core.fsync: introduce granular fsync control infrastructure

This commit introduces the infrastructure for the core.fsync
configuration knob. The repository components we want to sync
are identified by flags so that we can turn on or off syncing
for specific components.

If core.fsyncObjectFiles is set and the core.fsync configuration
also includes FSYNC_COMPONENT_LOOSE_OBJECT, we will fsync any
loose objects. This picks the strictest data integrity behavior
if core.fsync and core.fsyncObjectFiles are set to conflicting values.

This change introduces the currently unused fsync_component
helper, which will be used by a later patch that adds fsyncing to
the refs backend.

Actual configuration and documentation of the fsync components
list are in other patches in the series to separate review of
the underlying mechanism from the policy of how it's configured.

Helped-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Neeraj Singh <neerajsi@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 22 insertions, 4 deletions

diff --git a/write-or-die.c b/write-or-die.c
index 9faa5f9f56..c4fd91b5b4 100644
--- a/write-or-die.c
+++ b/write-or-die.c
@@ -56,21 +56,39 @@ void fprintf_or_die(FILE *f, const char *fmt, ...)
 	}
 }
 
-void fsync_or_die(int fd, const char *msg)
+static int maybe_fsync(int fd)
 {
 	if (use_fsync < 0)
 		use_fsync = git_env_bool("GIT_TEST_FSYNC", 1);
 	if (!use_fsync)
-		return;
+		return 0;
 
 	if (fsync_method == FSYNC_METHOD_WRITEOUT_ONLY &&
 	    git_fsync(fd, FSYNC_WRITEOUT_ONLY) >= 0)
-		return;
+		return 0;
+
+	return git_fsync(fd, FSYNC_HARDWARE_FLUSH);
+}
 
-	if (git_fsync(fd, FSYNC_HARDWARE_FLUSH) < 0)
+void fsync_or_die(int fd, const char *msg)
+{
+	if (maybe_fsync(fd) < 0)
 		die_errno("fsync error on '%s'", msg);
 }
 
+int fsync_component(enum fsync_component component, int fd)
+{
+	if (fsync_components & component)
+		return maybe_fsync(fd);
+	return 0;
+}
+
+void fsync_component_or_die(enum fsync_component component, int fd, const char *msg)
+{
+	if (fsync_components & component)
+		fsync_or_die(fd, msg);
+}
+
 void write_or_die(int fd, const void *buf, size_t count)
 {
 	if (write_in_full(fd, buf, count) < 0) {