refs.c: make ref_transaction_begin take an err argument

Add an err argument to _begin so that on non-fatal failures in future ref
backends we can report a nice error back to the caller.
While _begin can currently never fail for other reasons than OOM, in which
case we die() anyway, we may add other types of backends in the future.
For example, a hypothetical MySQL backend could fail in _begin with
"Can not connect to MySQL server. No route to host".

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/update-ref.c

@@ -365,7 +365,9 @@ int cmd_update_ref(int argc, const char **argv, const char *prefix)
 		die("Refusing to perform update with empty message.");
 
 	if (read_stdin) {
-		transaction = ref_transaction_begin();
+		transaction = ref_transaction_begin(&err);
+		if (!transaction)
+			die("%s", err.buf);
 		if (delete || no_deref || argc > 0)
 			usage_with_options(git_update_ref_usage, options);
 		if (end_null)
@@ -374,6 +376,7 @@ int cmd_update_ref(int argc, const char **argv, const char *prefix)
 		if (ref_transaction_commit(transaction, msg, &err))
 			die("%s", err.buf);
 		ref_transaction_free(transaction);
+		strbuf_release(&err);
 		return 0;
 	}
 

refs.c

@@ -3397,7 +3397,7 @@ struct ref_transaction {
 	size_t nr;
 };
 
-struct ref_transaction *ref_transaction_begin(void)
+struct ref_transaction *ref_transaction_begin(struct strbuf *err)
 {
 	return xcalloc(1, sizeof(struct ref_transaction));
 }

refs.h

@@ -262,7 +262,7 @@ enum action_on_err {
  * Begin a reference transaction.  The reference transaction must
  * be freed by calling ref_transaction_free().
  */
-struct ref_transaction *ref_transaction_begin(void);
+struct ref_transaction *ref_transaction_begin(struct strbuf *err);
 
 /*
  * The following functions add a reference check or update to a