core.fsyncmethod: add writeout-only mode

This commit introduces the `core.fsyncMethod` configuration
knob, which can currently be set to `fsync` or `writeout-only`.

The new writeout-only mode attempts to tell the operating system to
flush its in-memory page cache to the storage hardware without issuing a
CACHE_FLUSH command to the storage controller.

Writeout-only fsync is significantly faster than a vanilla fsync on
common hardware, since data is written to a disk-side cache rather than
all the way to a durable medium. Later changes in this patch series will
take advantage of this primitive to implement batching of hardware
flushes.

When git_fsync is called with FSYNC_WRITEOUT_ONLY, it may fail and the
caller is expected to do an ordinary fsync as needed.

On Apple platforms, the fsync system call does not issue a CACHE_FLUSH
directive to the storage controller. This change updates fsync to do
fcntl(F_FULLFSYNC) to make fsync actually durable. We maintain parity
with existing behavior on Apple platforms by setting the default value
of the new core.fsyncMethod option.

Signed-off-by: Neeraj Singh <neerajsi@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

wrapper.c

@@ -546,6 +546,70 @@ int xmkstemp_mode(char *filename_template, int mode)
 	return fd;
 }
 
+/*
+ * Some platforms return EINTR from fsync. Since fsync is invoked in some
+ * cases by a wrapper that dies on failure, do not expose EINTR to callers.
+ */
+static int fsync_loop(int fd)
+{
+	int err;
+
+	do {
+		err = fsync(fd);
+	} while (err < 0 && errno == EINTR);
+	return err;
+}
+
+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_loop(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:
+		/*
+		 * On macOS, a special fcntl is required to really flush the
+		 * caches within the storage controller. As of this writing,
+		 * this is a very expensive operation on Apple SSDs.
+		 */
+#ifdef __APPLE__
+		return fcntl(fd, F_FULLFSYNC);
+#else
+		return fsync_loop(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;