unpack-trees: skip lstats for deleted VFS entries in checkout (#865)

When core_virtualfilesystem is set and a branch switch deletes entries
(present in old tree, absent in new tree), deleted_entry() calls
verify_absent_if_directory() with 'ce' pointing to a tree entry from
traverse_trees(). This tree entry lacks CE_NEW_SKIP_WORKTREE because
that flag is only set on src_index entries by mark_new_skip_worktree().

The missing flag causes verify_absent_if_directory()'s fast-path to
fail, falling through to verify_absent_1() which lstats every such path.
In a VFS repo each lstat may trigger callbacks, creating placeholders.
On a large repo switching between LTS releases this produces tens of
thousands of placeholders that the VFS must then clean up when they are
deleted as part of the checkout.

Fix this by propagating CE_NEW_SKIP_WORKTREE from the index entry (old)
to the tree entry (ce) when core_virtualfilesystem is set. This allows
the existing fast-path to work, eliminating the unnecessary lstats
entirely.

Measured on a 2.8M file VFS repo (0% hydrated):
  Before: ~135s checkout, ~23k folder placeholders created
  After:  ~25s checkout, 0 folder placeholders created

* [x] This change only applies to the virtualization hook and VFS for
Git.

Author: dscho

t/t1093-virtualfilesystem.sh

@@ -368,6 +368,57 @@ test_expect_success 'folder with same prefix as file' '
 	test_cmp expected actual
 '
 
+test_expect_success 'checkout skips lstat for deleted skip-worktree entries in VFS mode' '
+	# When switching branches, entries present in the old tree but absent
+	# in the new tree go through deleted_entry() -> verify_absent_if_directory().
+	# Without the fix, the tree entry lacks CE_NEW_SKIP_WORKTREE (only
+	# src_index entries get that flag), so verify_absent_if_directory()
+	# falls through to verify_absent_1() which lstats the path. If a
+	# directory exists where the deleted file entry was (simulating a
+	# worst-case scenario), the lstat finds it and
+	# verify_clean_subdirectory() rejects the checkout due to untracked
+	# content inside.
+	#
+	# With the fix, verify_absent_if_directory() is skipped entirely
+	# when VFS mode is active — no lstat, no rejection, checkout completes.
+	#
+	# Set up two branches: main has dir1/ + dir2/, side has only dir1/
+	clean_repo &&
+
+	test_when_finished "rm -rf dir2/file1.txt && git -c core.virtualfilesystem= checkout main" &&
+
+	git -c core.virtualfilesystem= checkout -b side &&
+	git -c core.virtualfilesystem= rm -rf dir2 &&
+	git -c core.virtualfilesystem= commit -m "remove dir2" &&
+	git -c core.virtualfilesystem= checkout main &&
+
+	# Configure VFS hook that returns nothing (0% hydration)
+	write_script .git/hooks/virtualfilesystem <<-\EOF &&
+		printf ""
+	EOF
+
+	# Create a directory where the deleted file entry is, with
+	# untracked content inside. This would not happen with a real
+	# VFS because the VFS would report the file-to-directory change
+	# in the virtualfilesystem hook results, clearing skip-worktree.
+	# But it lets us verify that the lstat is not called: without
+	# the fix, verify_absent_1() lstats this path, finds a directory,
+	# and verify_clean_subdirectory() rejects the checkout because of
+	# the untracked file inside.
+	rm -f dir2/file1.txt &&
+	mkdir -p dir2/file1.txt &&
+	echo "untracked" >dir2/file1.txt/trap.txt &&
+
+	# Verify all entries are skip-worktree before checkout
+	git ls-files -v >actual &&
+	! grep "^H " actual &&
+
+	# Checkout to side branch. Without the fix this fails because
+	# verify_absent_1 finds untracked content in the directory at
+	# dir2/file1.txt. With the fix the lstat is skipped entirely.
+	git checkout side
+'
+
 test_expect_success MINGW,FSMONITOR_DAEMON 'virtualfilesystem hook disables built-in FSMonitor' '
 	clean_repo &&
 	test_config core.usebuiltinfsmonitor true &&

unpack-trees.c

@@ -2721,6 +2721,18 @@ static int deleted_entry(const struct cache_entry *ce,
 		if (verify_absent(ce, ERROR_WOULD_LOSE_UNTRACKED_REMOVED, o))
 			return -1;
 		return 0;
+	} else if (core_virtualfilesystem &&
+		   old->ce_flags & CE_NEW_SKIP_WORKTREE) {
+		/*
+		 * When core_virtualfilesystem is set, 'ce' may be a tree
+		 * entry from traverse_trees() that lacks CE_NEW_SKIP_WORKTREE
+		 * (only src_index entries get that flag from
+		 * mark_new_skip_worktree()). Propagate it from the index
+		 * entry so apply_sparse_checkout() preserves CE_SKIP_WORKTREE
+		 * later, and skip verify_absent_if_directory() entirely to
+		 * avoid unnecessary lstats on virtualized paths.
+		 */
+		((struct cache_entry *)ce)->ce_flags |= CE_NEW_SKIP_WORKTREE;
 	} else if (verify_absent_if_directory(ce, ERROR_WOULD_LOSE_UNTRACKED_REMOVED, o)) {
 		return -1;
 	}