[sheepdog] [PATCH v4 2/2] sheep: implement atomic create and write mechanism for config

[Hitoshi Mitake]

From: Hitoshi Mitake <mitake.hitoshi at gmail.com>

Because we cannot utilize the journaling mechanism for config, this
patch implements atomic creation and writing for it.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi at lab.ntt.co.jp>
---
v4: remove the unnecessary global variable

v3:
 - rephrase the message of check_tmp_config()
 - remove a redundant constant

v2: use access(2) for checking remained temporal file

 sheep/config.c |   30 ++++++++++++++++++++++++++++--
 1 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/sheep/config.c b/sheep/config.c
index 49916e7..6bc092b 100644
--- a/sheep/config.c
+++ b/sheep/config.c
@@ -36,10 +36,13 @@ char *config_path;
 static int write_config(void)
 {
 	int fd, ret;
+	char tmp_config_path[PATH_MAX];
 
-	fd = open(config_path, O_RDWR | O_CREAT | O_DSYNC, def_fmode);
+	snprintf(tmp_config_path, PATH_MAX, "%s.tmp", config_path);
+
+	fd = open(tmp_config_path, O_WRONLY | O_CREAT | O_SYNC, def_fmode);
 	if (fd < 0) {
-		sd_eprintf("failed to open config file, %m");
+		sd_eprintf("failed to temporal file for config, %m");
 		return SD_RES_EIO;
 	}
 
@@ -51,13 +54,36 @@ static int write_config(void)
 		ret = SD_RES_SUCCESS;
 	close(fd);
 
+	ret = rename(tmp_config_path, config_path);
+	if (ret < 0) {
+		sd_eprintf("failed to rename, %m");
+		ret = SD_RES_EIO;
+	}
+
+	unlink(tmp_config_path);
+
 	return ret;
 }
 
+static void check_tmp_config(void)
+{
+	char tmp_config_path[PATH_MAX];
+
+	snprintf(tmp_config_path, PATH_MAX, "%s.tmp", config_path);
+
+	if (!access(tmp_config_path, F_OK))
+		return;
+
+	sd_iprintf("remove temporal config file");
+	unlink(tmp_config_path);
+}
+
 int init_config_file(void)
 {
 	int fd, ret;
 
+	check_tmp_config();
+
 	fd = open(config_path, O_RDONLY);
 	if (fd < 0) {
 		if (errno != ENOENT) {
-- 
1.7.2.5