util: introduce sethostname_idempotent

Function queries system hostname and applies changes only when necessary. Also,
migrate all client of sethostname to sethostname_idempotent while at it.

src/core/hostname-setup.c

@@ -82,7 +82,7 @@ int hostname_setup(void) {
                 hn = "localhost";
         }
 
-        if (sethostname(hn, strlen(hn)) < 0) {
+        if (sethostname_idempotent(hn) < 0) {
                 log_warning("Failed to set hostname to <%s>: %m", hn);
                 return -errno;
         }

src/hostname/hostnamed.c

@@ -287,7 +287,7 @@ static int context_update_kernel_hostname(Context *c) {
         else
                 hn = "localhost";
 
-        if (sethostname(hn, strlen(hn)) < 0)
+        if (sethostname_idempotent(hn) < 0)
                 return -errno;
 
         return 0;

src/nspawn/nspawn.c

@@ -1289,7 +1289,7 @@ static int setup_hostname(void) {
         if (arg_share_system)
                 return 0;
 
-        if (sethostname(arg_machine, strlen(arg_machine)) < 0)
+        if (sethostname_idempotent(arg_machine) < 0)
                 return -errno;
 
         return 0;

src/shared/util.c

@@ -7175,3 +7175,23 @@ int free_and_strdup(char **p, const char *s) {
 
         return 0;
 }
+
+int sethostname_idempotent(const char *s) {
+        int r;
+        char buf[HOST_NAME_MAX + 1] = {};
+
+        assert(s);
+
+        r = gethostname(buf, sizeof(buf));
+        if (r < 0)
+                return -errno;
+
+        if (streq(buf, s))
+                return 0;
+
+        r = sethostname(buf, strlen(buf));
+        if (r < 0)
+                return -errno;
+
+        return 1;
+}

src/shared/util.h

@@ -997,3 +997,5 @@ int unquote_first_word(const char **p, char **ret);
 int unquote_many_words(const char **p, ...) _sentinel_;
 
 int free_and_strdup(char **p, const char *s);
+
+int sethostname_idempotent(const char *s);