Fix AsyncRequestBody.fromByteBuffers() never completes when no buffers are given

.changes/next-release/bugfix-AWSSDKforJavav2-17f90b1.json

@@ -0,0 +1,6 @@
+{
+    "type": "bugfix",
+    "category": "AWS SDK for Java v2",
+    "contributor": "",
+    "description": "Fixed AsyncRequestBody.fromByteBuffers() and related methods to properly complete when called with no buffers"
+}

core/sdk-core/src/main/java/software/amazon/awssdk/core/internal/async/ByteBuffersAsyncRequestBody.java

@@ -86,6 +86,10 @@ public void request(long n) {
                         if (n > 0) {
                             int i = index.getAndIncrement();
 
+                            if (buffers.length == 0 && completed.compareAndSet(false, true)) {
+                                s.onComplete();
+                            }
+
                             if (i >= buffers.length) {
                                 return;
                             }

core/sdk-core/src/test/java/software/amazon/awssdk/core/internal/async/ByteBuffersAsyncRequestBodyTest.java

@@ -81,6 +81,16 @@ public void subscriberIsMarkedAsCompleted() {
         assertEquals(1, subscriber.publishedResults.size());
     }
 
+    @Test
+    public void subscriberIsMarkedAsCompletedWhenNoBufferIsGiven() {
+        AsyncRequestBody requestBody = ByteBuffersAsyncRequestBody.of();
+        TestSubscriber subscriber = new TestSubscriber();
+
+        requestBody.subscribe(subscriber);
+        subscriber.request(1);
+        assertTrue(subscriber.onCompleteCalled);
+    }
+
     @Test
     public void subscriberIsMarkedAsCompletedWhenARequestIsMadeForMoreBuffersThanAreAvailable() {
         AsyncRequestBody requestBody = ByteBuffersAsyncRequestBody.from("Hello World!".getBytes(StandardCharsets.UTF_8));