Skip 'Expect: 100-continue' header for zero-length S3 streaming requests

.changes/next-release/bugfix-AmazonS3-bb097c9.json

@@ -0,0 +1,6 @@
+{
+    "type": "bugfix",
+    "category": "Amazon S3",
+    "contributor": "",
+    "description": "Fix StreamingRequestInterceptor to skip Expect: 100-continue header when PutObject or UploadPart requests have zero content length."
+}

services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/handlers/StreamingRequestInterceptor.java

@@ -19,6 +19,7 @@
 import software.amazon.awssdk.core.interceptor.Context;
 import software.amazon.awssdk.core.interceptor.ExecutionAttributes;
 import software.amazon.awssdk.core.interceptor.ExecutionInterceptor;
+import software.amazon.awssdk.core.sync.RequestBody;
 import software.amazon.awssdk.http.SdkHttpRequest;
 import software.amazon.awssdk.services.s3.model.PutObjectRequest;
 import software.amazon.awssdk.services.s3.model.UploadPartRequest;
@@ -32,9 +33,38 @@ public final class StreamingRequestInterceptor implements ExecutionInterceptor {
     @Override
     public SdkHttpRequest modifyHttpRequest(Context.ModifyHttpRequest context,
                                             ExecutionAttributes executionAttributes) {
-        if (context.request() instanceof PutObjectRequest || context.request() instanceof UploadPartRequest) {
+        if (shouldAddExpectContinueHeader(context)) {
             return context.httpRequest().toBuilder().putHeader("Expect", "100-continue").build();
         }
         return context.httpRequest();
     }
+
+    /**
+     * Determines whether to add 'Expect: 100-continue' header to streaming requests.
+     *
+     * Per RFC 9110 Section 10.1.1, clients MUST NOT send 100-continue for requests without content.
+     *
+     * Note: Empty Content length check currently applies to sync clients only. Sync HTTP clients (e.g., Apache HttpClient) may
+     * reuse connections, and sending empty content with Expect header can cause issues if the server has already closed the
+     * connection.
+     *
+     * @param context the HTTP request modification context
+     * @return true if Expect header should be added, false otherwise
+     */
+    private boolean shouldAddExpectContinueHeader(Context.ModifyHttpRequest context) {
+        // Must be a streaming request type
+        if (context.request() instanceof PutObjectRequest
+            || context.request() instanceof UploadPartRequest) {
+            // Zero Content length check
+            return context.requestBody()
+                          .flatMap(RequestBody::optionalContentLength)
+                          .map(length -> length != 0L)
+                          .orElse(true);
+        }
+        return false;
+    }
+
+
+
+
 }

services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/handlers/StreamingRequestInterceptorTest.java

@@ -26,7 +26,7 @@
 import software.amazon.awssdk.services.s3.model.UploadPartRequest;
 
 public class StreamingRequestInterceptorTest {
-    private final StreamingRequestInterceptor interceptor = new StreamingRequestInterceptor();
+    private StreamingRequestInterceptor interceptor = new StreamingRequestInterceptor();
 
     @Test
     public void modifyHttpRequest_setsExpect100Continue_whenSdkRequestIsPutObject() {
@@ -55,4 +55,44 @@ public void modifyHttpRequest_doesNotSetExpect_whenSdkRequestIsNotPutObject() {
 
         assertThat(modifiedRequest.firstMatchingHeader("Expect")).isNotPresent();
     }
+
+    @Test
+    public void modifyHttpRequest_doesNotSetExpect_whenPutObjectHasZeroContentLength() {
+        SdkHttpRequest modifiedRequest = interceptor.modifyHttpRequest(
+            modifyHttpRequestContext(PutObjectRequest.builder().build(), 0L),
+            new ExecutionAttributes());
+
+        assertThat(modifiedRequest.firstMatchingHeader("Expect"))
+            .as("Expect header should not be present for zero-length content per RFC 9110")
+            .isNotPresent();
+    }
+
+    @Test
+    public void modifyHttpRequest_doesNotSetExpect_whenUploadPartHasZeroContentLength() {
+        SdkHttpRequest modifiedRequest = interceptor.modifyHttpRequest(
+            modifyHttpRequestContext(UploadPartRequest.builder().build(), 0L),
+            new ExecutionAttributes());
+
+        assertThat(modifiedRequest.firstMatchingHeader("Expect"))
+            .as("Expect header should not be present for zero-length content per RFC 9110")
+            .isNotPresent();
+    }
+
+    @Test
+    public void modifyHttpRequest_setsExpect_whenPutObjectHasNonZeroContentLength() {
+        SdkHttpRequest modifiedRequest = interceptor.modifyHttpRequest(
+            modifyHttpRequestContext(PutObjectRequest.builder().build(), 1024L),
+            new ExecutionAttributes());
+
+        assertThat(modifiedRequest.firstMatchingHeader("Expect")).hasValue("100-continue");
+    }
+
+    @Test
+    public void modifyHttpRequest_setsExpect_whenUploadPartHasNonZeroContentLength() {
+        SdkHttpRequest modifiedRequest = interceptor.modifyHttpRequest(
+            modifyHttpRequestContext(UploadPartRequest.builder().build(), 5242880L),
+            new ExecutionAttributes());
+
+        assertThat(modifiedRequest.firstMatchingHeader("Expect")).hasValue("100-continue");
+    }
 }

services/s3/src/test/java/software/amazon/awssdk/services/s3/utils/InterceptorTestUtils.java

@@ -133,6 +133,41 @@ public SdkRequest request() {
         };
     }
 
+    /**
+     * Creates a ModifyHttpRequest context with a specific content length.
+     * Useful for testing scenarios where content length matters (e.g., RFC 9110 compliance).
+     *
+     * @param request the SDK request
+     * @param contentLength the content length in bytes
+     * @return a ModifyHttpRequest context with the specified content length
+     */
+    public static Context.ModifyHttpRequest modifyHttpRequestContext(SdkRequest request, long contentLength) {
+        Optional<RequestBody> requestBody = Optional.of(RequestBody.fromBytes(new byte[(int) contentLength]));
+        Optional<AsyncRequestBody> asyncRequestBody = Optional.of(AsyncRequestBody.fromBytes(new byte[(int) contentLength]));
+
+        return new Context.ModifyHttpRequest() {
+            @Override
+            public SdkHttpRequest httpRequest() {
+                return sdkHttpFullRequest();
+            }
+
+            @Override
+            public Optional<RequestBody> requestBody() {
+                return requestBody;
+            }
+
+            @Override
+            public Optional<AsyncRequestBody> asyncRequestBody() {
+                return asyncRequestBody;
+            }
+
+            @Override
+            public SdkRequest request() {
+                return request;
+            }
+        };
+    }
+
     public static Context.ModifyResponse modifyResponseContext(SdkRequest request, SdkResponse response, SdkHttpResponse sdkHttpResponse) {
         return new Context.ModifyResponse() {
             @Override