Fix multipart S3 composite checksum suffix

moto/s3/models.py

@@ -496,7 +496,9 @@ def complete(
         full_etag = md5_hash()
         full_etag.update(bytes(md5s))
         if checksum_algo:
-            encoded_checksum = compute_checksum(checksum, checksum_algo).decode("utf-8")
+            encoded_checksum = (
+                f"{compute_checksum(checksum, checksum_algo).decode('utf-8')}-{count}"
+            )
         else:
             encoded_checksum = None
         return total, f"{full_etag.hexdigest()}-{count}", encoded_checksum

tests/test_s3/test_s3_multipart.py

@@ -13,6 +13,7 @@
 import moto.s3.models as s3model
 from moto import mock_aws, settings
 from moto.s3.responses import DEFAULT_REGION_NAME
+from moto.s3.utils import compute_checksum
 from moto.settings import (
     S3_UPLOAD_PART_MIN_SIZE,
     get_s3_default_key_buffer_size,
@@ -159,6 +160,68 @@ def test_multipart_upload(key: str):
     assert response["Body"].read() == part1 + part2
 
 
+@mock_aws
+@reduced_min_part_size
+def test_multipart_upload_checksum_has_composite_suffix():
+    s3_resource = boto3.resource("s3", region_name=DEFAULT_REGION_NAME)
+    client = boto3.client("s3", region_name=DEFAULT_REGION_NAME)
+    bucket = s3_resource.create_bucket(Bucket=str(uuid4()))
+
+    part1 = b"0" * REDUCED_PART_SIZE
+    part2 = b"1"
+
+    multipart = client.create_multipart_upload(
+        Bucket=bucket.name,
+        Key="the-key",
+        ChecksumAlgorithm="SHA256",
+    )
+    up1 = client.upload_part(
+        Body=BytesIO(part1),
+        PartNumber=1,
+        Bucket=bucket.name,
+        Key="the-key",
+        UploadId=multipart["UploadId"],
+    )
+    up2 = client.upload_part(
+        Body=BytesIO(part2),
+        PartNumber=2,
+        Bucket=bucket.name,
+        Key="the-key",
+        UploadId=multipart["UploadId"],
+    )
+
+    client.complete_multipart_upload(
+        Bucket=bucket.name,
+        Key="the-key",
+        MultipartUpload={
+            "Parts": [
+                {"ETag": up1["ETag"], "PartNumber": 1},
+                {"ETag": up2["ETag"], "PartNumber": 2},
+            ]
+        },
+        UploadId=multipart["UploadId"],
+    )
+
+    response = client.get_object(
+        Bucket=bucket.name,
+        Key="the-key",
+        ChecksumMode="ENABLED",
+    )
+    checksum = response["ChecksumSHA256"]
+
+    expected_composite = (
+        compute_checksum(
+            compute_checksum(part1, "SHA256", encode_base64=False)
+            + compute_checksum(part2, "SHA256", encode_base64=False),
+            "SHA256",
+        ).decode("utf-8")
+        + "-2"
+    )
+
+    assert checksum == expected_composite
+    assert checksum.endswith("-2")
+
+
 @pytest.mark.aws_verified
 @s3_aws_verified
 def test_idempotent_multipart_upload(bucket_name=None):