shared: normalize the root domain to "." rather than ""

Let's make sure the root domain is normalized to ".", rather than then empty string, so that there's actually something
to see on screen. Normally, we don't append a trailing dot to normalized domain names, but do so in the one exception
of the root domain, taking inspiration from UNIX file system paths.

1 files changed, 19 insertions, 4 deletions

diff --git a/src/shared/dns-domain.c b/src/shared/dns-domain.c
index 3ad409fc29..7ef4ad3cf8 100644
--- a/src/shared/dns-domain.c
+++ b/src/shared/dns-domain.c
@@ -405,11 +405,17 @@ int dns_label_undo_idna(const char *encoded, size_t encoded_size, char *decoded,
 int dns_name_concat(const char *a, const char *b, char **_ret) {
         _cleanup_free_ char *ret = NULL;
         size_t n = 0, allocated = 0;
-        const char *p = a;
+        const char *p;
         bool first = true;
         int r;
 
-        assert(a);
+        if (a)
+                p = a;
+        else if (b) {
+                p = b;
+                b = NULL;
+        } else
+                goto finish;
 
         for (;;) {
                 char label[DNS_LABEL_MAX];
@@ -457,12 +463,21 @@ int dns_name_concat(const char *a, const char *b, char **_ret) {
                 n += r;
         }
 
+finish:
         if (n > DNS_HOSTNAME_MAX)
                 return -EINVAL;
 
         if (_ret) {
-                if (!GREEDY_REALLOC(ret, allocated, n + 1))
-                        return -ENOMEM;
+                if (n == 0) {
+                        /* Nothing appended? If so, generate at least a single dot, to indicate the DNS root domain */
+                        if (!GREEDY_REALLOC(ret, allocated, 2))
+                                return -ENOMEM;
+
+                        ret[n++] = '.';
+                } else {
+                        if (!GREEDY_REALLOC(ret, allocated, n + 1))
+                                return -ENOMEM;
+                }
 
                 ret[n] = 0;
                 *_ret = ret;