Fix error using _terms_enum on ip field (#112872)

When using an ipv6 prefix with shortened "0" entries like "23ec::", we
erroneously also matched any IP4 address. This change fixes that problem.

Closes #94378

Author: cbuescher

docs/changelog/112872.yaml

@@ -0,0 +1,6 @@
+pr: 112872
+summary: Fix parsing error in `_terms_enum` API
+area: Search
+type: bug
+issues:
+ - 94378

server/src/main/java/org/elasticsearch/index/mapper/IpPrefixAutomatonUtil.java

@@ -108,7 +108,7 @@ private static Automaton getIpv6Automaton(String ipPrefix) {
                 } else {
                     // potentially partial block
                     if (groupsAdded == 0 && ONLY_ZEROS.matcher(group).matches()) {
-                        // here we have a leading group with only "0" characters. If we would allow this to match
+                        // here we have a leading group with only "0" characters. If we allowed this to match
                         // ipv6 addresses, this would include things like 0000::127.0.0.1 (and all other ipv4 addresses).
                         // Allowing this would be counterintuitive, so "0*" prefixes should only expand
                         // to ipv4 addresses like "0.1.2.3" and we return with an automaton not matching anything here
@@ -129,7 +129,7 @@ private static Automaton getIpv6Automaton(String ipPrefix) {
 
     static Automaton automatonFromIPv6Group(String ipv6Group) {
         assert ipv6Group.length() > 0 && ipv6Group.length() <= 4 : "expected a full ipv6 group or prefix";
-        Automaton result = Automata.makeString("");
+        Automaton result = Automata.makeEmpty();
         for (int leadingZeros = 0; leadingZeros <= 4 - ipv6Group.length(); leadingZeros++) {
             int bytesAdded = 0;
             String padded = padWithZeros(ipv6Group, leadingZeros);

server/src/test/java/org/elasticsearch/index/mapper/IpPrefixAutomatonUtilTests.java

@@ -174,6 +174,12 @@ public void testBuildPrefixAutomaton() throws UnknownHostException {
             assertTrue(accepts(a, "255.27.240.24"));
             assertTrue(accepts(a, "255:a360::25bb:828f:ffff:ffff"));
         }
+        {
+            CompiledAutomaton a = buildIpPrefixAutomaton("23c9::");
+            assertTrue(accepts(a, "23c9::6063:7ac9:ffff:ffff"));
+            assertFalse(accepts(a, "0.0.0.0"));
+            assertFalse(accepts(a, "249.43.32.175"));
+        }
     }
 
     private static boolean accepts(CompiledAutomaton compiledAutomaton, String address) throws UnknownHostException {