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 <[email protected]>

Author: dscho

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.
-+
-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").
+	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.
++
+* `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.
 #
@@ -1907,6 +1909,10 @@ 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 @@ static void finish_bulk_checkin(struct bulk_checkin_state *state)
 	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

@@ -993,7 +993,13 @@ 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

@@ -330,6 +330,9 @@ 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, NTAPI, 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;
 	}