update-index: use the bulk-checkin infrastructure

The update-index functionality is used internally by 'git stash push' to
setup the internal stashed commit.

This change enables odb-transactions for update-index infrastructure to
speed up adding new objects to the object database by leveraging the
batch fsync functionality.

There is some risk with this change, since under batch fsync, the object
files will be in a tmp-objdir until update-index is complete, so callers
using the --stdin option will not see them until update-index is done.
This risk is mitigated by not keeping an ODB transaction open around
--stdin processing if in --verbose mode. Without --verbose mode,
a caller feeding update-index via --stdin wouldn't know when
update-index adds an object, event without an ODB transaction.

Signed-off-by: Neeraj Singh <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: neerajsi-msft

builtin/update-index.c

@@ -5,6 +5,7 @@
  */
 #define USE_THE_INDEX_COMPATIBILITY_MACROS
 #include "cache.h"
+#include "bulk-checkin.h"
 #include "config.h"
 #include "lockfile.h"
 #include "quote.h"
@@ -1116,6 +1117,12 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 	 */
 	parse_options_start(&ctx, argc, argv, prefix,
 			    options, PARSE_OPT_STOP_AT_NON_OPTION);
+
+	/*
+	 * Allow the object layer to optimize adding multiple objects in
+	 * a batch.
+	 */
+	begin_odb_transaction();
 	while (ctx.argc) {
 		if (parseopt_state != PARSE_OPT_DONE)
 			parseopt_state = parse_options_step(&ctx, options,
@@ -1167,6 +1174,17 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 		the_index.version = preferred_index_format;
 	}
 
+	/*
+	 * It is possible, though unlikely, that a caller could use the verbose
+	 * output to synchronize with addition of objects to the object
+	 * database. The current implementation of ODB transactions leaves
+	 * objects invisible while a transaction is active, so end the
+	 * transaction here if verbose output is enabled.
+	 */
+
+	if (verbose)
+		end_odb_transaction();
+
 	if (read_from_stdin) {
 		struct strbuf buf = STRBUF_INIT;
 		struct strbuf unquoted = STRBUF_INIT;
@@ -1190,6 +1208,12 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 		strbuf_release(&buf);
 	}
 
+	/*
+	 * By now we have added all of the new objects
+	 */
+	if (!verbose)
+		end_odb_transaction();
+
 	if (split_index > 0) {
 		if (git_config_get_split_index() == 0)
 			warning(_("core.splitIndex is set to false; "