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.

2 files changed, 36 insertions, 13 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;
diff --git a/src/test/test-dns-domain.c b/src/test/test-dns-domain.c
index 3b260ee75d..3efc61ad0a 100644
--- a/src/test/test-dns-domain.c
+++ b/src/test/test-dns-domain.c
@@ -186,7 +186,7 @@ static void test_dns_name_normalize_one(const char *what, const char *expect, in
 }
 
 static void test_dns_name_normalize(void) {
-        test_dns_name_normalize_one("", "", 0);
+        test_dns_name_normalize_one("", ".", 0);
         test_dns_name_normalize_one("f", "f", 0);
         test_dns_name_normalize_one("f.waldi", "f.waldi", 0);
         test_dns_name_normalize_one("f \\032.waldi", "f\\032\\032.waldi", 0);
@@ -194,7 +194,7 @@ static void test_dns_name_normalize(void) {
         test_dns_name_normalize_one("..", NULL, -EINVAL);
         test_dns_name_normalize_one(".foobar", NULL, -EINVAL);
         test_dns_name_normalize_one("foobar.", "foobar", 0);
-        test_dns_name_normalize_one(".", "", 0);
+        test_dns_name_normalize_one(".", ".", 0);
 }
 
 static void test_dns_name_equal_one(const char *a, const char *b, int ret) {
@@ -340,10 +340,18 @@ static void test_dns_name_concat_one(const char *a, const char *b, int r, const
 }
 
 static void test_dns_name_concat(void) {
+        test_dns_name_concat_one("", "", 0, ".");
+        test_dns_name_concat_one(".", "", 0, ".");
+        test_dns_name_concat_one("", ".", 0, ".");
+        test_dns_name_concat_one(".", ".", 0, ".");
         test_dns_name_concat_one("foo", "bar", 0, "foo.bar");
         test_dns_name_concat_one("foo.foo", "bar.bar", 0, "foo.foo.bar.bar");
         test_dns_name_concat_one("foo", NULL, 0, "foo");
+        test_dns_name_concat_one("foo", ".", 0, "foo");
         test_dns_name_concat_one("foo.", "bar.", 0, "foo.bar");
+        test_dns_name_concat_one(NULL, NULL, 0, ".");
+        test_dns_name_concat_one(NULL, ".", 0, ".");
+        test_dns_name_concat_one(NULL, "foo", 0, "foo");
 }
 
 static void test_dns_name_is_valid_one(const char *s, int ret) {
@@ -429,7 +437,7 @@ static void test_dns_service_join_one(const char *a, const char *b, const char *
         assert_se(dns_service_split(t, &x, &y, &z) >= 0);
         assert_se(streq_ptr(a, x));
         assert_se(streq_ptr(b, y));
-        assert_se(streq_ptr(c, z));
+        assert_se(dns_name_equal(c, z) > 0);
 }
 
 static void test_dns_service_join(void) {
@@ -460,18 +468,18 @@ static void test_dns_service_split_one(const char *joined, const char *a, const
 
         if (y) {
                 assert_se(dns_service_join(x, y, z, &t) == 0);
-                assert_se(streq_ptr(joined, t));
+                assert_se(dns_name_equal(joined, t) > 0);
         } else
-                assert_se(!x && streq_ptr(z, joined));
+                assert_se(!x && dns_name_equal(z, joined) > 0);
 }
 
 static void test_dns_service_split(void) {
-        test_dns_service_split_one("", NULL, NULL, "", 0);
+        test_dns_service_split_one("", NULL, NULL, ".", 0);
         test_dns_service_split_one("foo", NULL, NULL, "foo", 0);
         test_dns_service_split_one("foo.bar", NULL, NULL, "foo.bar", 0);
         test_dns_service_split_one("_foo.bar", NULL, NULL, "_foo.bar", 0);
-        test_dns_service_split_one("_foo._bar", NULL, "_foo._bar", "", 0);
-        test_dns_service_split_one("_meh._foo._bar", "_meh", "_foo._bar", "", 0);
+        test_dns_service_split_one("_foo._bar", NULL, "_foo._bar", ".", 0);
+        test_dns_service_split_one("_meh._foo._bar", "_meh", "_foo._bar", ".", 0);
         test_dns_service_split_one("Wuff\\032Wuff._foo._bar.waldo.com", "Wuff Wuff", "_foo._bar", "waldo.com", 0);
 }
 
@@ -490,7 +498,7 @@ static void test_dns_name_change_suffix(void) {
         test_dns_name_change_suffix_one("foo.bar.waldi.quux", "quux", "piff.paff", 1, "foo.bar.waldi.piff.paff");
         test_dns_name_change_suffix_one("foo.bar.waldi.quux", "", "piff.paff", 1, "foo.bar.waldi.quux.piff.paff");
         test_dns_name_change_suffix_one("", "", "piff.paff", 1, "piff.paff");
-        test_dns_name_change_suffix_one("", "", "", 1, "");
+        test_dns_name_change_suffix_one("", "", "", 1, ".");
         test_dns_name_change_suffix_one("a", "b", "c", 0, NULL);
 }