In ec983eb5d2 (core.fsyncobjectfiles: batched disk flushes, 2021-10-04),
we have introduced batched syncing of object files. This mode works by
only requesting a writeback of the page cache backing the file on
written files, followed by a single hardware-flush via a temporary file
created in the directory we want to flush. Given modern journaling file
systems, this pattern is expected to be durable.

While it's possible to reuse the `git_fsync()` helper to synchronize the
page cache only, there is no helper which would allow for doing a
hardware flush of a directory by creating a temporary file. Other
callers which want to follow the same pattern would thus have to repeat
this logic.

Extract a new helper `git_fsync_dir()` from the object files code which
neatly encapsulates this logic such that it can be reused.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 bulk-checkin.c    | 13 +++----------
 git-compat-util.h |  7 +++++++
 wrapper.c         | 21 +++++++++++++++++++++
 3 files changed, 31 insertions(+), 10 deletions(-)

diff --git a/bulk-checkin.c b/bulk-checkin.c
index 4deee1af46..e6ebdd1db5 100644
--- a/bulk-checkin.c
+++ b/bulk-checkin.c
@@ -98,16 +98,9 @@ static void do_batch_fsync(void)
 	 * 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 (needs_batch_fsync &&
+	    git_fsync_dir(get_object_directory()) < 0)
+		die_errno("fsyncing object directory");
 
 	if (bulk_fsync_objdir)
 		tmp_objdir_migrate(bulk_fsync_objdir);
diff --git a/git-compat-util.h b/git-compat-util.h
index 97f97178e7..f890bd07fd 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -1221,6 +1221,13 @@ enum fsync_action {
 
 int git_fsync(int fd, enum fsync_action action);
 
+/*
+ * Issue a full hardware flush against a temporary file in the given directory
+ * to ensure that all files inside that directory are durable before any renames
+ * occur.
+ */
+int git_fsync_dir(const char *path);
+
 /*
  * Preserves errno, prints a message, but gives no warning for ENOENT.
  * Returns 0 on success, which includes trying to unlink an object that does
diff --git a/wrapper.c b/wrapper.c
index e20df4f3a6..6c6cc8b74f 100644
--- a/wrapper.c
+++ b/wrapper.c
@@ -3,6 +3,7 @@
  */
 #include "cache.h"
 #include "config.h"
+#include "tempfile.h"
 
 static int memory_limit_check(size_t size, int gentle)
 {
@@ -601,6 +602,26 @@ int git_fsync(int fd, enum fsync_action action)
 	return 0;
 }
 
+int git_fsync_dir(const char *path)
+{
+	struct strbuf temp_path = STRBUF_INIT;
+	struct tempfile *temp;
+
+	strbuf_addf(&temp_path, "%s/bulk_fsync_XXXXXX", path);
+
+	temp = mks_tempfile(temp_path.buf);
+	if (!temp)
+		return -1;
+
+	if (git_fsync(get_tempfile_fd(temp), FSYNC_HARDWARE_FLUSH) < 0)
+		return -1;
+
+	delete_tempfile(&temp);
+	strbuf_release(&temp_path);
+
+	return 0;
+}
+
 static int warn_if_unremovable(const char *op, const char *file, int rc)
 {
 	int err;
-- 
2.33.1