maintenance(geometric): avoid deadlocks on Windows 10 (#6215)

This PR is a companion of https://github.com/gitgitgadget/git/pull/2103.

On Windows, `maintenance_task_geometric_repack()` opens pack index files
via `pack_geometry_init()` (which `mmap()`s the `.idx` files), then
spawns `git repack` as a child process without setting
`child.odb_to_close`. The parent's `mmap()`s prevent the child from
deleting old `.idx` files.

On Windows 10 builds before the POSIX delete semantics change (between
Build 17134.1304 and 18363.657, see
https://stackoverflow.com/a/60512798), this results in `Unlink of file
'.git/objects/pack/pack-<hash>.idx' failed. Should I try again?` during
fetch-triggered auto-maintenance with the geometric strategy.

The fix adds the missing `child.odb_to_close = the_repository->objects`
line, matching all other maintenance tasks.

The first commit introduces a `GIT_TEST_LEGACY_DELETE` environment
variable to simulate legacy (pre-POSIX) delete semantics on modern
Windows, so the regression test can verify the fix even on Windows 11.

This fixes https://github.com/git-for-windows/git/issues/6210.

Tested-by: Patryk Miś <foss@patrykmis.com>

builtin/gc.c

@@ -1590,6 +1590,7 @@ static int maintenance_task_geometric_repack(struct maintenance_run_opts *opts,
 	pack_geometry_split(&geometry);
 
 	child.git_cmd = 1;
+	child.odb_to_close = the_repository->objects;
 
 	strvec_pushl(&child.args, "repack", "-d", "-l", NULL);
 	if (geometry.split < geometry.pack_nr)

compat/mingw.c

@@ -554,20 +554,63 @@ static wchar_t *normalize_ntpath(wchar_t *wbuf)
 	return wbuf;
 }
 
+/*
+ * Use SetFileInformationByHandle(FileDispositionInfo) to force legacy
+ * (non-POSIX) delete semantics. On Windows 11, DeleteFileW() uses POSIX
+ * delete semantics internally, allowing deletion even with active
+ * MapViewOfFile views. This helper simulates Windows 10 behavior where
+ * deletion fails if a file mapping exists.
+ *
+ * Returns nonzero on success (like DeleteFileW), 0 on failure.
+ */
+static int legacy_delete_file(const wchar_t *wpathname)
+{
+	FILE_DISPOSITION_INFO fdi = { TRUE };
+	DWORD gle;
+	HANDLE h = CreateFileW(wpathname, DELETE,
+			       FILE_SHARE_READ | FILE_SHARE_WRITE |
+			       FILE_SHARE_DELETE,
+			       NULL, OPEN_EXISTING,
+			       FILE_FLAG_OPEN_REPARSE_POINT, NULL);
+	if (h == INVALID_HANDLE_VALUE)
+		return 0;
+
+	if (SetFileInformationByHandle(h, FileDispositionInfo,
+				       &fdi, sizeof(fdi))) {
+		CloseHandle(h);
+		return 1;
+	}
+	gle = GetLastError();
+	CloseHandle(h);
+	SetLastError(gle);
+	return 0;
+}
+
+static int try_delete_file(const wchar_t *wpathname, int use_legacy)
+{
+	if (use_legacy)
+		return legacy_delete_file(wpathname);
+	return DeleteFileW(wpathname);
+}
+
 int mingw_unlink(const char *pathname, int handle_in_use_error)
 {
+	static int use_legacy_delete = -1;
 	int tries = 0;
 	wchar_t wpathname[MAX_LONG_PATH];
 	if (xutftowcs_long_path(wpathname, pathname) < 0)
 		return -1;
 
-	if (DeleteFileW(wpathname))
+	if (use_legacy_delete < 0)
+		use_legacy_delete = !!getenv("GIT_TEST_LEGACY_DELETE");
+
+	if (try_delete_file(wpathname, use_legacy_delete))
 		return 0;
 
 	do {
 		/* read-only files cannot be removed */
 		_wchmod(wpathname, 0666);
-		if (!_wunlink(wpathname))
+		if (try_delete_file(wpathname, use_legacy_delete))
 			return 0;
 		if (!is_file_in_use_error(GetLastError()))
 			break;

t/t7900-maintenance.sh

@@ -532,7 +532,16 @@ run_and_verify_geometric_pack () {
 
 	# And verify that there are no loose objects anymore.
 	git count-objects -v >count &&
-	test_grep '^count: 0$' count
+	test_grep '^count: 0$' count &&
+
+	# Verify that no orphaned .idx files were left behind. On
+	# Windows, a missing odb_to_close causes the parent to hold
+	# mmap handles on .idx files, silently preventing their
+	# deletion by the child git-repack process.
+	ls .git/objects/pack/pack-*.idx .git/objects/pack/pack-*.pack |
+	sed "s/\.pack$/.idx/" |
+	sort | uniq -u >orphaned-idx &&
+	test_must_be_empty orphaned-idx
 }
 
 test_expect_success 'geometric repacking task' '
@@ -580,8 +589,19 @@ test_expect_success 'geometric repacking task' '
 
 		# And these two small packs should now be merged via the
 		# geometric repack. The large packfile should remain intact.
+		cp -R .git/objects .git/objects.save &&
 		run_and_verify_geometric_pack 2 &&
 
+		# On Windows, verify the same with legacy delete semantics
+		# that reject deletion of mmap-held .idx files.
+		if test_have_prereq MINGW
+		then
+			rm -rf .git/objects &&
+			mv .git/objects.save .git/objects &&
+			test_env GIT_TEST_LEGACY_DELETE=1 \
+				run_and_verify_geometric_pack 2
+		fi &&
+
 		# If we now add two more objects and repack twice we should
 		# then see another all-into-one repack. This time around
 		# though, as we have unreachable objects, we should also see a