From ed1e4f1e5e58d9e35e144e9c2e1d47a53cf66f99 Mon Sep 17 00:00:00 2001
From: yibole <yibole@amazon.com>
Date: Mon, 18 Aug 2025 09:58:31 -0700
Subject: [PATCH 01/14] added part count and content range validation for
 download

---
 .../MultipartDownloaderSubscriber.java        | 63 +++++++++++++++++++
 .../multipart/MultipartDownloadTestUtil.java  | 27 ++++++++
 ...ipartDownloaderSubscriberWiremockTest.java | 26 ++++++++
 3 files changed, 116 insertions(+)

diff --git a/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java b/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
index 1ccae234631d..edf50ff92ca2 100644
--- a/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
+++ b/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
@@ -23,6 +23,7 @@
 import org.reactivestreams.Subscription;
 import software.amazon.awssdk.annotations.SdkInternalApi;
 import software.amazon.awssdk.core.async.AsyncResponseTransformer;
+import software.amazon.awssdk.core.exception.SdkClientException;
 import software.amazon.awssdk.services.s3.S3AsyncClient;
 import software.amazon.awssdk.services.s3.model.GetObjectRequest;
 import software.amazon.awssdk.services.s3.model.GetObjectResponse;
@@ -76,6 +77,16 @@ public class MultipartDownloaderSubscriber implements Subscriber<AsyncResponseTr
      */
     private volatile String eTag;
 
+    /**
+     * The size of each part of the object being downloaded.
+     */
+    private volatile Long partSize;
+
+    /**
+     * The total size of the object being downloaded.
+     */
+    private volatile Long totalContentLength;
+
     /**
      * The Subscription lock
      */
@@ -117,6 +128,7 @@ public void onNext(AsyncResponseTransformer<GetObjectResponse, GetObjectResponse
 
         synchronized (lock) {
             if (totalParts != null && nextPartToGet > totalParts) {
+                validatePartsCount(completedParts.get());
                 log.debug(() -> String.format("Completing multipart download after a total of %d parts downloaded.", totalParts));
                 subscription.cancel();
                 return;
@@ -162,10 +174,20 @@ private void requestMoreIfNeeded(GetObjectResponse response) {
             totalParts = partCount;
         }
 
+        String actualContentRange = response.contentRange();
+        if (actualContentRange != null && partSize == null) {
+            getRangeInfo(actualContentRange);
+            log.debug(() -> String.format("Part size of the object to download: " + partSize));
+            log.debug(() -> String.format("Total Content Length of the object to download: " + totalContentLength));
+        }
+
+        validateContentRange(totalComplete, actualContentRange);
+
         synchronized (lock) {
             if (totalParts != null && totalParts > 1 && totalComplete < totalParts) {
                 subscription.request(1);
             } else {
+                validatePartsCount(completedParts.get());
                 log.debug(() -> String.format("Completing multipart download after a total of %d parts downloaded.", totalParts));
                 subscription.cancel();
             }
@@ -198,4 +220,45 @@ private GetObjectRequest nextRequest(int nextPartToGet) {
             }
         });
     }
+
+    private void validatePartsCount(int currentGetCount) {
+        if (totalParts != null && currentGetCount != totalParts) {
+            String errorMessage = "PartsCount validation failed. Expected " + totalParts + ", downloaded"
+                                  + " " + currentGetCount + " parts.";
+            log.error(() -> errorMessage);
+            subscription.cancel();
+            SdkClientException exception = SdkClientException.create(errorMessage);
+            onError(exception);
+        }
+    }
+
+    private void validateContentRange(int partNumber, String contentRange) {
+        if (contentRange == null) {
+            return;
+        }
+
+        long expectedStart = (partNumber - 1) * partSize;
+        long expectedEnd = partNumber == totalParts ? totalContentLength - 1 : expectedStart + partSize - 1;
+
+        String expectedContentRange = String.format("bytes %d-%d/%d", expectedStart, expectedEnd, totalContentLength);
+
+        if (!expectedContentRange.equals(contentRange)) {
+            String errorMessage = String.format(
+                "Content-Range validation failed for part %d. Expected: %s, Actual: %s",
+                partNumber, expectedContentRange, contentRange);
+            log.error(() -> errorMessage);
+            onError(SdkClientException.create(errorMessage));
+        }
+    }
+
+    private void getRangeInfo(String contentRange) {
+        String rangeInfo = contentRange.substring(6);
+        String[] parts = rangeInfo.split("/");
+
+        this.totalContentLength = Long.parseLong(parts[1]);
+        String[] rangeParts = parts[0].split("-");
+        long startByte = Long.parseLong(rangeParts[0]);
+        long endByte = Long.parseLong(rangeParts[1]);
+        this.partSize = endByte - startByte + 1;
+    }
 }
diff --git a/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloadTestUtil.java b/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloadTestUtil.java
index 708972b6b0d7..3df11de206b5 100644
--- a/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloadTestUtil.java
+++ b/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloadTestUtil.java
@@ -70,6 +70,21 @@ public byte[] stubForPart(String testBucket, String testKey,int part, int totalP
             aResponse()
                 .withHeader("x-amz-mp-parts-count", totalPart + "")
                 .withHeader("ETag", eTag)
+                .withHeader("Content-Length", String.valueOf(body.length))
+                .withHeader("Content-Range", contentRange(part, totalPart, partSize))
+                .withBody(body)));
+        return body;
+    }
+
+    public byte[] stubForPartwithWrongContentRange(String testBucket, String testKey,int part, int totalPart, int partSize) {
+        byte[] body = new byte[partSize];
+        random.nextBytes(body);
+        stubFor(get(urlEqualTo(String.format("/%s/%s?partNumber=%d", testBucket, testKey, part))).willReturn(
+            aResponse()
+                .withHeader("x-amz-mp-parts-count", totalPart + "")
+                .withHeader("ETag", eTag)
+                .withHeader("Content-Length", String.valueOf(body.length))
+                .withHeader("Content-Range", contentRange(part, totalPart, partSize + 1))
                 .withBody(body)));
         return body;
     }
@@ -95,4 +110,16 @@ public byte[] stubForPartSuccess(int part, int totalPart, int partSize) {
                             .withBody(body)));
         return body;
     }
+
+    private String contentRange(int part, int totalPart, int partSize) {
+        long totalObjectSize = (long) totalPart * partSize;
+        long startByte = (long) (part - 1) * partSize;
+        long endByte = startByte + partSize - 1;
+
+        if (part == totalPart) {
+            endByte = totalObjectSize - 1;
+        }
+
+        return String.format("bytes %d-%d/%d", startByte, endByte, totalObjectSize);
+    }
 }
diff --git a/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberWiremockTest.java b/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberWiremockTest.java
index 1c6eb666a9c2..6dcf24550756 100644
--- a/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberWiremockTest.java
+++ b/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberWiremockTest.java
@@ -160,6 +160,32 @@ <T> void errorOnThirdRequest_shouldCompleteExceptionallyOnlyPartsGreaterThanTwo(
         }
     }
 
+    @ParameterizedTest
+    @MethodSource("argumentsProvider")
+    <T> void wrongContentRangeOnSecondRequest_should(AsyncResponseTransformerTestSupplier<T> supplier,
+                                                     int amountOfPartToTest,
+                                                     int partSize) {
+        util.stubForPart(testBucket, testKey, 1, 3, partSize);
+        util.stubForPartwithWrongContentRange(testBucket, testKey, 2, 3, partSize);
+        util.stubForPart(testBucket, testKey, 3, 3, partSize);
+        //byte[] expectedBody = util.stubAllParts(testBucket, testKey, amountOfPartToTest, partSize);
+        AsyncResponseTransformer<GetObjectResponse, T> transformer = supplier.transformer();
+        AsyncResponseTransformer.SplitResult<GetObjectResponse, T> split = transformer.split(
+            SplittingTransformerConfiguration.builder()
+                                             .bufferSizeInBytes(1024 * 32L)
+                                             .build());
+        Subscriber<AsyncResponseTransformer<GetObjectResponse, GetObjectResponse>> subscriber = new MultipartDownloaderSubscriber(
+            s3AsyncClient,
+            GetObjectRequest.builder()
+                            .bucket(testBucket)
+                            .key(testKey)
+                            .build());
+
+        split.publisher().subscribe(subscriber);
+        T response = split.resultFuture().join();
+
+    }
+
     private static Stream<Arguments> argumentsProvider() {
         // amount of part, individual part size
         List<Pair<Integer, Integer>> partSizes = Arrays.asList(

From 391a506ca2a3e5226cbfa5795c0065fcd3bd6033 Mon Sep 17 00:00:00 2001
From: yibole <yibole@amazon.com>
Date: Mon, 18 Aug 2025 13:39:15 -0700
Subject: [PATCH 02/14] changelog added

---
 .changes/next-release/bugfix-AWSSDKforJavav2-17f90b1.json   | 6 ++++++
 .../archtests/CodingConventionWithSuppressionTest.java      | 2 ++
 2 files changed, 8 insertions(+)
 create mode 100644 .changes/next-release/bugfix-AWSSDKforJavav2-17f90b1.json

diff --git a/.changes/next-release/bugfix-AWSSDKforJavav2-17f90b1.json b/.changes/next-release/bugfix-AWSSDKforJavav2-17f90b1.json
new file mode 100644
index 000000000000..c6ce2c184d71
--- /dev/null
+++ b/.changes/next-release/bugfix-AWSSDKforJavav2-17f90b1.json
@@ -0,0 +1,6 @@
+{
+    "type": "bugfix",
+    "category": "AWS SDK for Java v2",
+    "contributor": "",
+    "description": "Added partCount and ContentRange validation for s3 transfer manager download request "
+}
diff --git a/test/architecture-tests/src/test/java/software/amazon/awssdk/archtests/CodingConventionWithSuppressionTest.java b/test/architecture-tests/src/test/java/software/amazon/awssdk/archtests/CodingConventionWithSuppressionTest.java
index d2edcaac742d..3795b30b5d3a 100644
--- a/test/architecture-tests/src/test/java/software/amazon/awssdk/archtests/CodingConventionWithSuppressionTest.java
+++ b/test/architecture-tests/src/test/java/software/amazon/awssdk/archtests/CodingConventionWithSuppressionTest.java
@@ -36,6 +36,7 @@
 import software.amazon.awssdk.core.sync.ResponseTransformer;
 import software.amazon.awssdk.metrics.publishers.emf.EmfMetricLoggingPublisher;
 import software.amazon.awssdk.metrics.publishers.emf.internal.MetricEmfConverter;
+import software.amazon.awssdk.services.s3.internal.multipart.MultipartDownloaderSubscriber;
 import software.amazon.awssdk.utils.Logger;
 
 /**
@@ -57,6 +58,7 @@ public class CodingConventionWithSuppressionTest {
     private static final Set<Pattern> ALLOWED_ERROR_LOG_SUPPRESSION = new HashSet<>(
         Arrays.asList(
             ArchUtils.classNameToPattern(EmfMetricLoggingPublisher.class),
+            ArchUtils.classNameToPattern(MultipartDownloaderSubscriber.class),
             ArchUtils.classWithInnerClassesToPattern(ResponseTransformer.class)));
 
     @Test

From fc956acb749f5e74a448bcc24df9f63c7a91ee69 Mon Sep 17 00:00:00 2001
From: yibole <yibole@amazon.com>
Date: Tue, 19 Aug 2025 15:16:36 -0700
Subject: [PATCH 03/14] remove contentRange validation since part size could
 different

---
 .../MultipartDownloaderSubscriber.java        | 49 -------------------
 ...ipartDownloaderSubscriberWiremockTest.java | 26 ----------
 2 files changed, 75 deletions(-)

diff --git a/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java b/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
index edf50ff92ca2..d6e490226b65 100644
--- a/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
+++ b/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
@@ -77,16 +77,6 @@ public class MultipartDownloaderSubscriber implements Subscriber<AsyncResponseTr
      */
     private volatile String eTag;
 
-    /**
-     * The size of each part of the object being downloaded.
-     */
-    private volatile Long partSize;
-
-    /**
-     * The total size of the object being downloaded.
-     */
-    private volatile Long totalContentLength;
-
     /**
      * The Subscription lock
      */
@@ -174,15 +164,6 @@ private void requestMoreIfNeeded(GetObjectResponse response) {
             totalParts = partCount;
         }
 
-        String actualContentRange = response.contentRange();
-        if (actualContentRange != null && partSize == null) {
-            getRangeInfo(actualContentRange);
-            log.debug(() -> String.format("Part size of the object to download: " + partSize));
-            log.debug(() -> String.format("Total Content Length of the object to download: " + totalContentLength));
-        }
-
-        validateContentRange(totalComplete, actualContentRange);
-
         synchronized (lock) {
             if (totalParts != null && totalParts > 1 && totalComplete < totalParts) {
                 subscription.request(1);
@@ -231,34 +212,4 @@ private void validatePartsCount(int currentGetCount) {
             onError(exception);
         }
     }
-
-    private void validateContentRange(int partNumber, String contentRange) {
-        if (contentRange == null) {
-            return;
-        }
-
-        long expectedStart = (partNumber - 1) * partSize;
-        long expectedEnd = partNumber == totalParts ? totalContentLength - 1 : expectedStart + partSize - 1;
-
-        String expectedContentRange = String.format("bytes %d-%d/%d", expectedStart, expectedEnd, totalContentLength);
-
-        if (!expectedContentRange.equals(contentRange)) {
-            String errorMessage = String.format(
-                "Content-Range validation failed for part %d. Expected: %s, Actual: %s",
-                partNumber, expectedContentRange, contentRange);
-            log.error(() -> errorMessage);
-            onError(SdkClientException.create(errorMessage));
-        }
-    }
-
-    private void getRangeInfo(String contentRange) {
-        String rangeInfo = contentRange.substring(6);
-        String[] parts = rangeInfo.split("/");
-
-        this.totalContentLength = Long.parseLong(parts[1]);
-        String[] rangeParts = parts[0].split("-");
-        long startByte = Long.parseLong(rangeParts[0]);
-        long endByte = Long.parseLong(rangeParts[1]);
-        this.partSize = endByte - startByte + 1;
-    }
 }
diff --git a/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberWiremockTest.java b/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberWiremockTest.java
index 6dcf24550756..1c6eb666a9c2 100644
--- a/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberWiremockTest.java
+++ b/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberWiremockTest.java
@@ -160,32 +160,6 @@ <T> void errorOnThirdRequest_shouldCompleteExceptionallyOnlyPartsGreaterThanTwo(
         }
     }
 
-    @ParameterizedTest
-    @MethodSource("argumentsProvider")
-    <T> void wrongContentRangeOnSecondRequest_should(AsyncResponseTransformerTestSupplier<T> supplier,
-                                                     int amountOfPartToTest,
-                                                     int partSize) {
-        util.stubForPart(testBucket, testKey, 1, 3, partSize);
-        util.stubForPartwithWrongContentRange(testBucket, testKey, 2, 3, partSize);
-        util.stubForPart(testBucket, testKey, 3, 3, partSize);
-        //byte[] expectedBody = util.stubAllParts(testBucket, testKey, amountOfPartToTest, partSize);
-        AsyncResponseTransformer<GetObjectResponse, T> transformer = supplier.transformer();
-        AsyncResponseTransformer.SplitResult<GetObjectResponse, T> split = transformer.split(
-            SplittingTransformerConfiguration.builder()
-                                             .bufferSizeInBytes(1024 * 32L)
-                                             .build());
-        Subscriber<AsyncResponseTransformer<GetObjectResponse, GetObjectResponse>> subscriber = new MultipartDownloaderSubscriber(
-            s3AsyncClient,
-            GetObjectRequest.builder()
-                            .bucket(testBucket)
-                            .key(testKey)
-                            .build());
-
-        split.publisher().subscribe(subscriber);
-        T response = split.resultFuture().join();
-
-    }
-
     private static Stream<Arguments> argumentsProvider() {
         // amount of part, individual part size
         List<Pair<Integer, Integer>> partSizes = Arrays.asList(

From c1acdd867d4ac825801aa5b1491de68183078c03 Mon Sep 17 00:00:00 2001
From: yibole <yibole@amazon.com>
Date: Wed, 20 Aug 2025 13:51:34 -0700
Subject: [PATCH 04/14] feedback addressed

---
 .../next-release/bugfix-AWSSDKforJavav2-17f90b1.json |  6 ------
 .changes/next-release/bugfix-AmazonS3-34da391.json   |  6 ++++++
 .../multipart/MultipartDownloaderSubscriber.java     | 12 ++++++------
 .../CodingConventionWithSuppressionTest.java         |  2 --
 4 files changed, 12 insertions(+), 14 deletions(-)
 delete mode 100644 .changes/next-release/bugfix-AWSSDKforJavav2-17f90b1.json
 create mode 100644 .changes/next-release/bugfix-AmazonS3-34da391.json

diff --git a/.changes/next-release/bugfix-AWSSDKforJavav2-17f90b1.json b/.changes/next-release/bugfix-AWSSDKforJavav2-17f90b1.json
deleted file mode 100644
index c6ce2c184d71..000000000000
--- a/.changes/next-release/bugfix-AWSSDKforJavav2-17f90b1.json
+++ /dev/null
@@ -1,6 +0,0 @@
-{
-    "type": "bugfix",
-    "category": "AWS SDK for Java v2",
-    "contributor": "",
-    "description": "Added partCount and ContentRange validation for s3 transfer manager download request "
-}
diff --git a/.changes/next-release/bugfix-AmazonS3-34da391.json b/.changes/next-release/bugfix-AmazonS3-34da391.json
new file mode 100644
index 000000000000..2d177a6c2849
--- /dev/null
+++ b/.changes/next-release/bugfix-AmazonS3-34da391.json
@@ -0,0 +1,6 @@
+{
+    "type": "bugfix",
+    "category": "Amazon S3",
+    "contributor": "",
+    "description": "Added additional validations for multipart download operations in the Java multipart S3 client"
+}
diff --git a/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java b/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
index d6e490226b65..9fa5cff11c2b 100644
--- a/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
+++ b/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
@@ -114,12 +114,13 @@ public void onNext(AsyncResponseTransformer<GetObjectResponse, GetObjectResponse
             throw new NullPointerException("onNext must not be called with null asyncResponseTransformer");
         }
 
-        int nextPartToGet = completedParts.get() + 1;
+        int currentPart = completedParts.get();
+        int nextPartToGet =  currentPart + 1;
 
         synchronized (lock) {
             if (totalParts != null && nextPartToGet > totalParts) {
-                validatePartsCount(completedParts.get());
                 log.debug(() -> String.format("Completing multipart download after a total of %d parts downloaded.", totalParts));
+                validatePartsCount(currentPart);
                 subscription.cancel();
                 return;
             }
@@ -168,7 +169,7 @@ private void requestMoreIfNeeded(GetObjectResponse response) {
             if (totalParts != null && totalParts > 1 && totalComplete < totalParts) {
                 subscription.request(1);
             } else {
-                validatePartsCount(completedParts.get());
+                validatePartsCount(totalComplete);
                 log.debug(() -> String.format("Completing multipart download after a total of %d parts downloaded.", totalParts));
                 subscription.cancel();
             }
@@ -204,9 +205,8 @@ private GetObjectRequest nextRequest(int nextPartToGet) {
 
     private void validatePartsCount(int currentGetCount) {
         if (totalParts != null && currentGetCount != totalParts) {
-            String errorMessage = "PartsCount validation failed. Expected " + totalParts + ", downloaded"
-                                  + " " + currentGetCount + " parts.";
-            log.error(() -> errorMessage);
+            String errorMessage = String.format("PartsCount validation failed. Expected %d, downloaded %d parts.", totalParts,
+                                                 currentGetCount);
             subscription.cancel();
             SdkClientException exception = SdkClientException.create(errorMessage);
             onError(exception);
diff --git a/test/architecture-tests/src/test/java/software/amazon/awssdk/archtests/CodingConventionWithSuppressionTest.java b/test/architecture-tests/src/test/java/software/amazon/awssdk/archtests/CodingConventionWithSuppressionTest.java
index 3795b30b5d3a..d2edcaac742d 100644
--- a/test/architecture-tests/src/test/java/software/amazon/awssdk/archtests/CodingConventionWithSuppressionTest.java
+++ b/test/architecture-tests/src/test/java/software/amazon/awssdk/archtests/CodingConventionWithSuppressionTest.java
@@ -36,7 +36,6 @@
 import software.amazon.awssdk.core.sync.ResponseTransformer;
 import software.amazon.awssdk.metrics.publishers.emf.EmfMetricLoggingPublisher;
 import software.amazon.awssdk.metrics.publishers.emf.internal.MetricEmfConverter;
-import software.amazon.awssdk.services.s3.internal.multipart.MultipartDownloaderSubscriber;
 import software.amazon.awssdk.utils.Logger;
 
 /**
@@ -58,7 +57,6 @@ public class CodingConventionWithSuppressionTest {
     private static final Set<Pattern> ALLOWED_ERROR_LOG_SUPPRESSION = new HashSet<>(
         Arrays.asList(
             ArchUtils.classNameToPattern(EmfMetricLoggingPublisher.class),
-            ArchUtils.classNameToPattern(MultipartDownloaderSubscriber.class),
             ArchUtils.classWithInnerClassesToPattern(ResponseTransformer.class)));
 
     @Test

From fd674d010637c6711649ef328724b76c1f705db6 Mon Sep 17 00:00:00 2001
From: yibole <yibole@amazon.com>
Date: Fri, 22 Aug 2025 14:49:56 -0700
Subject: [PATCH 05/14] delete unused utils

---
 .../multipart/MultipartDownloadTestUtil.java  | 26 -------------------
 1 file changed, 26 deletions(-)

diff --git a/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloadTestUtil.java b/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloadTestUtil.java
index 3df11de206b5..232f138c22f6 100644
--- a/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloadTestUtil.java
+++ b/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloadTestUtil.java
@@ -71,20 +71,6 @@ public byte[] stubForPart(String testBucket, String testKey,int part, int totalP
                 .withHeader("x-amz-mp-parts-count", totalPart + "")
                 .withHeader("ETag", eTag)
                 .withHeader("Content-Length", String.valueOf(body.length))
-                .withHeader("Content-Range", contentRange(part, totalPart, partSize))
-                .withBody(body)));
-        return body;
-    }
-
-    public byte[] stubForPartwithWrongContentRange(String testBucket, String testKey,int part, int totalPart, int partSize) {
-        byte[] body = new byte[partSize];
-        random.nextBytes(body);
-        stubFor(get(urlEqualTo(String.format("/%s/%s?partNumber=%d", testBucket, testKey, part))).willReturn(
-            aResponse()
-                .withHeader("x-amz-mp-parts-count", totalPart + "")
-                .withHeader("ETag", eTag)
-                .withHeader("Content-Length", String.valueOf(body.length))
-                .withHeader("Content-Range", contentRange(part, totalPart, partSize + 1))
                 .withBody(body)));
         return body;
     }
@@ -110,16 +96,4 @@ public byte[] stubForPartSuccess(int part, int totalPart, int partSize) {
                             .withBody(body)));
         return body;
     }
-
-    private String contentRange(int part, int totalPart, int partSize) {
-        long totalObjectSize = (long) totalPart * partSize;
-        long startByte = (long) (part - 1) * partSize;
-        long endByte = startByte + partSize - 1;
-
-        if (part == totalPart) {
-            endByte = totalObjectSize - 1;
-        }
-
-        return String.format("bytes %d-%d/%d", startByte, endByte, totalObjectSize);
-    }
 }

From 450368b5c7dfbfab1f64841e19242c93f08693fa Mon Sep 17 00:00:00 2001
From: yibole <yibole@amazon.com>
Date: Wed, 27 Aug 2025 09:26:59 -0700
Subject: [PATCH 06/14] change validation in futurn complete

---
 .../MultipartDownloaderSubscriber.java          | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java b/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
index 9fa5cff11c2b..49b625c98e96 100644
--- a/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
+++ b/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
@@ -61,6 +61,11 @@ public class MultipartDownloaderSubscriber implements Subscriber<AsyncResponseTr
      */
     private final AtomicInteger completedParts;
 
+    /**
+     * The total number of getObject calls made. This tracks how many times we've actually called getObject.
+     */
+    private final AtomicInteger getObjectCallCount;
+
     /**
      * The subscription received from the publisher this subscriber subscribes to.
      */
@@ -95,6 +100,7 @@ public MultipartDownloaderSubscriber(S3AsyncClient s3, GetObjectRequest getObjec
         this.s3 = s3;
         this.getObjectRequest = getObjectRequest;
         this.completedParts = new AtomicInteger(completedParts);
+        this.getObjectCallCount = new AtomicInteger(completedParts);
     }
 
     @Override
@@ -120,7 +126,6 @@ public void onNext(AsyncResponseTransformer<GetObjectResponse, GetObjectResponse
         synchronized (lock) {
             if (totalParts != null && nextPartToGet > totalParts) {
                 log.debug(() -> String.format("Completing multipart download after a total of %d parts downloaded.", totalParts));
-                validatePartsCount(currentPart);
                 subscription.cancel();
                 return;
             }
@@ -129,6 +134,7 @@ public void onNext(AsyncResponseTransformer<GetObjectResponse, GetObjectResponse
         GetObjectRequest actualRequest = nextRequest(nextPartToGet);
         log.debug(() -> "Sending GetObjectRequest for next part with partNumber=" + nextPartToGet);
         CompletableFuture<GetObjectResponse> getObjectFuture = s3.getObject(actualRequest, asyncResponseTransformer);
+        getObjectCallCount.incrementAndGet();
         getObjectFutures.add(getObjectFuture);
         getObjectFuture.whenComplete((response, error) -> {
             if (error != null) {
@@ -169,7 +175,6 @@ private void requestMoreIfNeeded(GetObjectResponse response) {
             if (totalParts != null && totalParts > 1 && totalComplete < totalParts) {
                 subscription.request(1);
             } else {
-                validatePartsCount(totalComplete);
                 log.debug(() -> String.format("Completing multipart download after a total of %d parts downloaded.", totalParts));
                 subscription.cancel();
             }
@@ -187,6 +192,7 @@ public void onError(Throwable t) {
 
     @Override
     public void onComplete() {
+        validatePartsCount();
         future.complete(null);
     }
 
@@ -203,10 +209,11 @@ private GetObjectRequest nextRequest(int nextPartToGet) {
         });
     }
 
-    private void validatePartsCount(int currentGetCount) {
-        if (totalParts != null && currentGetCount != totalParts) {
+    private void validatePartsCount() {
+        int actualGetCount = getObjectCallCount.get();
+        if (totalParts != null && actualGetCount != totalParts) {
             String errorMessage = String.format("PartsCount validation failed. Expected %d, downloaded %d parts.", totalParts,
-                                                 currentGetCount);
+                                                actualGetCount);
             subscription.cancel();
             SdkClientException exception = SdkClientException.create(errorMessage);
             onError(exception);

From a272a7a9f37cce6358110d274ccaad79aa962292 Mon Sep 17 00:00:00 2001
From: yibole <yibole@amazon.com>
Date: Wed, 27 Aug 2025 10:26:23 -0700
Subject: [PATCH 07/14] added unit test

---
 ...artDownloaderSubscriberMockClientTest.java | 123 ++++++++++++++++++
 1 file changed, 123 insertions(+)
 create mode 100644 services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberMockClientTest.java

diff --git a/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberMockClientTest.java b/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberMockClientTest.java
new file mode 100644
index 000000000000..ee21419ac80a
--- /dev/null
+++ b/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberMockClientTest.java
@@ -0,0 +1,123 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License").
+ * You may not use this file except in compliance with the License.
+ * A copy of the License is located at
+ *
+ *  http://aws.amazon.com/apache2.0
+ *
+ * or in the "license" file accompanying this file. This file is distributed
+ * on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+ * express or implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ */
+
+package software.amazon.awssdk.services.s3.internal.multipart;
+
+
+import static org.junit.Assert.assertThrows;
+import static org.junit.Assert.assertTrue;
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.when;
+
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.TimeUnit;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.mockito.Mock;
+import org.mockito.MockitoAnnotations;
+import org.reactivestreams.Subscription;
+import software.amazon.awssdk.core.async.AsyncResponseTransformer;
+import software.amazon.awssdk.core.exception.SdkClientException;
+import software.amazon.awssdk.services.s3.S3AsyncClient;
+import software.amazon.awssdk.services.s3.model.GetObjectRequest;
+import software.amazon.awssdk.services.s3.model.GetObjectResponse;
+
+public class MultipartDownloaderSubscriberMockClientTest {
+    @Mock
+    private S3AsyncClient s3Client;
+
+    @Mock
+    private Subscription subscription;
+
+    @Mock
+    private AsyncResponseTransformer<GetObjectResponse, GetObjectResponse> responseTransformer;
+
+    private GetObjectRequest getObjectRequest;
+    private MultipartDownloaderSubscriber subscriber;
+
+    @BeforeEach
+    void setUp() {
+        MockitoAnnotations.openMocks(this);
+        getObjectRequest = GetObjectRequest.builder()
+                                           .bucket("test-bucket")
+                                           .key("test-key")
+                                           .build();
+    }
+
+    @Test
+    void testValidationPassesWhenCallCountMatchesTotalParts() throws InterruptedException {
+        subscriber = new MultipartDownloaderSubscriber(s3Client, getObjectRequest);
+        GetObjectResponse response1 = createMockResponse(3, "etag1");
+        GetObjectResponse response2 = createMockResponse(3, "etag2");
+        GetObjectResponse response3 = createMockResponse(3, "etag3");
+
+        CompletableFuture<GetObjectResponse> future1 = CompletableFuture.completedFuture(response1);
+        CompletableFuture<GetObjectResponse> future2 = CompletableFuture.completedFuture(response2);
+        CompletableFuture<GetObjectResponse> future3 = CompletableFuture.completedFuture(response3);
+
+        when(s3Client.getObject(any(GetObjectRequest.class), eq(responseTransformer)))
+            .thenReturn(future1, future2, future3);
+
+        subscriber.onSubscribe(subscription);
+        subscriber.onNext(responseTransformer);
+        subscriber.onNext(responseTransformer);
+        subscriber.onNext(responseTransformer);
+        Thread.sleep(100);
+
+        subscriber.onComplete();
+
+        assertDoesNotThrow(() -> subscriber.future().get(1, TimeUnit.SECONDS));
+    }
+
+    @Test
+    void testValidationFailsWhenCallCountExceedsTotalParts() throws InterruptedException {
+        subscriber = new MultipartDownloaderSubscriber(s3Client, getObjectRequest);
+        GetObjectResponse response1 = createMockResponse(3, "etag1");
+        GetObjectResponse response2 = createMockResponse(3, "etag2");
+
+        CompletableFuture<GetObjectResponse> future1 = CompletableFuture.completedFuture(response1);
+        CompletableFuture<GetObjectResponse> future2 = CompletableFuture.completedFuture(response2);
+
+        when(s3Client.getObject(any(GetObjectRequest.class), eq(responseTransformer)))
+            .thenReturn(future1, future2);
+
+        subscriber.onSubscribe(subscription);
+        subscriber.onNext(responseTransformer);
+        subscriber.onNext(responseTransformer);
+        Thread.sleep(100);
+
+        subscriber.onComplete();
+
+        ExecutionException exception = assertThrows(ExecutionException.class,
+                                                    () -> subscriber.future().get(1, TimeUnit.SECONDS));
+        assertTrue(exception.getCause() instanceof SdkClientException);
+        assertTrue(exception.getCause().getMessage().contains("PartsCount validation failed"));
+        assertTrue(exception.getCause().getMessage().contains("Expected 3, downloaded 2 parts"));
+
+    }
+
+    private GetObjectResponse createMockResponse(int partsCount, String etag) {
+        GetObjectResponse.Builder builder = GetObjectResponse.builder()
+                                                             .eTag(etag)
+                                                             .contentLength(1024L);
+
+        builder.partsCount(partsCount);
+        return builder.build();
+    }
+
+}

From 13de014212768f5f28990a06ec93152fdb45b675 Mon Sep 17 00:00:00 2001
From: yibole <yibole@amazon.com>
Date: Wed, 27 Aug 2025 10:38:39 -0700
Subject: [PATCH 08/14] change test name

---
 ...ltipartDownloaderSubscriberPartCountValidationTest.java} | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)
 rename services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/{MultipartDownloaderSubscriberMockClientTest.java => MultipartDownloaderSubscriberPartCountValidationTest.java} (95%)

diff --git a/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberMockClientTest.java b/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberPartCountValidationTest.java
similarity index 95%
rename from services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberMockClientTest.java
rename to services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberPartCountValidationTest.java
index ee21419ac80a..915013a1dc83 100644
--- a/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberMockClientTest.java
+++ b/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberPartCountValidationTest.java
@@ -37,7 +37,7 @@
 import software.amazon.awssdk.services.s3.model.GetObjectRequest;
 import software.amazon.awssdk.services.s3.model.GetObjectResponse;
 
-public class MultipartDownloaderSubscriberMockClientTest {
+public class MultipartDownloaderSubscriberPartCountValidationTest {
     @Mock
     private S3AsyncClient s3Client;
 
@@ -60,7 +60,7 @@ void setUp() {
     }
 
     @Test
-    void testValidationPassesWhenCallCountMatchesTotalParts() throws InterruptedException {
+    void callCountMatchesTotalParts_shouldPass() throws InterruptedException {
         subscriber = new MultipartDownloaderSubscriber(s3Client, getObjectRequest);
         GetObjectResponse response1 = createMockResponse(3, "etag1");
         GetObjectResponse response2 = createMockResponse(3, "etag2");
@@ -85,7 +85,7 @@ void testValidationPassesWhenCallCountMatchesTotalParts() throws InterruptedExce
     }
 
     @Test
-    void testValidationFailsWhenCallCountExceedsTotalParts() throws InterruptedException {
+    void callCountLessThanTotalParts_shouldThrowException() throws InterruptedException {
         subscriber = new MultipartDownloaderSubscriber(s3Client, getObjectRequest);
         GetObjectResponse response1 = createMockResponse(3, "etag1");
         GetObjectResponse response2 = createMockResponse(3, "etag2");

From a556358db1e14f3ca6d168a5a9aba07445cd8d03 Mon Sep 17 00:00:00 2001
From: yibole <yibole@amazon.com>
Date: Wed, 27 Aug 2025 12:30:24 -0700
Subject: [PATCH 09/14] minor change

---
 .../s3/internal/multipart/MultipartDownloaderSubscriber.java   | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java b/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
index 49b625c98e96..4a389fcfdbd1 100644
--- a/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
+++ b/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
@@ -120,8 +120,7 @@ public void onNext(AsyncResponseTransformer<GetObjectResponse, GetObjectResponse
             throw new NullPointerException("onNext must not be called with null asyncResponseTransformer");
         }
 
-        int currentPart = completedParts.get();
-        int nextPartToGet =  currentPart + 1;
+        int nextPartToGet =  completedParts.get() + 1;
 
         synchronized (lock) {
             if (totalParts != null && nextPartToGet > totalParts) {

From 72e94faaa8ba55f44034edcf9ee26bb9543f4f7f Mon Sep 17 00:00:00 2001
From: yibole <yibole@amazon.com>
Date: Wed, 27 Aug 2025 12:30:58 -0700
Subject: [PATCH 10/14] minor change

---
 .../s3/internal/multipart/MultipartDownloaderSubscriber.java    | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java b/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
index 4a389fcfdbd1..27fab95c9649 100644
--- a/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
+++ b/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
@@ -120,7 +120,7 @@ public void onNext(AsyncResponseTransformer<GetObjectResponse, GetObjectResponse
             throw new NullPointerException("onNext must not be called with null asyncResponseTransformer");
         }
 
-        int nextPartToGet =  completedParts.get() + 1;
+        int nextPartToGet = completedParts.get() + 1;
 
         synchronized (lock) {
             if (totalParts != null && nextPartToGet > totalParts) {

From 6eaf6dfb6d56b5f8b3067d15c89b6475c5467d2d Mon Sep 17 00:00:00 2001
From: yibole <yibole@amazon.com>
Date: Wed, 10 Sep 2025 17:37:35 -0700
Subject: [PATCH 11/14] Integration test added

---
 .../MultipartDownloaderSubscriber.java        | 14 ++++--
 ...3MultipartClientGetObjectWiremockTest.java | 44 +++++++++++++++++++
 2 files changed, 54 insertions(+), 4 deletions(-)

diff --git a/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java b/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
index 27fab95c9649..0d3145814079 100644
--- a/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
+++ b/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
@@ -138,7 +138,7 @@ public void onNext(AsyncResponseTransformer<GetObjectResponse, GetObjectResponse
         getObjectFuture.whenComplete((response, error) -> {
             if (error != null) {
                 log.debug(() -> "Error encountered during GetObjectRequest with partNumber=" + nextPartToGet);
-                onError(error);
+                handleError(error);
                 return;
             }
             requestMoreIfNeeded(response);
@@ -174,12 +174,20 @@ private void requestMoreIfNeeded(GetObjectResponse response) {
             if (totalParts != null && totalParts > 1 && totalComplete < totalParts) {
                 subscription.request(1);
             } else {
+                validatePartsCount();
                 log.debug(() -> String.format("Completing multipart download after a total of %d parts downloaded.", totalParts));
                 subscription.cancel();
             }
         }
     }
 
+    /**
+     * The method used by the Subscriber itself when error occured.
+     */
+    private void handleError(Throwable t) {
+        onError(t);
+    }
+
     @Override
     public void onError(Throwable t) {
         CompletableFuture<GetObjectResponse> partFuture;
@@ -191,7 +199,6 @@ public void onError(Throwable t) {
 
     @Override
     public void onComplete() {
-        validatePartsCount();
         future.complete(null);
     }
 
@@ -213,9 +220,8 @@ private void validatePartsCount() {
         if (totalParts != null && actualGetCount != totalParts) {
             String errorMessage = String.format("PartsCount validation failed. Expected %d, downloaded %d parts.", totalParts,
                                                 actualGetCount);
-            subscription.cancel();
             SdkClientException exception = SdkClientException.create(errorMessage);
-            onError(exception);
+            handleError(exception);
         }
     }
 }
diff --git a/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/S3MultipartClientGetObjectWiremockTest.java b/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/S3MultipartClientGetObjectWiremockTest.java
index 9fd58e6a5fa0..449f245203c9 100644
--- a/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/S3MultipartClientGetObjectWiremockTest.java
+++ b/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/S3MultipartClientGetObjectWiremockTest.java
@@ -34,6 +34,7 @@
 import static org.junit.jupiter.params.provider.Arguments.arguments;
 import static software.amazon.awssdk.services.s3.internal.multipart.utils.MultipartDownloadTestUtils.internalErrorBody;
 import static software.amazon.awssdk.services.s3.internal.multipart.utils.MultipartDownloadTestUtils.transformersSuppliers;
+import static software.amazon.awssdk.services.s3.multipart.S3MultipartExecutionAttribute.MULTIPART_DOWNLOAD_RESUME_CONTEXT;
 
 import com.github.tomakehurst.wiremock.http.Fault;
 import com.github.tomakehurst.wiremock.junit5.WireMockRuntimeInfo;
@@ -58,6 +59,7 @@
 import software.amazon.awssdk.auth.credentials.StaticCredentialsProvider;
 import software.amazon.awssdk.core.SplittingTransformerConfiguration;
 import software.amazon.awssdk.core.async.AsyncResponseTransformer;
+import software.amazon.awssdk.core.exception.SdkClientException;
 import software.amazon.awssdk.core.internal.async.ByteArrayAsyncResponseTransformer;
 import software.amazon.awssdk.core.internal.async.FileAsyncResponseTransformer;
 import software.amazon.awssdk.core.internal.async.InputStreamResponseTransformer;
@@ -67,6 +69,7 @@
 import software.amazon.awssdk.regions.Region;
 import software.amazon.awssdk.services.s3.S3AsyncClient;
 import software.amazon.awssdk.services.s3.internal.multipart.utils.MultipartDownloadTestUtils;
+import software.amazon.awssdk.services.s3.model.GetObjectRequest;
 import software.amazon.awssdk.services.s3.model.GetObjectResponse;
 import software.amazon.awssdk.services.s3.model.S3Exception;
 import software.amazon.awssdk.services.s3.utils.AsyncResponseTransformerTestSupplier;
@@ -144,6 +147,47 @@ public <T> void errorOnThirdPart_shouldCompleteExceptionallyOnlyPartsGreaterThan
         }
     }
 
+    @ParameterizedTest
+    @MethodSource("partSizeAndTransformerParams")
+    public <T> void partCountValidationFailure_shouldThrowException(
+        AsyncResponseTransformerTestSupplier<T> supplier,
+        int partSize) {
+
+        // To trigger the partCount failure, the resumeContext is used to initialize the actualGetCount larger than the
+        // totalPart number set in the response. This won't happen in real scenario, just to test if the error can be surfaced
+        // to the user if the validation fails.
+        MultipartDownloadResumeContext resumeContext = new MultipartDownloadResumeContext();
+        resumeContext.addCompletedPart(1);
+        resumeContext.addCompletedPart(2);
+        resumeContext.addCompletedPart(3);
+        resumeContext.addToBytesToLastCompletedParts(3 * partSize);
+
+        GetObjectRequest request = GetObjectRequest.builder()
+                                                   .bucket(BUCKET)
+                                                   .key(KEY)
+                                                   .overrideConfiguration(config -> config
+                                                       .putExecutionAttribute(
+                                                           MULTIPART_DOWNLOAD_RESUME_CONTEXT,
+                                                           resumeContext))
+                                                   .build();
+
+        util.stubForPart(BUCKET, KEY, 4, 2, partSize);
+
+        // Skip the lazy transformer since the error won't surface unless the content is consumed
+        AsyncResponseTransformer<GetObjectResponse, T> transformer = supplier.transformer();
+        if (transformer instanceof InputStreamResponseTransformer || transformer instanceof PublisherAsyncResponseTransformer) {
+            return;
+        }
+
+        assertThatThrownBy(() -> {
+            T res = multipartClient.getObject(request, transformer).join();
+            supplier.body(res);
+        }).isInstanceOf(CompletionException.class)
+          .hasCauseInstanceOf(SdkClientException.class)
+          .hasMessageContaining("PartsCount validation failed. Expected 2, downloaded 4 parts");
+
+    }
+
     @ParameterizedTest
     @MethodSource("nonRetryableResponseTransformers")
     public <T> void errorOnFirstPart_shouldFail(AsyncResponseTransformerTestSupplier<T> supplier) {

From 749fc3c1f5e1ada6b4522db06066faebd3cfbc86 Mon Sep 17 00:00:00 2001
From: yibole <yibole@amazon.com>
Date: Wed, 10 Sep 2025 19:28:54 -0700
Subject: [PATCH 12/14] Unit test fixed

---
 ...DownloaderSubscriberPartCountValidationTest.java | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberPartCountValidationTest.java b/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberPartCountValidationTest.java
index 915013a1dc83..8ff1eeac0ad5 100644
--- a/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberPartCountValidationTest.java
+++ b/services/s3/src/test/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriberPartCountValidationTest.java
@@ -85,20 +85,17 @@ void callCountMatchesTotalParts_shouldPass() throws InterruptedException {
     }
 
     @Test
-    void callCountLessThanTotalParts_shouldThrowException() throws InterruptedException {
-        subscriber = new MultipartDownloaderSubscriber(s3Client, getObjectRequest);
-        GetObjectResponse response1 = createMockResponse(3, "etag1");
-        GetObjectResponse response2 = createMockResponse(3, "etag2");
+    void callCountMoreThanTotalParts_shouldThrowException() throws InterruptedException {
+        subscriber = new MultipartDownloaderSubscriber(s3Client, getObjectRequest, 3);
+        GetObjectResponse response1 = createMockResponse(2, "etag1");
 
         CompletableFuture<GetObjectResponse> future1 = CompletableFuture.completedFuture(response1);
-        CompletableFuture<GetObjectResponse> future2 = CompletableFuture.completedFuture(response2);
 
         when(s3Client.getObject(any(GetObjectRequest.class), eq(responseTransformer)))
-            .thenReturn(future1, future2);
+            .thenReturn(future1);
 
         subscriber.onSubscribe(subscription);
         subscriber.onNext(responseTransformer);
-        subscriber.onNext(responseTransformer);
         Thread.sleep(100);
 
         subscriber.onComplete();
@@ -107,7 +104,7 @@ void callCountLessThanTotalParts_shouldThrowException() throws InterruptedExcept
                                                     () -> subscriber.future().get(1, TimeUnit.SECONDS));
         assertTrue(exception.getCause() instanceof SdkClientException);
         assertTrue(exception.getCause().getMessage().contains("PartsCount validation failed"));
-        assertTrue(exception.getCause().getMessage().contains("Expected 3, downloaded 2 parts"));
+        assertTrue(exception.getCause().getMessage().contains("Expected 2, downloaded 4 parts"));
 
     }
 

From a3c55cb00bbf49ebe317d2b79eb3734de460d734 Mon Sep 17 00:00:00 2001
From: yibole <yibole@amazon.com>
Date: Thu, 11 Sep 2025 16:01:41 -0700
Subject: [PATCH 13/14] swap method body of onError and handleError

---
 .../multipart/MultipartDownloaderSubscriber.java       | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java b/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
index 0d3145814079..13443d2da6b9 100644
--- a/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
+++ b/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
@@ -185,11 +185,6 @@ private void requestMoreIfNeeded(GetObjectResponse response) {
      * The method used by the Subscriber itself when error occured.
      */
     private void handleError(Throwable t) {
-        onError(t);
-    }
-
-    @Override
-    public void onError(Throwable t) {
         CompletableFuture<GetObjectResponse> partFuture;
         while ((partFuture = getObjectFutures.poll()) != null) {
             partFuture.cancel(true);
@@ -197,6 +192,11 @@ public void onError(Throwable t) {
         future.completeExceptionally(t);
     }
 
+    @Override
+    public void onError(Throwable t) {
+        handleError(t);
+    }
+
     @Override
     public void onComplete() {
         future.complete(null);

From 80ae743f2ef9ef05399264f48af1240da90973ad Mon Sep 17 00:00:00 2001
From: yibole <yibole@amazon.com>
Date: Thu, 11 Sep 2025 16:03:44 -0700
Subject: [PATCH 14/14] minor change

---
 .../multipart/MultipartDownloaderSubscriber.java       | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java b/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
index 13443d2da6b9..7466bda5b2a3 100644
--- a/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
+++ b/services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/MultipartDownloaderSubscriber.java
@@ -181,6 +181,11 @@ private void requestMoreIfNeeded(GetObjectResponse response) {
         }
     }
 
+    @Override
+    public void onError(Throwable t) {
+        handleError(t);
+    }
+
     /**
      * The method used by the Subscriber itself when error occured.
      */
@@ -192,11 +197,6 @@ private void handleError(Throwable t) {
         future.completeExceptionally(t);
     }
 
-    @Override
-    public void onError(Throwable t) {
-        handleError(t);
-    }
-
     @Override
     public void onComplete() {
         future.complete(null);