Don't over-allocate in HeapBufferedAsyncEntityConsumer in order to consume the response

CHANGELOG.md

@@ -71,6 +71,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Fixed
 - Fix 'org.apache.hc.core5.http.ParseException: Invalid protocol version' under JDK 16+ ([#4827](https://github.com/opensearch-project/OpenSearch/pull/4827))
 - Fix compression support for h2c protocol ([#4944](https://github.com/opensearch-project/OpenSearch/pull/4944))
+- Don't over-allocate in HeapBufferedAsyncEntityConsumer in order to consume the response ([#9993](https://github.com/opensearch-project/OpenSearch/pull/9993))
 
 ### Security
 

client/rest/src/main/java/org/opensearch/client/nio/HeapBufferedAsyncEntityConsumer.java

@@ -86,25 +86,29 @@
             return;
         }
 
+        int len = src.limit();
+        if (len < 0) {
+            len = 4096;
+        } else if (len > bufferLimitBytes) {
+            throw new ContentTooLongException(
+                "entity content is too long [" + len + "] for the configured buffer limit [" + bufferLimitBytes + "]"
+            );
+        }
+
         ByteArrayBuffer buffer = bufferRef.get();
         if (buffer == null) {
-            buffer = new ByteArrayBuffer(bufferLimitBytes);
+            buffer = new ByteArrayBuffer(len);
             if (bufferRef.compareAndSet(null, buffer) == false) {
                 buffer = bufferRef.get();
             }
         }
 
-        int len = src.limit();
         if (buffer.length() + len > bufferLimitBytes) {
             throw new ContentTooLongException(
                 "entity content is too long [" + len + "] for the configured buffer limit [" + bufferLimitBytes + "]"
             );
         }
 
-        if (len < 0) {
-            len = 4096;
-        }
-
         if (src.hasArray()) {
             buffer.append(src.array(), src.arrayOffset() + src.position(), src.remaining());
         } else {
@@ -136,4 +140,12 @@
             buffer = null;
         }
     }
+
+    /**
+     * Gets current byte buffer instance
+     * @return byte buffer instance
+     */
+    ByteArrayBuffer getBuffer() {
+        return bufferRef.get();
+    }
 }

client/rest/src/test/java/org/opensearch/client/nio/HeapBufferedAsyncEntityConsumerTests.java

@@ -0,0 +1,71 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.client.nio;
+
+import org.apache.hc.core5.http.ContentTooLongException;
+import org.opensearch.client.RestClientTestCase;
+import org.junit.After;
+import org.junit.Before;
+
+import java.io.IOException;
+import java.nio.ByteBuffer;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.junit.Assert.assertThrows;
+
+public class HeapBufferedAsyncEntityConsumerTests extends RestClientTestCase {
+    private static final int BUFFER_LIMIT = 100 * 1024 * 1024 /* 100Mb */;
+    private HeapBufferedAsyncEntityConsumer consumer;
+
+    @Before
+    public void setUp() {
+        consumer = new HeapBufferedAsyncEntityConsumer(BUFFER_LIMIT);
+    }
+
+    @After
+    public void tearDown() {
+        consumer.releaseResources();
+    }
+
+    public void testConsumerAllocatesBufferLimit() throws IOException {
+        consumer.consume(randomByteBufferOfLength(1000).flip());
+        assertThat(consumer.getBuffer().capacity(), equalTo(1000));
+    }
+
+    public void testConsumerAllocatesEmptyBuffer() throws IOException {
+        consumer.consume(ByteBuffer.allocate(0).flip());
+        assertThat(consumer.getBuffer().capacity(), equalTo(0));
+    }
+
+    public void testConsumerExpandsBufferLimits() throws IOException {
+        consumer.consume(randomByteBufferOfLength(1000).flip());
+        consumer.consume(randomByteBufferOfLength(2000).flip());
+        consumer.consume(randomByteBufferOfLength(3000).flip());
+        assertThat(consumer.getBuffer().capacity(), equalTo(6000));
+    }
+
+    public void testConsumerAllocatesLimit() throws IOException {
+        consumer.consume(randomByteBufferOfLength(BUFFER_LIMIT).flip());
+        assertThat(consumer.getBuffer().capacity(), equalTo(BUFFER_LIMIT));
+    }
+
+    public void testConsumerFailsToAllocateOverLimit() throws IOException {
+        assertThrows(ContentTooLongException.class, () -> consumer.consume(randomByteBufferOfLength(BUFFER_LIMIT + 1).flip()));
+    }
+
+    public void testConsumerFailsToExpandOverLimit() throws IOException {
+        consumer.consume(randomByteBufferOfLength(BUFFER_LIMIT).flip());
+        assertThrows(ContentTooLongException.class, () -> consumer.consume(randomByteBufferOfLength(1).flip()));
+    }
+
+    private static ByteBuffer randomByteBufferOfLength(int length) {
+        return ByteBuffer.allocate(length).put(randomBytesOfLength(length));
+    }
+}