                           | 121 ++++++++++++++++++++-
 wt-status.c                              |   9 ++
 6 files changed, 268 insertions(+), 9 deletions(-)


base-commit: e6ebfd0e8cbbd10878070c8a356b5ad1b3ca464e
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1157%2Fvdye%2Fsparse%2Fread-tree-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1157/vdye/sparse/read-tree-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1157

Range-diff vs v1:

 1:  90da1f9f33a ! 1:  744668eeece sparse-index: prevent repo root from becoming sparse
     @@ Commit message
          non-cone sparse-checkouts), the new check does not cause additional changes
          to how sparse patterns are applied.
      
     +    Helped-by: Derrick Stolee <derrickstolee@github.com>
          Signed-off-by: Victoria Dye <vdye@github.com>
      
       ## dir.c ##
     @@ dir.c: static int path_in_sparse_checkout_1(const char *path,
      +	 * patterns, or the patterns are of the wrong type.
       	 */
      -	if (init_sparse_checkout_patterns(istate) ||
     -+	if (!strlen(path) ||
     ++	if (!*path ||
      +	    init_sparse_checkout_patterns(istate) ||
       	    (require_cone_mode &&
       	     !istate->sparse_checkout_patterns->use_cone_patterns))
 2:  c21c9b9be34 = 2:  f0cff03b95d status: fix nested sparse directory diff in sparse index
 3:  ac42ae21d4a = 3:  ffe0b6aff2b read-tree: expand sparse checkout test coverage
 4:  5ee193bfa87 = 4:  cb7e0cf419c read-tree: integrate with sparse index
 5:  bea482b6b28 = 5:  4f05fa70209 read-tree: narrow scope of index expansion for '--prefix'
 6:  9fdcab038b2 = 6:  94c2aad2f93 read-tree: make two-way merge sparse-aware
 7:  1502e9acb32 = 7:  c4080e99d6e read-tree: make three-way merge sparse-aware

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