nspawn: skip symlink to a combined cgroup hierarchy if it already exists

If a symlink to a combined cgroup hierarchy already exists and points to
the right path, skip it. This avoids an error when the cgroups are set
manually before calling nspawn.

src/shared/util.c

@@ -2765,6 +2765,28 @@ int symlink_atomic(const char *from, const char *to) {
         return 0;
 }
 
+int symlink_idempotent(const char *from, const char *to) {
+        _cleanup_free_ char *p = NULL;
+        int r;
+
+        assert(from);
+        assert(to);
+
+        if (symlink(from, to) < 0) {
+                if (errno != EEXIST)
+                        return -errno;
+
+                r = readlink_malloc(to, &p);
+                if (r < 0)
+                        return r;
+
+                if (!streq(p, from))
+                        return -EINVAL;
+        }
+
+        return 0;
+}
+
 int mknod_atomic(const char *path, mode_t mode, dev_t dev) {
         _cleanup_free_ char *t = NULL;
         int r;

src/shared/util.h

@@ -404,6 +404,7 @@ bool machine_name_is_valid(const char *s) _pure_;
 
 char* strshorten(char *s, size_t l);
 
+int symlink_idempotent(const char *from, const char *to);
 
 int symlink_atomic(const char *from, const char *to);
 int mknod_atomic(const char *path, mode_t mode, dev_t dev);