Fix template alias parsing livelock (elastic#112217) (elastic#112245)

* Fix template alias parsing livelock

This commit fixes an issue with templates parsing alias definitions that can cause the ES thread to
hang indefinitely. Due to the malformed alias definition, the parsing gets into a loop which never
exits. In this commit a null check in both the component template and alias parsing code is added,
which prevents the looping.

docs/changelog/112217.yaml

@@ -0,0 +1,5 @@
+pr: 112217
+summary: Fix template alias parsing livelock
+area: Indices APIs
+type: bug
+issues: []

server/src/main/java/org/elasticsearch/cluster/metadata/AliasMetadata.java

@@ -396,6 +396,8 @@ public static AliasMetadata fromXContent(XContentParser parser) throws IOExcepti
                     } else if ("is_hidden".equals(currentFieldName)) {
                         builder.isHidden(parser.booleanValue());
                     }
+                } else if (token == null) {
+                    throw new IllegalArgumentException("unexpected null token while parsing alias");
                 }
             }
             return builder.build();

server/src/main/java/org/elasticsearch/cluster/metadata/Template.java

@@ -70,7 +70,11 @@ public class Template implements SimpleDiffable<Template>, ToXContentObject {
         }, MAPPINGS, ObjectParser.ValueType.VALUE_OBJECT_ARRAY);
         PARSER.declareObject(ConstructingObjectParser.optionalConstructorArg(), (p, c) -> {
             Map<String, AliasMetadata> aliasMap = new HashMap<>();
-            while ((p.nextToken()) != XContentParser.Token.END_OBJECT) {
+            XContentParser.Token token;
+            while ((token = p.nextToken()) != XContentParser.Token.END_OBJECT) {
+                if (token == null) {
+                    break;
+                }
                 AliasMetadata alias = AliasMetadata.Builder.fromXContent(p);
                 aliasMap.put(alias.alias(), alias);
             }

server/src/test/java/org/elasticsearch/cluster/metadata/ComponentTemplateTests.java

@@ -24,6 +24,7 @@
 import org.elasticsearch.xcontent.XContentBuilder;
 import org.elasticsearch.xcontent.XContentFactory;
 import org.elasticsearch.xcontent.XContentParser;
+import org.elasticsearch.xcontent.XContentParserConfiguration;
 import org.elasticsearch.xcontent.XContentType;
 
 import java.io.IOException;
@@ -307,4 +308,22 @@ public void testXContentSerializationWithRolloverAndEffectiveRetention() throws
             assertThat(serialized, not(containsString("effective_retention")));
         }
     }
+
+    public void testHangingParsing() throws IOException {
+        String cutDown = """
+            {
+              "template": {
+                "aliases": {
+                  "foo": "bar"
+                },
+                "food": "eggplant"
+              },
+              "potato": true
+            }
+            """;
+
+        try (XContentParser parser = XContentType.JSON.xContent().createParser(XContentParserConfiguration.EMPTY, cutDown)) {
+            expectThrows(Exception.class, () -> ComponentTemplate.parse(parser));
+        }
+    }
 }