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>

config.c

@@ -1600,6 +1600,18 @@ static int git_default_core_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.fsyncmethod")) {
+		if (!value)
+			return config_error_nonbool(var);
+		if (!strcmp(value, "fsync"))
+			fsync_method = FSYNC_METHOD_FSYNC;
+		else if (!strcmp(value, "writeout-only"))
+			fsync_method = FSYNC_METHOD_WRITEOUT_ONLY;
+		else
+			warning(_("ignoring unknown core.fsyncMethod value '%s'"), value);
+
+	}
+
 	if (!strcmp(var, "core.fsyncobjectfiles")) {
 		fsync_object_files = git_config_bool(var, value);
 		return 0;