[PATCH 2/4] object-file: only flush batched fsync writes when enabled

[Justin Tobler <jltobler@gmail.com>]

When committing an ODB transaction for the "files" backend, a full
hardware flush is issued against a temporary file before the staged
objects are migrated into the main object database. This flush acts as a
barrier for objects that have been written with batched fsync, where
`odb_transaction_files_fsync()` only issues a writeout request for each
object instead of fully syncing it.

The flush is issued whenever the transaction has a temporary object
directory though, regardless of whether batched fsync is actually
enabled. For transactions configured for git-receive-pack(1), the
temporary object directory is always created, so the flush is also
performed when objects are synced individually or not synced at all. In
those cases there are no pending writeout requests and the flush is
unnecessary. Only issue the flush when batched fsync is enabled
accordingly.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
---
 object-file.c | 43 +++++++++++++++++++++++++------------------
 1 file changed, 25 insertions(+), 18 deletions(-)

diff --git a/object-file.c b/object-file.c
index 201d9d6490..da189111bd 100644
--- a/object-file.c
+++ b/object-file.c
@@ -1330,6 +1330,29 @@ static int write_pack_lockfiles(struct odb_transaction_files *transaction)
 	return ret;
 }
 
+static void flush_batched_fsync(struct repository *repo)
+{
+	struct strbuf temp_path = STRBUF_INIT;
+	struct tempfile *temp;
+
+	/*
+	 * Issue a full hardware flush against a temporary file to ensure
+	 * that all objects are durable before any renames occur. The code in
+	 * odb_transaction_files_fsync has already issued a writeout
+	 * request, but it has not flushed any writeback cache in the storage
+	 * hardware or any filesystem logs. This fsync call acts as a barrier
+	 * to ensure that the data in each new object file is durable before
+	 * the final name is visible.
+	 */
+	strbuf_addf(&temp_path, "%s/bulk_fsync_XXXXXX",
+		    repo_get_object_directory(repo));
+	temp = xmks_tempfile(temp_path.buf);
+	fsync_or_die(get_tempfile_fd(temp), get_tempfile_path(temp));
+
+	strbuf_release(&temp_path);
+	delete_tempfile(&temp);
+}
+
 static int odb_transaction_files_commit(struct odb_transaction *base)
 {
 	struct odb_transaction_files *transaction =
@@ -1339,24 +1362,8 @@ static int odb_transaction_files_commit(struct odb_transaction *base)
 	flush_packfile_transaction(transaction);
 
 	if (transaction->objdir) {
-		struct strbuf temp_path = STRBUF_INIT;
-		struct tempfile *temp;
-
-		/*
-		 * Issue a full hardware flush against a temporary file to ensure
-		 * that all objects are durable before any renames occur. The code in
-		 * odb_transaction_files_fsync has already issued a writeout
-		 * request, but it has not flushed any writeback cache in the storage
-		 * hardware or any filesystem logs. This fsync call acts as a barrier
-		 * to ensure that the data in each new object file is durable before
-		 * the final name is visible.
-		 */
-		strbuf_addf(&temp_path, "%s/bulk_fsync_XXXXXX",
-			    repo_get_object_directory(transaction->base.source->odb->repo));
-		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 (batch_fsync_enabled(FSYNC_COMPONENT_LOOSE_OBJECT))
+			flush_batched_fsync(transaction->base.source->odb->repo);
 
 		if (write_pack_lockfiles(transaction))
 			return error(_("unable to lock packfiles"));
-- 
2.56.0.173.gf57c9a7388