[libvirt] [PATCH] Make saving domain XML more robust

[Jiri Denemark]

When saving domain XML (either config or status) we just overwrite old
content of the file. In case something fails during that process (e.g.
disk gets full) we lose both old and new content. This patch makes the
process more robust by writing the new content into a separate file and
only if that succeeds the original file is atomically replaced with the
new one.
---
 src/conf/domain_conf.c |   25 +++++++++++++++++++++----
 1 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
index f9007ce..04c2b1c 100644
--- a/src/conf/domain_conf.c
+++ b/src/conf/domain_conf.c
@@ -11025,6 +11025,7 @@ int virDomainSaveXML(const char *configDir,
                      const char *xml)
 {
     char *configFile = NULL;
+    char *newfile = NULL;
     int fd = -1, ret = -1;
     size_t towrite;
 
@@ -11038,12 +11039,17 @@ int virDomainSaveXML(const char *configDir,
         goto cleanup;
     }
 
-    if ((fd = open(configFile,
+    if (virAsprintf(&newfile, "%s.new", configFile) < 0) {
+        virReportOOMError();
+        goto cleanup;
+    }
+
+    if ((fd = open(newfile,
                    O_WRONLY | O_CREAT | O_TRUNC,
                    S_IRUSR | S_IWUSR )) < 0) {
         virReportSystemError(errno,
                              _("cannot create config file '%s'"),
-                             configFile);
+                             newfile);
         goto cleanup;
     }
 
@@ -11053,14 +11059,21 @@ int virDomainSaveXML(const char *configDir,
     if (safewrite(fd, xml, towrite) < 0) {
         virReportSystemError(errno,
                              _("cannot write config file '%s'"),
-                             configFile);
+                             newfile);
         goto cleanup;
     }
 
     if (VIR_CLOSE(fd) < 0) {
         virReportSystemError(errno,
                              _("cannot save config file '%s'"),
-                             configFile);
+                             newfile);
+        goto cleanup;
+    }
+
+    if (rename(newfile, configFile) < 0) {
+        virReportSystemError(errno,
+                             _("cannot rename config file '%s' as '%s'"),
+                             newfile, configFile);
         goto cleanup;
     }
 
@@ -11068,6 +11081,10 @@ int virDomainSaveXML(const char *configDir,
  cleanup:
     VIR_FORCE_CLOSE(fd);
 
+    if (newfile) {
+        unlink(newfile);
+        VIR_FREE(newfile);
+    }
     VIR_FREE(configFile);
     return ret;
 }
-- 
1.7.7