resolved: on negative NODATA replies, properly deal with empty non-terminals

empty non-terminals generally lack NSEC RRs, which means we can deduce their existance only from the fact that there
are other RRs that contain them in their suffix. Specifically, the NSEC proof for NODATA on ENTs works by sending the
NSEC whose next name is a suffix of the queried name to the client. Use this information properly.

src/shared/dns-domain.c

@@ -1157,22 +1157,20 @@ finish:
         return 0;
 }
 
-int dns_name_suffix(const char *name, unsigned n_labels, const char **ret) {
-        const char* labels[DNS_N_LABELS_MAX+1];
-        unsigned n = 0;
+static int dns_name_build_suffix_table(const char *name, const char*table[]) {
         const char *p;
+        unsigned n = 0;
         int r;
 
         assert(name);
-        assert(ret);
+        assert(table);
 
         p = name;
         for (;;) {
                 if (n > DNS_N_LABELS_MAX)
                         return -EINVAL;
 
-                labels[n] = p;
-
+                table[n] = p;
                 r = dns_name_parent(&p);
                 if (r < 0)
                         return r;
@@ -1182,7 +1180,21 @@ int dns_name_suffix(const char *name, unsigned n_labels, const char **ret) {
                 n++;
         }
 
-        if (n < n_labels)
+        return (int) n;
+}
+
+int dns_name_suffix(const char *name, unsigned n_labels, const char **ret) {
+        const char* labels[DNS_N_LABELS_MAX+1];
+        int n;
+
+        assert(name);
+        assert(ret);
+
+        n = dns_name_build_suffix_table(name, labels);
+        if (n < 0)
+                return n;
+
+        if ((unsigned) n < n_labels)
                 return -EINVAL;
 
         *ret = labels[n - n_labels];
@@ -1245,3 +1257,49 @@ int dns_name_equal_skip(const char *a, unsigned n_labels, const char *b) {
 
         return dns_name_equal(a, b);
 }
+
+int dns_name_common_suffix(const char *a, const char *b, const char **ret) {
+        const char *a_labels[DNS_N_LABELS_MAX+1], *b_labels[DNS_N_LABELS_MAX+1];
+        int n = 0, m = 0, k = 0, r, q;
+
+        assert(a);
+        assert(b);
+        assert(ret);
+
+        /* Determines the common suffix of domain names a and b */
+
+        n = dns_name_build_suffix_table(a, a_labels);
+        if (n < 0)
+                return n;
+
+        m = dns_name_build_suffix_table(b, b_labels);
+        if (m < 0)
+                return m;
+
+        for (;;) {
+                char la[DNS_LABEL_MAX], lb[DNS_LABEL_MAX];
+                const char *x, *y;
+
+                if (k >= n || k >= m) {
+                        *ret = a_labels[n - k];
+                        return 0;
+                }
+
+                x = a_labels[n - 1 - k];
+                r = dns_label_unescape_undo_idna(&x, la, sizeof(la));
+                if (r < 0)
+                        return r;
+
+                y = b_labels[m - 1 - k];
+                q = dns_label_unescape_undo_idna(&y, lb, sizeof(lb));
+                if (q < 0)
+                        return q;
+
+                if (r != q || ascii_strcasecmp_n(la, lb, r) != 0) {
+                        *ret = a_labels[n - k];
+                        return 0;
+                }
+
+                k++;
+        }
+}

src/shared/dns-domain.h

@@ -106,3 +106,5 @@ int dns_name_count_labels(const char *name);
 
 int dns_name_skip(const char *a, unsigned n_labels, const char **ret);
 int dns_name_equal_skip(const char *a, unsigned n_labels, const char *b);
+
+int dns_name_common_suffix(const char *a, const char *b, const char **ret);