[Rest Api Compatibility] Allow first empty line for msearch

The support for this buggy behaviour was removed in elastic#41011
however since this is a change that affects a request shape it should
still be available to use it in rest api compatibility

relates elastic#51816

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

@@ -53,7 +53,9 @@ public class MultiSearchRequest extends ActionRequest implements CompositeIndice
     private static final DeprecationLogger deprecationLogger = DeprecationLogger.getLogger(RestSearchAction.class);
     public static final String TYPES_DEPRECATION_MESSAGE = "[types removal]" +
         " Specifying types in search requests is deprecated.";
-
+    public static final String FIRST_LINE_EMPTY_DEPRECATION_MESSAGE =
+        "support for empty first line before any action metadata in msearch API is deprecated " +
+            "and will be removed in the next major version";
     public static final int MAX_CONCURRENT_SEARCH_REQUESTS_DEFAULT = 0;
 
     private int maxConcurrentSearchRequests = 0;
@@ -185,6 +187,13 @@ public static void readMultiLineFormat(BytesReference data,
             if (nextMarker == -1) {
                 break;
             }
+            // support first line with \n
+            if (restApiVersion == RestApiVersion.V_7 && nextMarker == 0) {
+                deprecationLogger.compatibleApiWarning("msearch_first_line_empty", FIRST_LINE_EMPTY_DEPRECATION_MESSAGE);
+                from = nextMarker + 1;
+                continue;
+            }
+
             SearchRequest searchRequest = new SearchRequest();
             if (indices != null) {
                 searchRequest.indices(indices);

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

@@ -209,10 +209,15 @@ public void testMsearchTerminatedByNewline() throws Exception {
         assertEquals(3, msearchRequest.requests().size());
     }
 
+    private MultiSearchRequest parseMultiSearchRequestFromString(String request, RestApiVersion restApiVersion) throws IOException {
+        return parseMultiSearchRequest(createRestRequest(request.getBytes(StandardCharsets.UTF_8), restApiVersion));
+    }
+
     private MultiSearchRequest parseMultiSearchRequest(String sample) throws IOException {
-        byte[] data = StreamsUtils.copyToBytesFromClasspath(sample);
-        RestRequest restRequest = new FakeRestRequest.Builder(xContentRegistry())
-            .withContent(new BytesArray(data), XContentType.JSON).build();
+        return parseMultiSearchRequest(createRestRequest(sample, null));
+    }
+
+    private MultiSearchRequest parseMultiSearchRequest(RestRequest restRequest) throws IOException {
 
         MultiSearchRequest request = new MultiSearchRequest();
         RestMultiSearchAction.parseMultiLineRequest(restRequest, SearchRequest.DEFAULT_INDICES_OPTIONS, true,
@@ -223,6 +228,26 @@ private MultiSearchRequest parseMultiSearchRequest(String sample) throws IOExcep
         return request;
     }
 
+    private RestRequest createRestRequest(String sample, RestApiVersion restApiVersion) throws IOException {
+        byte[] data = StreamsUtils.copyToBytesFromClasspath(sample);
+        return createRestRequest(data, restApiVersion);
+    }
+
+    private FakeRestRequest createRestRequest(byte[] data, RestApiVersion restApiVersion) {
+        if (restApiVersion != null) {
+            final List<String> contentTypeHeader =
+                Collections.singletonList(compatibleMediaType(XContentType.VND_JSON, RestApiVersion.V_7));
+            return new FakeRestRequest.Builder(xContentRegistry())
+                .withHeaders(Map.of("Content-Type", contentTypeHeader, "Accept", contentTypeHeader))
+                .withContent(new BytesArray(data), null)
+                .build();
+        } else {
+            return new FakeRestRequest.Builder(xContentRegistry())
+                .withContent(new BytesArray(data), XContentType.JSON).build();
+        }
+    }
+
+
     @Override
     protected NamedXContentRegistry xContentRegistry() {
         return new NamedXContentRegistry(singletonList(new NamedXContentRegistry.Entry(QueryBuilder.class,
@@ -271,6 +296,48 @@ public void testWritingExpandWildcards() throws IOException {
             randomBoolean(), randomBoolean(), randomBoolean()), "none");
     }
 
+    public void testEmptyFirstLine1() throws Exception {
+        MultiSearchRequest request = parseMultiSearchRequestFromString(
+            "\n" +
+                "\n" +
+                "{ \"query\": {\"match_all\": {}}}\n" +
+                "{}\n" +
+                "{ \"query\": {\"match_all\": {}}}\n" +
+                "\n" +
+                "{ \"query\": {\"match_all\": {}}}\n" +
+                "{}\n" +
+                "{ \"query\": {\"match_all\": {}}}\n",
+            RestApiVersion.V_7);
+        assertThat(request.requests().size(), equalTo(4));
+        for (SearchRequest searchRequest : request.requests()) {
+            assertThat(searchRequest.indices().length, equalTo(0));
+            assertThat(searchRequest.source().query(), instanceOf(MatchAllQueryBuilder.class));
+        }
+        assertWarnings("support for empty first line before any action metadata in msearch API is deprecated and will be removed " +
+            "in the next major version");
+    }
+
+    public void testEmptyFirstLine2() throws Exception {
+        MultiSearchRequest request = parseMultiSearchRequestFromString(
+            "\n" +
+                "{}\n" +
+                "{ \"query\": {\"match_all\": {}}}\n" +
+                "\n" +
+                "{ \"query\": {\"match_all\": {}}}\n" +
+                "{}\n" +
+                "{ \"query\": {\"match_all\": {}}}\n" +
+                "\n" +
+                "{ \"query\": {\"match_all\": {}}}\n",
+            RestApiVersion.V_7);
+        assertThat(request.requests().size(), equalTo(4));
+        for (SearchRequest searchRequest : request.requests()) {
+            assertThat(searchRequest.indices().length, equalTo(0));
+            assertThat(searchRequest.source().query(), instanceOf(MatchAllQueryBuilder.class));
+        }
+        assertWarnings("support for empty first line before any action metadata in msearch API is deprecated and will be removed " +
+            "in the next major version");
+    }
+
     private void assertExpandWildcardsValue(IndicesOptions options, String expectedValue) throws IOException {
         SearchRequest request = new SearchRequest();
         request.indicesOptions(options);

server/src/test/resources/org/elasticsearch/action/search/msearch-empty-first-line1.json

@@ -0,0 +1,9 @@
+
+
+{ "query": {"match_all": {}}}
+{}
+{ "query": {"match_all": {}}}
+
+{ "query": {"match_all": {}}}
+{}
+{ "query": {"match_all": {}}}

server/src/test/resources/org/elasticsearch/action/search/msearch-empty-first-line2.json

@@ -0,0 +1,10 @@
+
+
+{}
+{ "query": {"match_all": {}}}
+
+{ "query": {"match_all": {}}}
+{}
+{ "query": {"match_all": {}}}
+
+{ "query": {"match_all": {}}}