Support empty first line in msearch request body (elastic#41011)

With elastic#41442 we have deprecated support for empty line before any action metadata in msearch API. With this commit we remove support for such empty line, in place of parsing it as empty action metadata, which was previously not supported although the following items could have an empty line representing their corresponding action metadata. This way we make all times equal.

Relates to elastic#39841

server/src/main/java/org/elasticsearch/action/search/MultiSearchRequest.java

@@ -183,12 +183,6 @@ public static void readMultiLineFormat(BytesReference data,
             if (nextMarker == -1) {
                 break;
             }
-            // support first line with \n
-            if (nextMarker == 0) {
-                from = nextMarker + 1;
-                continue;
-            }
-
             SearchRequest searchRequest = new SearchRequest();
             if (indices != null) {
                 searchRequest.indices(indices);

server/src/test/java/org/elasticsearch/action/search/MultiSearchRequestTests.java

@@ -48,6 +48,7 @@
 import static org.elasticsearch.search.RandomSearchRequestGenerator.randomSearchRequest;
 import static org.elasticsearch.test.EqualsHashCodeTestUtils.checkEqualsAndHashCode;
 import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.instanceOf;
 import static org.hamcrest.Matchers.is;
 import static org.hamcrest.Matchers.nullValue;
 
@@ -180,6 +181,15 @@ public void testSimpleAdd4() throws Exception {
         assertThat(request.requests().get(2).routing(), equalTo("123"));
     }
 
+    public void testNoMetadata() throws Exception {
+        MultiSearchRequest request = parseMultiSearchRequest("/org/elasticsearch/action/search/msearch-no-metadata.json");
+        assertThat(request.requests().size(), equalTo(4));
+        for (SearchRequest searchRequest : request.requests()) {
+            assertThat(searchRequest.indices().length, equalTo(0));
+            assertThat(searchRequest.source().query(), instanceOf(MatchAllQueryBuilder.class));
+        }
+    }
+
     public void testResponseErrorToXContent() {
         long tookInMillis = randomIntBetween(1, 1000);
         MultiSearchResponse response = new MultiSearchResponse(

server/src/test/resources/org/elasticsearch/action/search/msearch-no-metadata.json

@@ -0,0 +1,8 @@
+
+{ "query": {"match_all": {}}}
+
+{ "query": {"match_all": {}}}
+
+{ "query": {"match_all": {}}}
+
+{ "query": {"match_all": {}}}