sequencer: lib'ify write_message()

Instead of dying there, let the caller high up in the callchain
notice the error and handle it (by dying, still).

The only caller of write_message(), do_pick_commit() already checks
the return value and passes it on to its callers, so its caller must
be already prepared to handle error returns, and with this step, we
make it notice an error return from this function.

So this is a safe conversion to make write_message() callable
from new callers that want it not to die, without changing the
external behaviour of anything existing.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -180,17 +180,20 @@ static void print_advice(int show_hint, struct replay_opts *opts)
 	}
 }
 
-static void write_message(struct strbuf *msgbuf, const char *filename)
+static int write_message(struct strbuf *msgbuf, const char *filename)
 {
 	static struct lock_file msg_file;
 
-	int msg_fd = hold_lock_file_for_update(&msg_file, filename,
-					       LOCK_DIE_ON_ERROR);
+	int msg_fd = hold_lock_file_for_update(&msg_file, filename, 0);
+	if (msg_fd < 0)
+		return error_errno(_("Could not lock '%s'"), filename);
 	if (write_in_full(msg_fd, msgbuf->buf, msgbuf->len) < 0)
-		die_errno(_("Could not write to %s"), filename);
+		return error_errno(_("Could not write to %s"), filename);
 	strbuf_release(msgbuf);
 	if (commit_lock_file(&msg_file) < 0)
-		die(_("Error wrapping up %s."), filename);
+		return error(_("Error wrapping up %s."), filename);
+
+	return 0;
 }
 
 static struct tree *empty_tree(void)
@@ -564,16 +567,16 @@ static int do_pick_commit(struct commit *commit, struct replay_opts *opts)
 					 head, &msgbuf, opts);
 		if (res < 0)
 			return res;
-		write_message(&msgbuf, git_path_merge_msg());
+		res |= write_message(&msgbuf, git_path_merge_msg());
 	} else {
 		struct commit_list *common = NULL;
 		struct commit_list *remotes = NULL;
 
-		write_message(&msgbuf, git_path_merge_msg());
+		res = write_message(&msgbuf, git_path_merge_msg());
 
 		commit_list_insert(base, &common);
 		commit_list_insert(next, &remotes);
-		res = try_merge_command(opts->strategy, opts->xopts_nr, opts->xopts,
+		res |= try_merge_command(opts->strategy, opts->xopts_nr, opts->xopts,
 					common, sha1_to_hex(head), remotes);
 		free_commit_list(common);
 		free_commit_list(remotes);