[PATCH v3 3/6] core.fsyncobjectfiles: add windows support for batch mode

["Neeraj Singh via GitGitGadget" <gitgitgadget@gmail.com>]

From: Neeraj Singh <neerajsi@microsoft.com>

This commit adds a win32 implementation for fsync_no_flush that is
called git_fsync. The 'NtFlushBuffersFileEx' function being called is
available since Windows 8. If the function is not available, we
return -1 and Git falls back to doing a full fsync.

The operating system is told to flush data only without a hardware
flush primitive. A later full fsync will cause the metadata log
to be flushed and then the disk cache to be flushed on NTFS and
ReFS. Other filesystems will treat this as a full flush operation.

I added a new file here for this system call so as not to conflict with
downstream changes in the git-for-windows repository related to fscache.

Signed-off-by: Neeraj Singh <neerajsi@microsoft.com>
---
 compat/mingw.h                      |  3 +++
 compat/win32/flush.c                | 29 +++++++++++++++++++++++++++++
 config.mak.uname                    |  2 ++
 contrib/buildsystems/CMakeLists.txt |  3 ++-
 wrapper.c                           |  4 ++++
 5 files changed, 40 insertions(+), 1 deletion(-)
 create mode 100644 compat/win32/flush.c

diff --git a/compat/mingw.h b/compat/mingw.h
index c9a52ad64a6..6074a3d3ced 100644
--- a/compat/mingw.h
+++ b/compat/mingw.h
@@ -329,6 +329,9 @@ 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;
 };
diff --git a/compat/win32/flush.c b/compat/win32/flush.c
new file mode 100644
index 00000000000..c013920ce37
--- /dev/null
+++ b/compat/win32/flush.c
@@ -0,0 +1,29 @@
+#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;
+       }
+
+       /* See https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/ntifs/nf-ntifs-ntflushbuffersfileex */
+       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;
+}
diff --git a/config.mak.uname b/config.mak.uname
index e6d482fbcc6..34c93314a50 100644
--- a/config.mak.uname
+++ b/config.mak.uname
@@ -451,6 +451,7 @@ 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 \
@@ -626,6 +627,7 @@ ifneq (,$(findstring MINGW,$(uname_S)))
 	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
diff --git a/contrib/buildsystems/CMakeLists.txt b/contrib/buildsystems/CMakeLists.txt
index 171b4124afe..b573a5ee122 100644
--- a/contrib/buildsystems/CMakeLists.txt
+++ b/contrib/buildsystems/CMakeLists.txt
@@ -261,7 +261,8 @@ 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)
diff --git a/wrapper.c b/wrapper.c
index cffe24d307a..a9647018b68 100644
--- a/wrapper.c
+++ b/wrapper.c
@@ -565,6 +565,10 @@ int git_fsync(int fd, enum fsync_action action)
 						 SYNC_FILE_RANGE_WAIT_AFTER);
 #endif
 
+#ifdef fsync_no_flush
+		return fsync_no_flush(fd);
+#endif
+
 		errno = ENOSYS;
 		return -1;
 	}
-- 
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