Sparse index: integrate with 'git stash'

This series, in combination with the sparse index integrations of reset [1], update-index [2], checkout-index [2], clean [2], and read-tree [3], allows most subcommands of 'git stash' to use the sparse index end-to-end without index expansion.

Like the earlier series, this series starts with new tests ensuring compatibility of the sparse index with non-sparse index full and sparse checkouts [1/6]. Next, sparse index is trivially enabled [2/6]. Functionally, sparse index-enabled sparse-checkouts remain compatible with non-sparse index sparse-checkouts, but there are still some cases where the index (or a temporary index) is expanded unnecessarily. These cases are fixed in three parts:

* First, 'git stash -u' is made sparse index-compatible by ensuring the "temporary" index holding the stashed, untracked files is created as a sparse index whenever possible (per repo settings & 'is_sparse_index_allowed()'). Patch [3/6] exposes 'is_sparse_index_allowed()' to files outside of 'sparse-index.c', then patch [4/6] uses that function to mark the temporary index sparse when appropriate.
* Next, 'git stash (apply|pop)' are made sparse index-compatible by changing their internal merge function from 'merge_recursive_generic()' (which constructs "fake" commits from the working tree, stash, and stash base) to 'merge_ort_nonrecursive()' (which operates on the trees directly) in patch [5/6]. The use of the non-recursive 'merge-ort' helps us to avoid sparse index expansion associated with 'merge-recursive', as well as avoid the unused/unnecessary complexity of a recursive merge.
* Finally, while patch 5 skips index expansion for most cases of 'git stash (apply|pop)', applying a stash that includes untracked files still expands the index. This is a result of an internal 'read-tree' execution (specifically in its 'unpack_trees' call) creating a result index that is never sparse in-core, thus forcing the index to be unnecessarily collapsed and re-expanded in 'do_write_locked_index()'. In patch [6/6], 'unpack_trees' is updated to set the default sparsity of the resultant index to "sparse" if allowed by repo settings and 'is_sparse_index_allowed()' (similar to the change in patch 4).

Performance results (from the 'p2000' tests):

(git stash &&
git stash pop) master this series
---------------------------------------------------------------------

full-v3                      4.07(2.42+1.34)   3.98(2.42+1.32) -2.2%
full-v4                      4.05(2.46+1.31)   4.00(2.49+1.29) -1.2%
sparse-v3                    7.48(4.81+2.57)   1.53(0.26+1.61) -79.5%
sparse-v4                    7.35(4.74+2.54)   1.59(0.27+1.63) -78.4%

(echo >>new &&
git stash -u &&
git stash pop) master this series
---------------------------------------------------------------------

full-v3                      4.21(2.62+1.45)   4.11(2.55+1.44) -2.4%
full-v4                      4.11(2.51+1.41)   4.02(2.49+1.41) -2.2%
sparse-v3                    7.35(4.64+2.66)   1.70(0.32+1.64) -76.9%
sparse-v4                    7.74(4.87+2.83)   1.70(0.32+1.66) -78.0%

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

* Replaced use of 'merge_recursive_generic' with 'merge_ort_nonrecursive' in 'do_apply_stash()'
* Rebased on top of 'master'

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

* Added quotes to the "$WITHOUT_UNTRACKED_TXT" when testing for it in 'ensure_not_expanded' (in 't/t1092-sparse-checkout-compatibility.sh')
* Moved the 'stash' test in 't1092' elsewhere in the file, so that it doesn't conflict (even trivially) with the also-in-flight 'git show' integration
* Moved the 'ensure_not_expended' tests for 'checkout-index' back to original location

[1]
https://lore.kernel.org/git/[email protected]/
[2]
https://lore.kernel.org/git/[email protected]/
[3]
https://lore.kernel.org/git/[email protected]/

Thanks!

-Victoria

Victoria Dye (6):
stash: expand sparse-checkout compatibility testing
stash: integrate with sparse index
sparse-index: expose 'is_sparse_index_allowed()'
read-cache: set sparsity when index is new
stash: apply stash using 'merge_ort_nonrecursive()'
unpack-trees: preserve index sparsity

 builtin/stash.c                          | 33 ++++++++--
 read-cache.c                             | 18 +++++-
 sparse-index.c                           |  2 +-
 sparse-index.h                           |  1 +
 t/perf/p2000-sparse-operations.sh        |  2 +
 t/t1092-sparse-checkout-compatibility.sh | 78 +++++++++++++++++++++++-
 unpack-trees.c                           |  6 ++
 7 files changed, 131 insertions(+), 9 deletions(-)

base-commit: 0f828332d5ac36fc63b7d8202652efa152809856
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1171%2Fvdye%2Fsparse%2Fstash-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1171/vdye/sparse/stash-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/1171

Range-diff vs v2:

 1:  8ea986cb249 = 1:  4e0a09f7a3c stash: expand sparse-checkout compatibility testing
 2:  b3e3f0298fb = 2:  7be484a8c0b stash: integrate with sparse index
 3:  73f04e95400 = 3:  6f00fca9267 sparse-index: expose 'is_sparse_index_allowed()'
 4:  42550f39a75 = 4:  bb092c075f4 read-cache: set sparsity when index is new
 5:  4537d473b93 < -:  ----------- merge-recursive: add merge function arg to 'merge_recursive_generic'
 6:  22fee0732ad ! 5:  e733c2fd9f4 stash: merge applied stash with merge-ort

@@ Metadata
Author: Victoria Dye <[email protected]>

## Commit message ##
- stash: merge applied stash with merge-ort
+ stash: apply stash using 'merge_ort_nonrecursive()'

- Change the merge function used in 'do_apply_stash()' from 'merge_recursive'
- to 'merge_ort_recursive'. In addition to aligning with the default merge
- strategy used by 'git merge' (6a5fb96672 (Change default merge backend from
- recursive to ort, 2021-08-04)), this allows 'git stash <apply|pop>' to
- operate without expanding the index by default. Update tests in 't1092'
- verifying index expansion for 'git stash' accordingly.
+ Update 'stash' to use 'merge_ort_nonrecursive()' to apply a stash to the
+ current working tree. When 'git stash apply' was converted from its shell
+ script implementation to a builtin in 8a0fc8d19d (stash: convert apply to
+ builtin, 2019-02-25), 'merge_recursive_generic()' was used to merge a stash
+ into the working tree as part of 'git stash (apply|pop)'. However, with the
+ single merge base used in 'do_apply_stash()', the commit wrapping done by
+ 'merge_recursive_generic()' is not only unnecessary, but misleading (the
+ *real* merge base is labeled "constructed merge base"). Therefore, a
+ non-recursive merge of the working tree, stashed tree, and stash base tree
+ is more appropriate.
+
+ There are two options for a non-recursive merge-then-update-worktree
+ function: 'merge_trees()' and 'merge_ort_nonrecursive()'. Use
+ 'merge_ort_nonrecursive()' to align with the default merge strategy used by
+ 'git merge' (6a5fb96672 (Change default merge backend from recursive to ort,
+ 2021-08-04)) and, because merge-ort does not operate in-place on the index,
+ avoid unnecessary index expansion. Update tests in 't1092' verifying index
+ expansion for 'git stash' accordingly.

@@ builtin/stash.c
#include "strvec.h"
#include "run-command.h"
#include "dir.h"
+@@ builtin/stash.c: static void unstage_changes_unless_new(struct object_id *orig_tree)
+ static int do_apply_stash(const char *prefix, struct stash_info *info,
+ int index, int quiet)
+ {
+- int ret;
++ int clean, ret;
+ int has_index = index;
+ struct merge_options o;
+ struct object_id c_tree;
+ struct object_id index_tree;
+- struct commit *result;
+- const struct object_id *bases[1];
++ struct tree *head, *merge, *merge_base;
++ struct lock_file lock = LOCK_INIT;
+
+ read_cache_preload(NULL);
+ if (refresh_and_write_cache(REFRESH_QUIET, 0, 0))
+@@ builtin/stash.c: static int do_apply_stash(const char *prefix, struct stash_info *info, +
+ o.branch1 = "Updated upstream";
+ o.branch2 = "Stashed changes";
++ o.ancestor = "Stash base";
+
+ if (oideq(&info->b_tree, &c_tree))
+ o.branch1 = "Version stash was based on";
@@ builtin/stash.c: static int do_apply_stash(const char *prefix, struct stash_info *info,
- bases[0] = &info->b_tree;
+ if (o.verbosity >= 3)
+ printf_ln(_("Merging %s with %s"), o.branch1, o.branch2);
+
+- bases[0] = &info->b_tree;
++ head = lookup_tree(o.repo, &c_tree);
++ merge = lookup_tree(o.repo, &info->w_tree);
++ merge_base = lookup_tree(o.repo, &info->b_tree);
++
++ repo_hold_locked_index(o.repo, &lock, LOCK_DIE_ON_ERROR);
++ clean = merge_ort_nonrecursive(&o, head, merge, merge_base);
++
++ /*
++ * If 'clean' >= 0, reverse the value for 'ret' so 'ret' is 0 when the
++ * merge was clean, and nonzero if the merge was unclean or encountered
++ * an error.
++ */
++ ret = clean >= 0 ? !clean : clean;
++
++ if (ret < 0)
++ rollback_lock_file(&lock);
++ else if (write_locked_index(o.repo->index, &lock,
++ COMMIT_LOCK | SKIP_IF_UNCHANGED))
++ ret = error(_("could not write index"));

- ret = merge_recursive_generic(&o, &c_tree, &info->w_tree, 1, bases,
-- merge_recursive, &result);
-+ merge_ort_recursive, &result);
+- ret = merge_recursive_generic(&o, &c_tree, &info->w_tree, 1, bases,
+- &result);
if (ret) {
rerere(0);

7: 3179018a8cb = 6: 4b4c38fcc03 unpack-trees: preserve index sparsity

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