[PATCH v3 6/6] unpack-trees: preserve index sparsity

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

From: Victoria Dye <vdye@github.com>

When unpacking trees, set the default sparsity of the resultant index based
on repo settings and 'is_sparse_index_allowed()'.

Normally, when executing 'unpack_trees', the output index is marked sparse
when (and only when) it unpacks a sparse directory. However, an index may be
"sparse" even if it contains no sparse directories - when all files fall
inside the sparse-checkout definition or otherwise have SKIP_WORKTREE
disabled. Therefore, the output index may be marked "full" even when it is
"sparse", resulting in unnecessary 'ensure_full_index' calls when writing to
disk. Avoid this by setting the "default" index sparsity to match what is
expected for the repository.

As a consequence of this fix, the (non-merge) 'read-tree' performed when
applying a stash with untracked files no longer expands the index. Update
the corresponding test in 't1092'.

Signed-off-by: Victoria Dye <vdye@github.com>
---
 t/t1092-sparse-checkout-compatibility.sh | 2 +-
 unpack-trees.c                           | 6 ++++++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index aaf4d880dbc..19221c14225 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -1392,7 +1392,7 @@ test_expect_success 'sparse-index is not expanded: stash' '
 	ensure_not_expanded stash -u &&
 	(
 		WITHOUT_UNTRACKED_TXT=1 &&
-		! ensure_not_expanded stash pop
+		ensure_not_expanded stash pop
 	) &&
 
 	ensure_not_expanded stash create &&
diff --git a/unpack-trees.c b/unpack-trees.c
index 7f528d35cc2..a1d0ff3a4d3 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -11,6 +11,7 @@
 #include "refs.h"
 #include "attr.h"
 #include "split-index.h"
+#include "sparse-index.h"
 #include "submodule.h"
 #include "submodule-config.h"
 #include "fsmonitor.h"
@@ -1839,6 +1840,11 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	o->result.fsmonitor_last_update =
 		xstrdup_or_null(o->src_index->fsmonitor_last_update);
 
+	if (!o->src_index->initialized &&
+	    !repo->settings.command_requires_full_index &&
+	    is_sparse_index_allowed(&o->result, 0))
+		o->result.sparse_index = 1;
+
 	/*
 	 * Sparse checkout loop #1: set NEW_SKIP_WORKTREE on existing entries
 	 */
-- 
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