Merge branch 'ns/batched-fsync'

This merges the topic branch (specifically backported onto v2.33.1 to
allow for integrating into Git for Windows' `main` branch) that strikes
a better balance between safety and speed: rather than `fsync()`ing each
and every loose object file, we now offer to do it in a batch.

This will become the new default in Git for Windows.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Documentation/config/core.txt

@@ -576,12 +576,29 @@ core.whitespace::
   errors. The default tab width is 8. Allowed values are 1 to 63.
 
 core.fsyncObjectFiles::
-	This boolean will enable 'fsync()' when writing object files.
+	A value indicating the level of effort Git will expend in
+	trying to make objects added to the repo durable in the event
+	of an unclean system shutdown. This setting currently only
+	controls loose objects in the object store, so updates to any
+	refs or the index may not be equally durable.
 +
-This is a total waste of time and effort on a filesystem that orders
-data writes properly, but can be useful for filesystems that do not use
-journalling (traditional UNIX filesystems) or that only journal metadata
-and not file contents (OS X's HFS+, or Linux ext3 with "data=writeback").
+* `false` allows data to remain in file system caches according to
+  operating system policy, whence it may be lost if the system loses power
+  or crashes.
+* `true` triggers a data integrity flush for each loose object added to the
+  object store. This is the safest setting that is likely to ensure durability
+  across all operating systems and file systems that honor the 'fsync' system
+  call. However, this setting comes with a significant performance cost on
+  common hardware. Git does not currently fsync parent directories for
+  newly-added files, so some filesystems may still allow data to be lost on
+  system crash.
+* `batch` enables an experimental mode that uses interfaces available in some
+  operating systems to write loose object data with a minimal set of FLUSH
+  CACHE (or equivalent) commands sent to the storage controller. If the
+  operating system interfaces are not available, this mode behaves the same as
+  `true`. This mode is expected to be as safe as `true` on macOS for repos
+  stored on HFS+ or APFS filesystems and on Windows for repos stored on NTFS or
+  ReFS.
 
 core.preloadIndex::
 	Enable parallel index preload for operations like 'git diff'

Makefile

@@ -405,6 +405,8 @@ all::
 #
 # Define HAVE_CLOCK_MONOTONIC if your platform has CLOCK_MONOTONIC.
 #
+# Define HAVE_SYNC_FILE_RANGE if your platform has sync_file_range.
+#
 # Define NEEDS_LIBRT if your platform requires linking with librt (glibc version
 # before 2.17) for clock_gettime and CLOCK_MONOTONIC.
 #
@@ -1909,6 +1911,10 @@ ifdef HAVE_CLOCK_MONOTONIC
 	BASIC_CFLAGS += -DHAVE_CLOCK_MONOTONIC
 endif
 
+ifdef HAVE_SYNC_FILE_RANGE
+	BASIC_CFLAGS += -DHAVE_SYNC_FILE_RANGE
+endif
+
 ifdef NEEDS_LIBRT
 	EXTLIBS += -lrt
 endif

builtin/unpack-objects.c

@@ -1,5 +1,6 @@
 #include "builtin.h"
 #include "cache.h"
+#include "bulk-checkin.h"
 #include "config.h"
 #include "object-store.h"
 #include "object.h"
@@ -503,10 +504,12 @@ static void unpack_all(void)
 	if (!quiet)
 		progress = start_progress(_("Unpacking objects"), nr_objects);
 	CALLOC_ARRAY(obj_list, nr_objects);
+	plug_bulk_checkin();
 	for (i = 0; i < nr_objects; i++) {
 		unpack_one(i);
 		display_progress(progress, i + 1);
 	}
+	unplug_bulk_checkin();
 	stop_progress(&progress);
 
 	if (delta_list)

builtin/update-index.c

@@ -5,6 +5,7 @@
  */
 #define USE_THE_INDEX_COMPATIBILITY_MACROS
 #include "cache.h"
+#include "bulk-checkin.h"
 #include "config.h"
 #include "lockfile.h"
 #include "quote.h"
@@ -1088,6 +1089,9 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 
 	the_index.updated_skipworktree = 1;
 
+	/* we might be adding many objects to the object database */
+	plug_bulk_checkin();
+
 	/*
 	 * Custom copy of parse_options() because we want to handle
 	 * filename arguments as they come.
@@ -1168,6 +1172,8 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 		strbuf_release(&buf);
 	}
 
+	/* by now we must have added all of the new objects */
+	unplug_bulk_checkin();
 	if (split_index > 0) {
 		if (git_config_get_split_index() == 0)
 			warning(_("core.splitIndex is set to false; "

bulk-checkin.c

@@ -3,16 +3,22 @@
  */
 #include "cache.h"
 #include "bulk-checkin.h"
+#include "lockfile.h"
 #include "repository.h"
 #include "csum-file.h"
 #include "pack.h"
 #include "strbuf.h"
+#include "string-list.h"
+#include "tmp-objdir.h"
 #include "packfile.h"
 #include "object-store.h"
 
-static struct bulk_checkin_state {
-	unsigned plugged:1;
+static int bulk_checkin_plugged;
+static int needs_batch_fsync;
 
+static struct tmp_objdir *bulk_fsync_objdir;
+
+static struct bulk_checkin_state {
 	char *pack_tmp_name;
 	struct hashfile *f;
 	off_t offset;
@@ -21,7 +27,7 @@ static struct bulk_checkin_state {
 	struct pack_idx_entry **written;
 	uint32_t alloc_written;
 	uint32_t nr_written;
-} state;
+} bulk_checkin_state;
 
 static void finish_tmp_packfile(struct strbuf *basename,
 				const char *pack_tmp_name,
@@ -79,6 +85,34 @@ clear_exit:
 	reprepare_packed_git(the_repository);
 }
 
+/*
+ * Cleanup after batch-mode fsync_object_files.
+ */
+static void do_batch_fsync(void)
+{
+	/*
+	 * Issue a full hardware flush against a temporary file to ensure
+	 * that all objects are durable before any renames occur.  The code in
+	 * fsync_loose_object_bulk_checkin has already issued a writeout
+	 * request, but it has not flushed any writeback cache in the storage
+	 * hardware.
+	 */
+
+	if (needs_batch_fsync) {
+		struct strbuf temp_path = STRBUF_INIT;
+		struct tempfile *temp;
+
+		strbuf_addf(&temp_path, "%s/bulk_fsync_XXXXXX", get_object_directory());
+		temp = xmks_tempfile(temp_path.buf);
+		fsync_or_die(get_tempfile_fd(temp), get_tempfile_path(temp));
+		delete_tempfile(&temp);
+		strbuf_release(&temp_path);
+	}
+
+	if (bulk_fsync_objdir)
+		tmp_objdir_migrate(bulk_fsync_objdir);
+}
+
 static int already_written(struct bulk_checkin_state *state, struct object_id *oid)
 {
 	int i;
@@ -273,25 +307,61 @@ static int deflate_to_pack(struct bulk_checkin_state *state,
 	return 0;
 }
 
+void fsync_loose_object_bulk_checkin(int fd)
+{
+	assert(fsync_object_files == FSYNC_OBJECT_FILES_BATCH);
+
+	/*
+	 * If we have a plugged bulk checkin, we issue a call that
+	 * cleans the filesystem page cache but avoids a hardware flush
+	 * command. Later on we will issue a single hardware flush
+	 * before as part of do_batch_fsync.
+	 */
+	if (bulk_checkin_plugged &&
+	    git_fsync(fd, FSYNC_WRITEOUT_ONLY) >= 0) {
+		if (!needs_batch_fsync)
+			needs_batch_fsync = 1;
+	} else {
+		fsync_or_die(fd, "loose object file");
+	}
+}
+
 int index_bulk_checkin(struct object_id *oid,
 		       int fd, size_t size, enum object_type type,
 		       const char *path, unsigned flags)
 {
-	int status = deflate_to_pack(&state, oid, fd, size, type,
+	int status = deflate_to_pack(&bulk_checkin_state, oid, fd, size, type,
 				     path, flags);
-	if (!state.plugged)
-		finish_bulk_checkin(&state);
+	if (!bulk_checkin_plugged)
+		finish_bulk_checkin(&bulk_checkin_state);
 	return status;
 }
 
 void plug_bulk_checkin(void)
 {
-	state.plugged = 1;
+	assert(!bulk_checkin_plugged);
+
+	/*
+	 * A temporary object directory is used to hold the files
+	 * while they are not fsynced.
+	 */
+	if (fsync_object_files == FSYNC_OBJECT_FILES_BATCH) {
+		bulk_fsync_objdir = tmp_objdir_create("bulk-fsync");
+		if (!bulk_fsync_objdir)
+			die(_("Could not create temporary object directory for core.fsyncobjectfiles=batch"));
+
+		tmp_objdir_replace_primary_odb(bulk_fsync_objdir, 0);
+	}
+
+	bulk_checkin_plugged = 1;
 }
 
 void unplug_bulk_checkin(void)
 {
-	state.plugged = 0;
-	if (state.f)
-		finish_bulk_checkin(&state);
+	assert(bulk_checkin_plugged);
+	bulk_checkin_plugged = 0;
+	if (bulk_checkin_state.f)
+		finish_bulk_checkin(&bulk_checkin_state);
+
+	do_batch_fsync();
 }

bulk-checkin.h

@@ -6,6 +6,8 @@
 
 #include "cache.h"
 
+void fsync_loose_object_bulk_checkin(int fd);
+
 int index_bulk_checkin(struct object_id *oid,
 		       int fd, size_t size, enum object_type type,
 		       const char *path, unsigned flags);

cache.h

@@ -995,7 +995,13 @@ void reset_shared_repository(void);
 extern int read_replace_refs;
 extern char *git_replace_ref_base;
 
-extern int fsync_object_files;
+enum fsync_object_files_mode {
+    FSYNC_OBJECT_FILES_OFF,
+    FSYNC_OBJECT_FILES_ON,
+    FSYNC_OBJECT_FILES_BATCH
+};
+
+extern enum fsync_object_files_mode fsync_object_files;
 extern int use_fsync;
 extern int core_preload_index;
 extern int precomposed_unicode;

compat/mingw.h

@@ -332,6 +332,9 @@ int mingw_getpagesize(void);
 #define getpagesize mingw_getpagesize
 #endif
 
+int win32_fsync_no_flush(int fd);
+#define fsync_no_flush win32_fsync_no_flush
+
 struct rlimit {
 	unsigned int rlim_cur;
 };

compat/win32/flush.c

@@ -0,0 +1,28 @@
+#include "../../git-compat-util.h"
+#include <winternl.h>
+#include "lazyload.h"
+
+int win32_fsync_no_flush(int fd)
+{
+       IO_STATUS_BLOCK io_status;
+
+#define FLUSH_FLAGS_FILE_DATA_ONLY 1
+
+       DECLARE_PROC_ADDR(ntdll.dll, NTSTATUS, NtFlushBuffersFileEx,
+			 HANDLE FileHandle, ULONG Flags, PVOID Parameters, ULONG ParameterSize,
+			 PIO_STATUS_BLOCK IoStatusBlock);
+
+       if (!INIT_PROC_ADDR(NtFlushBuffersFileEx)) {
+		errno = ENOSYS;
+		return -1;
+       }
+
+       memset(&io_status, 0, sizeof(io_status));
+       if (NtFlushBuffersFileEx((HANDLE)_get_osfhandle(fd), FLUSH_FLAGS_FILE_DATA_ONLY,
+				NULL, 0, &io_status)) {
+		errno = EINVAL;
+		return -1;
+       }
+
+       return 0;
+}

config.c

@@ -1491,7 +1491,12 @@ int git_default_core_config(const char *var, const char *value, void *cb)
 	}
 
 	if (!strcmp(var, "core.fsyncobjectfiles")) {
-		fsync_object_files = git_config_bool(var, value);
+		if (value && !strcmp(value, "batch"))
+			fsync_object_files = FSYNC_OBJECT_FILES_BATCH;
+		else if (git_config_bool(var, value))
+			fsync_object_files = FSYNC_OBJECT_FILES_ON;
+		else
+			fsync_object_files = FSYNC_OBJECT_FILES_OFF;
 		return 0;
 	}
 

config.mak.uname

@@ -57,6 +57,7 @@ ifeq ($(uname_S),Linux)
 	HAVE_CLOCK_MONOTONIC = YesPlease
 	# -lrt is needed for clock_gettime on glibc <= 2.16
 	NEEDS_LIBRT = YesPlease
+	HAVE_SYNC_FILE_RANGE = YesPlease
 	HAVE_GETDELIM = YesPlease
 	FREAD_READS_DIRECTORIES = UnfortunatelyYes
 	BASIC_CFLAGS += -DHAVE_SYSINFO
@@ -475,6 +476,7 @@ endif
 	CFLAGS =
 	BASIC_CFLAGS = -nologo -I. -Icompat/vcbuild/include -DWIN32 -D_CONSOLE -DHAVE_STRING_H -D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_DEPRECATE
 	COMPAT_OBJS = compat/msvc.o compat/winansi.o \
+		compat/win32/flush.o \
 		compat/win32/path-utils.o \
 		compat/win32/pthread.o compat/win32/syslog.o \
 		compat/win32/trace2_win32_process_info.o \
@@ -662,6 +664,7 @@ ifeq ($(uname_S),MINGW)
 	COMPAT_CFLAGS += -DSTRIP_EXTENSION=\".exe\"
 	COMPAT_OBJS += compat/mingw.o compat/winansi.o \
 		compat/win32/trace2_win32_process_info.o \
+		compat/win32/flush.o \
 		compat/win32/path-utils.o \
 		compat/win32/pthread.o compat/win32/syslog.o \
 		compat/win32/dirent.o compat/win32/fscache.o

configure.ac

@@ -1095,6 +1095,14 @@ AC_COMPILE_IFELSE([CLOCK_MONOTONIC_SRC],
 	[AC_MSG_RESULT([no])
 	HAVE_CLOCK_MONOTONIC=])
 GIT_CONF_SUBST([HAVE_CLOCK_MONOTONIC])
+
+#
+# Define HAVE_SYNC_FILE_RANGE=YesPlease if sync_file_range is available.
+GIT_CHECK_FUNC(sync_file_range,
+	[HAVE_SYNC_FILE_RANGE=YesPlease],
+	[HAVE_SYNC_FILE_RANGE])
+GIT_CONF_SUBST([HAVE_SYNC_FILE_RANGE])
+
 #
 # Define NO_SETITIMER if you don't have setitimer.
 GIT_CHECK_FUNC(setitimer,

contrib/buildsystems/CMakeLists.txt

@@ -286,7 +286,8 @@ if(CMAKE_SYSTEM_NAME STREQUAL "Windows")
 				NOGDI OBJECT_CREATION_MODE=1 __USE_MINGW_ANSI_STDIO=0
 				USE_NED_ALLOCATOR OVERRIDE_STRDUP MMAP_PREVENTS_DELETE USE_WIN32_MMAP
 				UNICODE _UNICODE HAVE_WPGMPTR ENSURE_MSYSTEM_IS_SET)
-	list(APPEND compat_SOURCES compat/mingw.c compat/winansi.c compat/win32/path-utils.c
+	list(APPEND compat_SOURCES compat/mingw.c compat/winansi.c
+		compat/win32/flush.c compat/win32/path-utils.c
 		compat/win32/pthread.c compat/win32mmap.c compat/win32/syslog.c
 		compat/win32/trace2_win32_process_info.c compat/win32/dirent.c
 		compat/nedmalloc/nedmalloc.c compat/strdup.c compat/win32/fscache.c)

environment.c

@@ -42,7 +42,7 @@ const char *git_attributes_file;
 const char *git_hooks_path;
 int zlib_compression_level = Z_BEST_SPEED;
 int pack_compression_level = Z_DEFAULT_COMPRESSION;
-int fsync_object_files;
+enum fsync_object_files_mode fsync_object_files;
 int use_fsync = -1;
 size_t packed_git_window_size = DEFAULT_PACKED_GIT_WINDOW_SIZE;
 size_t packed_git_limit = DEFAULT_PACKED_GIT_LIMIT;

git-compat-util.h

@@ -1284,6 +1284,13 @@ __attribute__((format (printf, 1, 2))) NORETURN
 void BUG(const char *fmt, ...);
 #endif
 
+enum fsync_action {
+    FSYNC_WRITEOUT_ONLY,
+    FSYNC_HARDWARE_FLUSH
+};
+
+int git_fsync(int fd, enum fsync_action action);
+
 /*
  * Preserves errno, prints a message, but gives no warning for ENOENT.
  * Returns 0 on success, which includes trying to unlink an object that does

object-file.c

@@ -1850,8 +1850,18 @@ int hash_object_file(const struct git_hash_algo *algo, const void *buf,
 static void close_loose_object(int fd)
 {
 	if (!the_repository->objects->odb->will_destroy) {
-		if (fsync_object_files)
+		switch (fsync_object_files) {
+		case FSYNC_OBJECT_FILES_OFF:
+			break;
+		case FSYNC_OBJECT_FILES_ON:
 			fsync_or_die(fd, "loose object file");
+			break;
+		case FSYNC_OBJECT_FILES_BATCH:
+			fsync_loose_object_bulk_checkin(fd);
+			break;
+		default:
+			BUG("Invalid fsync_object_files mode.");
+		}
 	}
 
 	if (close(fd) != 0)

t/lib-unique-files.sh

@@ -0,0 +1,36 @@
+# Helper to create files with unique contents
+
+
+# Create multiple files with unique contents. Takes the number of
+# directories, the number of files in each directory, and the base
+# directory.
+#
+# test_create_unique_files 2 3 my_dir -- Creates 2 directories with 3 files
+#					 each in my_dir, all with unique
+#					 contents.
+
+test_create_unique_files() {
+	test "$#" -ne 3 && BUG "3 param"
+
+	local dirs=$1
+	local files=$2
+	local basedir=$3
+	local counter=0
+	test_tick
+	local basedata=$test_tick
+
+
+	rm -rf $basedir
+
+	for i in $(test_seq $dirs)
+	do
+		local dir=$basedir/dir$i
+
+		mkdir -p "$dir"
+		for j in $(test_seq $files)
+		do
+			counter=$((counter + 1))
+			echo "$basedata.$counter"  >"$dir/file$j.txt"
+		done
+	done
+}

t/perf/p3700-add.sh

@@ -0,0 +1,43 @@
+#!/bin/sh
+#
+# This test measures the performance of adding new files to the object database
+# and index. The test was originally added to measure the effect of the
+# core.fsyncObjectFiles=batch mode, which is why we are testing different values
+# of that setting explicitly and creating a lot of unique objects.
+
+test_description="Tests performance of add"
+
+. ./perf-lib.sh
+
+. $TEST_DIRECTORY/lib-unique-files.sh
+
+test_perf_default_repo
+test_checkout_worktree
+
+dir_count=10
+files_per_dir=50
+total_files=$((dir_count * files_per_dir))
+
+# We need to create the files each time we run the perf test, but
+# we do not want to measure the cost of creating the files, so run
+# the tet once.
+if test "${GIT_PERF_REPEAT_COUNT-1}" -ne 1
+then
+	echo "warning: Setting GIT_PERF_REPEAT_COUNT=1" >&2
+	GIT_PERF_REPEAT_COUNT=1
+fi
+
+for m in false true batch
+do
+	test_expect_success "create the files for core.fsyncObjectFiles=$m" '
+		git reset --hard &&
+		# create files across directories
+		test_create_unique_files $dir_count $files_per_dir files
+	'
+
+	test_perf "add $total_files files (core.fsyncObjectFiles=$m)" "
+		git -c core.fsyncobjectfiles=$m add files
+	"
+done
+
+test_done

t/perf/p3900-stash.sh

@@ -0,0 +1,46 @@
+#!/bin/sh
+#
+# This test measures the performance of adding new files to the object database
+# and index. The test was originally added to measure the effect of the
+# core.fsyncObjectFiles=batch mode, which is why we are testing different values
+# of that setting explicitly and creating a lot of unique objects.
+
+test_description="Tests performance of stash"
+
+. ./perf-lib.sh
+
+. $TEST_DIRECTORY/lib-unique-files.sh
+
+test_perf_default_repo
+test_checkout_worktree
+
+dir_count=10
+files_per_dir=50
+total_files=$((dir_count * files_per_dir))
+
+# We need to create the files each time we run the perf test, but
+# we do not want to measure the cost of creating the files, so run
+# the tet once.
+if test "${GIT_PERF_REPEAT_COUNT-1}" -ne 1
+then
+	echo "warning: Setting GIT_PERF_REPEAT_COUNT=1" >&2
+	GIT_PERF_REPEAT_COUNT=1
+fi
+
+for m in false true batch
+do
+	test_expect_success "create the files for core.fsyncObjectFiles=$m" '
+		git reset --hard &&
+		# create files across directories
+		test_create_unique_files $dir_count $files_per_dir files
+	'
+
+	# We only stash files in the 'files' subdirectory since
+	# the perf test infrastructure creates files in the
+	# current working directory that need to be preserved
+	test_perf "stash 500 files (core.fsyncObjectFiles=$m)" "
+		git -c core.fsyncobjectfiles=$m stash push -u -- files
+	"
+done
+
+test_done

t/t3700-add.sh

@@ -8,6 +8,8 @@ test_description='Test of git add, including the -- option.'
 TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
+. $TEST_DIRECTORY/lib-unique-files.sh
+
 # Test the file mode "$1" of the file "$2" in the index.
 test_mode_in_index () {
 	case "$(git ls-files -s "$2")" in
@@ -34,6 +36,24 @@ test_expect_success \
     'Test that "git add -- -q" works' \
     'touch -- -q && git add -- -q'
 
+test_expect_success 'git add: core.fsyncobjectfiles=batch' "
+	test_create_unique_files 2 4 fsync-files &&
+	git -c core.fsyncobjectfiles=batch add -- ./fsync-files/ &&
+	rm -f fsynced_files &&
+	git ls-files --stage fsync-files/ > fsynced_files &&
+	test_line_count = 8 fsynced_files &&
+	awk -- '{print \$2}' fsynced_files | xargs -n1 git cat-file -e
+"
+
+test_expect_success 'git update-index: core.fsyncobjectfiles=batch' "
+	test_create_unique_files 2 4 fsync-files2 &&
+	find fsync-files2 ! -type d -print | xargs git -c core.fsyncobjectfiles=batch update-index --add -- &&
+	rm -f fsynced_files2 &&
+	git ls-files --stage fsync-files2/ > fsynced_files2 &&
+	test_line_count = 8 fsynced_files2 &&
+	awk -- '{print \$2}' fsynced_files2 | xargs -n1 git cat-file -e
+"
+
 test_expect_success \
 	'git add: Test that executable bit is not used if core.filemode=0' \
 	'git config core.filemode 0 &&

t/t3903-stash.sh

@@ -9,6 +9,7 @@ GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
 export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 
 . ./test-lib.sh
+. $TEST_DIRECTORY/lib-unique-files.sh
 
 test_expect_success 'usage on cmd and subcommand invalid option' '
 	test_expect_code 129 git stash --invalid-option 2>usage &&
@@ -1323,6 +1324,19 @@ test_expect_success 'stash handles skip-worktree entries nicely' '
 	git rev-parse --verify refs/stash:A.t
 '
 
+test_expect_success 'stash with core.fsyncobjectfiles=batch' "
+	test_create_unique_files 2 4 fsync-files &&
+	git -c core.fsyncobjectfiles=batch stash push -u -- ./fsync-files/ &&
+	rm -f fsynced_files &&
+
+	# The files were untracked, so use the third parent,
+	# which contains the untracked files
+	git ls-tree -r stash^3 -- ./fsync-files/ > fsynced_files &&
+	test_line_count = 8 fsynced_files &&
+	awk -- '{print \$3}' fsynced_files | xargs -n1 git cat-file -e
+"
+
+
 test_expect_success 'stash -c stash.useBuiltin=false warning ' '
 	expected="stash.useBuiltin support has been removed" &&
 

t/t5300-pack-object.sh

@@ -162,23 +162,23 @@ test_expect_success 'pack-objects with bogus arguments' '
 
 check_unpack () {
 	test_when_finished "rm -rf git2" &&
-	git init --bare git2 &&
-	git -C git2 unpack-objects -n <"$1".pack &&
-	git -C git2 unpack-objects <"$1".pack &&
-	(cd .git && find objects -type f -print) |
-	while read path
-	do
-		cmp git2/$path .git/$path || {
-			echo $path differs.
-			return 1
-		}
-	done
+	git $2 init --bare git2 &&
+	(
+		git $2 -C git2 unpack-objects -n <"$1".pack &&
+		git $2 -C git2 unpack-objects <"$1".pack &&
+		git $2 -C git2 cat-file --batch-check="%(objectname)"
+	) <obj-list >current &&
+	cmp obj-list current
 }
 
 test_expect_success 'unpack without delta' '
 	check_unpack test-1-${packname_1}
 '
 
+test_expect_success 'unpack without delta (core.fsyncobjectfiles=batch)' '
+	check_unpack test-1-${packname_1} "-c core.fsyncobjectfiles=batch"
+'
+
 test_expect_success 'pack with REF_DELTA' '
 	packname_2=$(git pack-objects --progress test-2 <obj-list 2>stderr) &&
 	check_deltas stderr -gt 0
@@ -188,6 +188,10 @@ test_expect_success 'unpack with REF_DELTA' '
 	check_unpack test-2-${packname_2}
 '
 
+test_expect_success 'unpack with REF_DELTA (core.fsyncobjectfiles=batch)' '
+       check_unpack test-2-${packname_2} "-c core.fsyncobjectfiles=batch"
+'
+
 test_expect_success 'pack with OFS_DELTA' '
 	packname_3=$(git pack-objects --progress --delta-base-offset test-3 \
 			<obj-list 2>stderr) &&
@@ -198,6 +202,10 @@ test_expect_success 'unpack with OFS_DELTA' '
 	check_unpack test-3-${packname_3}
 '
 
+test_expect_success 'unpack with OFS_DELTA (core.fsyncobjectfiles=batch)' '
+       check_unpack test-3-${packname_3} "-c core.fsyncobjectfiles=batch"
+'
+
 test_expect_success 'compare delta flavors' '
 	perl -e '\''
 		defined($_ = -s $_) or die for @ARGV;

wrapper.c

@@ -546,6 +546,54 @@ int xmkstemp_mode(char *filename_template, int mode)
 	return fd;
 }
 
+int git_fsync(int fd, enum fsync_action action)
+{
+	switch (action) {
+	case FSYNC_WRITEOUT_ONLY:
+
+#ifdef __APPLE__
+		/*
+		 * on macOS, fsync just causes filesystem cache writeback but does not
+		 * flush hardware caches.
+		 */
+		return fsync(fd);
+#endif
+
+#ifdef HAVE_SYNC_FILE_RANGE
+		/*
+		 * On linux 2.6.17 and above, sync_file_range is the way to issue
+		 * a writeback without a hardware flush. An offset of 0 and size of 0
+		 * indicates writeout of the entire file and the wait flags ensure that all
+		 * dirty data is written to the disk (potentially in a disk-side cache)
+		 * before we continue.
+		 */
+
+		return sync_file_range(fd, 0, 0, SYNC_FILE_RANGE_WAIT_BEFORE |
+						 SYNC_FILE_RANGE_WRITE |
+						 SYNC_FILE_RANGE_WAIT_AFTER);
+#endif
+
+#ifdef fsync_no_flush
+		return fsync_no_flush(fd);
+#endif
+
+		errno = ENOSYS;
+		return -1;
+
+	case FSYNC_HARDWARE_FLUSH:
+
+#ifdef __APPLE__
+		return fcntl(fd, F_FULLFSYNC);
+#else
+		return fsync(fd);
+#endif
+
+	default:
+		BUG("unexpected git_fsync(%d) call", action);
+	}
+
+}
+
 static int warn_if_unremovable(const char *op, const char *file, int rc)
 {
 	int err;

write-or-die.c

@@ -62,7 +62,7 @@ void fsync_or_die(int fd, const char *msg)
 		use_fsync = git_env_bool("GIT_TEST_FSYNC", 1);
 	if (!use_fsync)
 		return;
-	while (fsync(fd) < 0) {
+	while (git_fsync(fd, FSYNC_HARDWARE_FLUSH) < 0) {
 		if (errno != EINTR)
 			die_errno("fsync error on '%s'", msg);
 	}