[2383] Clarify @ in names

If it is alone, it means the origin (or root, if the absolute-name
constructor is used). If it is not alone, it is taken verbatim, without
any special meaning.

Add some tests to verify this.

Remove unused enum value. It was a relict from non-exact port of bind9
code.

src/lib/dns/name.cc

@@ -123,11 +123,7 @@ typedef enum {
     ft_ordinary,                // parsing an ordinary label
     ft_initialescape,           // just found '\'
     ft_escape,                  // begin of handling a '\'-escaped sequence
-    ft_escdecimal,              // parsing a '\DDD' octet.
-
-    // Unused at this moment.  We'll revisit this when we support master file
-    // parser where @ is used to mean an origin name.
-    ft_at
+    ft_escdecimal               // parsing a '\DDD' octet.
 } ft_state;
 
 // The parser of name from a string. It is a template, because
@@ -283,7 +279,7 @@ stringParse(Iterator s, Iterator send, bool downcase, Offsets& offsets,
                       string(orig_s, send));
         }
         assert(s == send);
-        if (state != ft_ordinary && state != ft_at) {
+        if (state != ft_ordinary) {
             isc_throw(IncompleteName,
                       "incomplete textual name in " <<
                       (empty ? "<empty>" : string(orig_s, send)));

src/lib/dns/tests/name_unittest.cc

@@ -295,6 +295,30 @@ TEST_F(NameTest, combinedTooLong) {
                          &Name::ROOT_NAME()));
 }
 
+// Test the handling of @ in the name. If it is alone, it is the origin (when
+// it exists) or the root. If it is somewhere else, it has no special meaning.
+TEST_F(NameTest, atSign) {
+    // If it is alone, it is the origin
+    EXPECT_EQ(origin_name, Name("@", 1, &origin_name));
+    EXPECT_THROW(Name("@", 1, NULL), MissingNameOrigin);
+    EXPECT_EQ(Name::ROOT_NAME(), Name("@"));
+
+    // It is not alone. It is taken verbatim. We check the name converted
+    // back to the textual form, since checking it agains other name object
+    // may be wrong -- if we create it wrong the same way as the tested
+    // object.
+    EXPECT_EQ("\\@.", Name("@.").toText());
+    EXPECT_EQ("\\@.", Name("@.", 2, NULL).toText());
+    EXPECT_EQ("\\@something.", Name("@something").toText());
+    EXPECT_EQ("something\\@.", Name("something@").toText());
+    EXPECT_EQ("\\@x.example.com.", Name("@x", 2, &origin_name).toText());
+    EXPECT_EQ("x\\@.example.com.", Name("x@", 2, &origin_name).toText());
+
+    // An escaped at-sign isn't active
+    EXPECT_EQ("\\@.", Name("\\@").toText());
+    EXPECT_EQ("\\@.example.com.", Name("\\@", 2, &origin_name).toText());
+}
+
 TEST_F(NameTest, fromWire) {
     //
     // test cases derived from BIND9 tests.
@@ -593,7 +617,6 @@ TEST_F(NameTest, downcase) {
     // confirm the calling object is actually modified
     example_name_upper.downcase();
     compareInWireFormat(example_name_upper, example_name);
-    
 }
 
 TEST_F(NameTest, at) {