Validate metdata on `_msearch` (#35938)

MultiSearchRequests issues through `_msearch` now validate all keys
in the metadata section. Previously unknown keys were ignored
while now an exception is thrown.

Closes #35869

docs/reference/migration/migrate_7_0/api.asciidoc

@@ -107,3 +107,9 @@ object has been removed in favor of the top level `created` field.
 ==== Source filtering url parameters `_source_include` and `_source_exclude` have been removed
 
 The deprecated in 6.x url parameters are now removed. Use `_source_includes` and `_source_excludes` instead.
+
+[float]
+==== Multi Search Request metadata validation
+
+MultiSearchRequests issued through `_msearch` now validate all keys in the metadata section. Previously unknown keys were ignored
+while now an exception is thrown.

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

@@ -211,6 +211,10 @@ public class MultiSearchRequest extends ActionRequest implements CompositeIndice
                 try (InputStream stream = data.slice(from, nextMarker - from).streamInput();
                      XContentParser parser = xContent.createParser(registry, LoggingDeprecationHandler.INSTANCE, stream)) {
                     Map<String, Object> source = parser.map();
+                    Object expandWildcards = null;
+                    Object ignoreUnavailable = null;
+                    Object ignoreThrottled = null;
+                    Object allowNoIndices = null;
                     for (Map.Entry<String, Object> entry : source.entrySet()) {
                         Object value = entry.getValue();
                         if ("index".equals(entry.getKey()) || "indices".equals(entry.getKey())) {
@@ -230,12 +234,20 @@ public class MultiSearchRequest extends ActionRequest implements CompositeIndice
                             searchRequest.routing(nodeStringValue(value, null));
                         } else if ("allow_partial_search_results".equals(entry.getKey())) {
                             searchRequest.allowPartialSearchResults(nodeBooleanValue(value, null));
+                        } else if ("expand_wildcards".equals(entry.getKey()) || "expandWildcards".equals(entry.getKey())) {
+                            expandWildcards = value;
+                        } else if ("ignore_unavailable".equals(entry.getKey()) || "ignoreUnavailable".equals(entry.getKey())) {
+                            ignoreUnavailable = value;
+                        } else if ("allow_no_indices".equals(entry.getKey()) || "allowNoIndices".equals(entry.getKey())) {
+                            allowNoIndices = value;
+                        } else if ("ignore_throttled".equals(entry.getKey()) || "ignoreThrottled".equals(entry.getKey())) {
+                            ignoreThrottled = value;
                         } else {
-                            // TODO we should not be lenient here and fail if there is any unknown key in the source map
+                            throw new IllegalArgumentException("key [" + entry.getKey() + "] is not supported in the metadata section");
                         }
-
                     }
-                    defaultOptions = IndicesOptions.fromMap(source, defaultOptions);
+                    defaultOptions = IndicesOptions.fromParameters(expandWildcards, ignoreUnavailable, allowNoIndices, ignoreThrottled,
+                        defaultOptions);
                 }
             }
             searchRequest.indicesOptions(defaultOptions);

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

@@ -90,6 +90,16 @@ public class MultiSearchRequestTests extends ESTestCase {
         assertThat(request.requests().get(7).types().length, equalTo(0));
     }
 
+    public void testFailWithUnknownKey() {
+        final String requestContent = "{\"index\":\"test\", \"ignore_unavailable\" : true, \"unknown_key\" : \"open,closed\"}}\r\n" +
+            "{\"query\" : {\"match_all\" :{}}}\r\n";
+        FakeRestRequest restRequest = new FakeRestRequest.Builder(xContentRegistry())
+            .withContent(new BytesArray(requestContent), XContentType.JSON).build();
+        IllegalArgumentException ex = expectThrows(IllegalArgumentException.class,
+            () -> RestMultiSearchAction.parseRequest(restRequest, true));
+        assertEquals("key [unknown_key] is not supported in the metadata section", ex.getMessage());
+    }
+
     public void testSimpleAddWithCarriageReturn() throws Exception {
         final String requestContent = "{\"index\":\"test\", \"ignore_unavailable\" : true, \"expand_wildcards\" : \"open,closed\"}}\r\n" +
             "{\"query\" : {\"match_all\" :{}}}\r\n";