[BUG] Streaming bulk request hangs (#16158)

Signed-off-by: Andriy Redko <andriy.redko@aiven.io>
(cherry picked from commit 1ee858f)
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

CHANGELOG.md

@@ -48,6 +48,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Avoid infinite loop when `flat_object` field contains invalid token ([#15985](https://github.com/opensearch-project/OpenSearch/pull/15985))
 - Fix infinite loop in nested agg ([#15931](https://github.com/opensearch-project/OpenSearch/pull/15931))
 - Fix race condition in node-join and node-left ([#15521](https://github.com/opensearch-project/OpenSearch/pull/15521))
+- Streaming bulk request hangs ([#16158](https://github.com/opensearch-project/OpenSearch/pull/16158))
 
 ### Security
 

plugins/transport-reactor-netty4/src/javaRestTest/java/org/opensearch/rest/ReactorNetty4StreamingIT.java

@@ -21,6 +21,7 @@
 import java.nio.ByteBuffer;
 import java.nio.charset.StandardCharsets;
 import java.time.Duration;
+import java.util.Locale;
 import java.util.stream.IntStream;
 import java.util.stream.Stream;
 
@@ -297,4 +298,31 @@ public void testStreamingBadStream() throws IOException {
         assertThat(streamingResponse.getStatusLine().getStatusCode(), equalTo(200));
         assertThat(streamingResponse.getWarnings(), empty());
     }
+
+    public void testStreamingLargeDocument() throws IOException {
+        final Stream<String> stream = Stream.of(
+            String.format(
+                Locale.getDefault(),
+                "{ \"index\": { \"_index\": \"test-streaming\", \"_id\": \"1\" } }\n{ \"name\": \"%s\"  }\n",
+                randomAlphaOfLength(5000)
+            )
+        );
+
+        final Duration delay = Duration.ofMillis(1);
+        final StreamingRequest<ByteBuffer> streamingRequest = new StreamingRequest<>(
+            "POST",
+            "/_bulk/stream",
+            Flux.fromStream(stream).map(s -> ByteBuffer.wrap(s.getBytes(StandardCharsets.UTF_8)))
+        );
+
+        final StreamingResponse<ByteBuffer> streamingResponse = client().streamRequest(streamingRequest);
+
+        StepVerifier.create(Flux.from(streamingResponse.getBody()).map(b -> new String(b.array(), StandardCharsets.UTF_8)))
+            .expectNextMatches(s -> s.contains("\"type\":\"illegal_argument_exception\""))
+            .expectComplete()
+            .verify();
+
+        assertThat(streamingResponse.getStatusLine().getStatusCode(), equalTo(200));
+        assertThat(streamingResponse.getWarnings(), empty());
+    }
 }

server/src/main/java/org/opensearch/rest/RestController.java

@@ -709,7 +709,7 @@ public void sendResponse(RestResponse response) {
                 prepareResponse(response.status(), Map.of("Content-Type", List.of(response.contentType())));
             }
 
-            Mono.ignoreElements(this).then(Mono.just(response)).subscribe(delegate::sendResponse);
+            Mono.from(this).ignoreElement().then(Mono.just(response)).subscribe(delegate::sendResponse);
         }
 
         @Override