moto: s3: support partNumber for head_object

To support it, we need to keep multipart info in the key itself when
completing multipart upload.

Fixes #2154

Signed-off-by: Ruslan Kuprieiev <ruslan@iterative.ai>

tests/test_s3/test_s3.py

@@ -1671,6 +1671,42 @@ def test_boto3_multipart_etag():
     resp['ETag'].should.equal(EXPECTED_ETAG)
 
 
+@mock_s3
+@reduced_min_part_size
+def test_boto3_multipart_part_size():
+    s3 = boto3.client('s3', region_name='us-east-1')
+    s3.create_bucket(Bucket='mybucket')
+
+    mpu = s3.create_multipart_upload(Bucket='mybucket', Key='the-key')
+    mpu_id = mpu["UploadId"]
+
+    parts = []
+    n_parts = 10
+    for i in range(1, n_parts + 1):
+        part_size = 5 * 1024 * 1024
+        body = b'1' * part_size
+        part = s3.upload_part(
+            Bucket='mybucket',
+            Key='the-key',
+            PartNumber=i,
+            UploadId=mpu_id,
+            Body=body,
+            ContentLength=len(body),
+        )
+        parts.append({"PartNumber": i, "ETag": part["ETag"]})
+
+    s3.complete_multipart_upload(
+        Bucket='mybucket',
+        Key='the-key',
+        UploadId=mpu_id,
+        MultipartUpload={"Parts": parts},
+    )
+
+    for i in range(1, n_parts + 1):
+        obj = s3.head_object(Bucket='mybucket', Key='the-key', PartNumber=i)
+        assert obj["ContentLength"] == part_size
+
+
 @mock_s3
 def test_boto3_put_object_with_tagging():
     s3 = boto3.client('s3', region_name='us-east-1')