core.fsync: new option to harden the index

This commit introduces the new ability for the user to harden
the index. In the event of a system crash, the index must be
durable for the user to actually find a file that has been added
to the repo and then deleted from the working tree.

We use the presence of the COMMIT_LOCK flag and absence of the
alternate_index_output as a proxy for determining whether we're
updating the persistent index of the repo or some temporary
index. We don't sync these temporary indexes.

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

cache.h

@@ -1004,6 +1004,7 @@ enum fsync_component {
 	FSYNC_COMPONENT_PACK			= 1 << 1,
 	FSYNC_COMPONENT_PACK_METADATA		= 1 << 2,
 	FSYNC_COMPONENT_COMMIT_GRAPH		= 1 << 3,
+	FSYNC_COMPONENT_INDEX			= 1 << 4,
 };
 
 #define FSYNC_COMPONENTS_DEFAULT (FSYNC_COMPONENT_PACK | \

config.c

@@ -1331,6 +1331,7 @@ static const struct fsync_component_name {
 	{ "pack", FSYNC_COMPONENT_PACK },
 	{ "pack-metadata", FSYNC_COMPONENT_PACK_METADATA },
 	{ "commit-graph", FSYNC_COMPONENT_COMMIT_GRAPH },
+	{ "index", FSYNC_COMPONENT_INDEX },
 };
 
 static enum fsync_component parse_fsync_components(const char *var, const char *string)

read-cache.c

@@ -2842,7 +2842,7 @@ static int record_ieot(void)
  * rely on it.
  */
 static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
-			  int strip_extensions)
+			  int strip_extensions, unsigned flags)
 {
 	uint64_t start = getnanotime();
 	struct hashfile *f;
@@ -2856,6 +2856,7 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
 	struct strbuf previous_name_buf = STRBUF_INIT, *previous_name;
 	int drop_cache_tree = istate->drop_cache_tree;
 	off_t offset;
+	int csum_fsync_flag;
 	int ieot_entries = 1;
 	struct index_entry_offset_table *ieot = NULL;
 	int nr, nr_threads;
@@ -3089,7 +3090,13 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
 			return -1;
 	}
 
-	finalize_hashfile(f, istate->oid.hash, FSYNC_COMPONENT_NONE, CSUM_HASH_IN_STREAM);
+	csum_fsync_flag = 0;
+	if (!alternate_index_output && (flags & COMMIT_LOCK))
+		csum_fsync_flag = CSUM_FSYNC;
+
+	finalize_hashfile(f, istate->oid.hash, FSYNC_COMPONENT_INDEX,
+			  CSUM_HASH_IN_STREAM | csum_fsync_flag);
+
 	if (close_tempfile_gently(tempfile)) {
 		error(_("could not close '%s'"), get_tempfile_path(tempfile));
 		return -1;
@@ -3144,7 +3151,7 @@ static int do_write_locked_index(struct index_state *istate, struct lock_file *l
 	 */
 	trace2_region_enter_printf("index", "do_write_index", the_repository,
 				   "%s", get_lock_file_path(lock));
-	ret = do_write_index(istate, lock->tempfile, 0);
+	ret = do_write_index(istate, lock->tempfile, 0, flags);
 	trace2_region_leave_printf("index", "do_write_index", the_repository,
 				   "%s", get_lock_file_path(lock));
 
@@ -3238,7 +3245,7 @@ static int clean_shared_index_files(const char *current_hex)
 }
 
 static int write_shared_index(struct index_state *istate,
-			      struct tempfile **temp)
+			      struct tempfile **temp, unsigned flags)
 {
 	struct split_index *si = istate->split_index;
 	int ret, was_full = !istate->sparse_index;
@@ -3248,7 +3255,7 @@ static int write_shared_index(struct index_state *istate,
 
 	trace2_region_enter_printf("index", "shared/do_write_index",
 				   the_repository, "%s", get_tempfile_path(*temp));
-	ret = do_write_index(si->base, *temp, 1);
+	ret = do_write_index(si->base, *temp, 1, flags);
 	trace2_region_leave_printf("index", "shared/do_write_index",
 				   the_repository, "%s", get_tempfile_path(*temp));
 
@@ -3357,7 +3364,7 @@ int write_locked_index(struct index_state *istate, struct lock_file *lock,
 			ret = do_write_locked_index(istate, lock, flags);
 			goto out;
 		}
-		ret = write_shared_index(istate, &temp);
+		ret = write_shared_index(istate, &temp, flags);
 
 		saved_errno = errno;
 		if (is_tempfile_active(temp))