Invalid JSON request body caused endless loop (#26680)

Request bodys that only consists of a String value can lead to endless loops in the
parser of several rest requests like e.g. `_count`. Up to 5.2 this seems to have been caught 
in the logic guessing the content type of the request, but since then it causes the node to
block. This change introduces checks for receiving a valid xContent object before starting the
parsing in RestActions#parseTopLevelQueryBuilder().

Closes #26083

core/src/main/java/org/elasticsearch/rest/action/RestActions.java

@@ -243,6 +243,15 @@ public RestResponse buildResponse(NodesResponse response, XContentBuilder builde
     private static QueryBuilder parseTopLevelQueryBuilder(XContentParser parser) {
         try {
             QueryBuilder queryBuilder = null;
+            XContentParser.Token first = parser.nextToken();
+            if (first == null) {
+                return null;
+            } else if (first != XContentParser.Token.START_OBJECT) {
+                throw new ParsingException(
+                    parser.getTokenLocation(), "Expected [" + XContentParser.Token.START_OBJECT +
+                    "] but found [" + first + "]", parser.getTokenLocation()
+                );
+            }
             for (XContentParser.Token token = parser.nextToken(); token != XContentParser.Token.END_OBJECT; token = parser.nextToken()) {
                 if (token == XContentParser.Token.FIELD_NAME) {
                     String fieldName = parser.currentName();

core/src/test/java/org/elasticsearch/rest/action/RestActionsTests.java

@@ -19,6 +19,8 @@
 
 package org.elasticsearch.rest.action;
 
+import com.fasterxml.jackson.core.io.JsonEOFException;
+import java.util.Arrays;
 import org.elasticsearch.common.ParsingException;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.xcontent.NamedXContentRegistry;
@@ -59,10 +61,32 @@ public void testParseTopLevelBuilder() throws IOException {
     }
 
     public void testParseTopLevelBuilderEmptyObject() throws IOException {
-        String requestBody = "{}";
-        try (XContentParser parser = createParser(JsonXContent.jsonXContent, requestBody)) {
-            QueryBuilder query = RestActions.getQueryContent(parser);
-            assertNull(query);
+        for (String requestBody : Arrays.asList("{}", "")) {
+            try (XContentParser parser = createParser(JsonXContent.jsonXContent, requestBody)) {
+                QueryBuilder query = RestActions.getQueryContent(parser);
+                assertNull(query);
+            }
+        }
+    }
+
+    public void testParseTopLevelBuilderMalformedJson() throws IOException {
+        for (String requestBody : Arrays.asList("\"\"", "\"someString\"", "\"{\"")) {
+            try (XContentParser parser = createParser(JsonXContent.jsonXContent, requestBody)) {
+                ParsingException exception =
+                    expectThrows(ParsingException.class, () -> RestActions.getQueryContent(parser));
+                assertEquals("Expected [START_OBJECT] but found [VALUE_STRING]", exception.getMessage());
+            }
+        }
+    }
+
+    public void testParseTopLevelBuilderIncompleteJson() throws IOException {
+        for (String requestBody : Arrays.asList("{", "{ \"query\" :")) {
+            try (XContentParser parser = createParser(JsonXContent.jsonXContent, requestBody)) {
+                ParsingException exception =
+                    expectThrows(ParsingException.class, () -> RestActions.getQueryContent(parser));
+                assertEquals("Failed to parse", exception.getMessage());
+                assertEquals(JsonEOFException.class, exception.getRootCause().getClass());
+            }
         }
     }