[sheepdog] [PATCH v3 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>
---
v3:
 - rephrase the message of check_tmp_config()
 - remove a redundant constant

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

 sheep/config.c |   29 ++++++++++++++++++++++++++---
 1 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/sheep/config.c b/sheep/config.c
index 49916e7..6e4ba6b 100644
--- a/sheep/config.c
+++ b/sheep/config.c
@@ -29,7 +29,7 @@ static struct sheepdog_config {
 	uint64_t space;
 } config;
 
-char *config_path;
+char *config_path, *tmp_config_path;
 
 #define CONFIG_PATH "/config"
 
@@ -37,9 +37,9 @@ static int write_config(void)
 {
 	int fd, ret;
 
-	fd = open(config_path, O_RDWR | O_CREAT | O_DSYNC, def_fmode);
+	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 +51,32 @@ 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)
+{
+	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) {
@@ -119,6 +138,10 @@ void init_config_path(const char *base_path)
 
 	config_path = xzalloc(len);
 	snprintf(config_path, len, "%s" CONFIG_PATH, base_path);
+
+	len = strlen(base_path) + strlen(CONFIG_PATH ".tmp") + 1;
+	tmp_config_path = xzalloc(len);
+	snprintf(tmp_config_path, len, "%s" CONFIG_PATH ".tmp", base_path);
 }
 
 int set_cluster_ctime(uint64_t ct)
-- 
1.7.2.5