[sheepdog] [PATCH 2/3] util:add a new API atomic_create_and_write()

[Hitoshi Mitake]

This patch adds a new API atomic_create_and_write(). As the name
implies, the API creates and writes data to a file.

atomic_create_and_write() creates a file path.tmp (path is a
parameter) internally. During the initialization process of sheep,
every user of this API should detect and clean temporal files.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi at lab.ntt.co.jp>
---
 include/util.h |    1 +
 lib/util.c     |   39 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 0 deletions(-)

diff --git a/include/util.h b/include/util.h
index 38efb8b..9faa21b 100644
--- a/include/util.h
+++ b/include/util.h
@@ -96,6 +96,7 @@ bool is_xattr_enabled(const char *path);
 void trim_zero_sectors(void *buf, uint64_t *offset, uint32_t *len);
 void untrim_zero_sectors(void *buf, uint64_t offset, uint32_t len,
 			 uint32_t requested_len);
+int atomic_create_and_write(const char *path, char *buf, size_t len);
 
 #ifdef assert
 #undef assert
diff --git a/lib/util.c b/lib/util.c
index aac0fa9..498fad8 100644
--- a/lib/util.c
+++ b/lib/util.c
@@ -22,6 +22,7 @@
 #include <ctype.h>
 #include <signal.h>
 #include <sys/xattr.h>
+#include <fcntl.h>
 
 #include "util.h"
 #include "logger.h"
@@ -434,3 +435,41 @@ bool is_xattr_enabled(const char *path)
 
 	return !(ret == -1 && errno == ENOTSUP);
 }
+
+int atomic_create_and_write(const char *path, char *buf, size_t len)
+{
+	int fd, ret;
+	char tmp_path[PATH_MAX];
+
+	snprintf(tmp_path, PATH_MAX, "%s.tmp", path);
+	if (!access(tmp_path, F_OK))
+		panic("temporal file: %s exists, this implies invalid usage of"
+			" atomic_create_and_write() of sheepdog", tmp_path);
+
+	fd = open(tmp_path, O_WRONLY | O_CREAT | O_SYNC,
+		S_IWUSR | S_IRUSR | S_IWGRP | S_IRGRP);
+	if (fd < 0) {
+		sd_eprintf("failed to temporal file for config, %m");
+		ret = -1;
+		goto end;
+	}
+
+	ret = xwrite(fd, buf, len);
+	if (ret != len) {
+		sd_eprintf("failed to write config data, %m");
+		ret = -1;
+		goto close_fd;
+	}
+
+	ret = rename(tmp_path, path);
+	if (ret < 0) {
+		sd_eprintf("failed to rename, %m");
+		ret = -1;
+	}
+
+close_fd:
+	close(fd);
+end:
+	return ret;
+}
+
-- 
1.7.2.5