add if match statement to download

Author: sbiscigl

Diff for: src/aws-cpp-sdk-transfer/include/aws/transfer/TransferHandle.h

@@ -297,6 +297,12 @@ namespace Aws
             const Aws::String GetVersionId() const { std::lock_guard<std::mutex> locker(m_getterSetterLock); return m_versionId; }
             void SetVersionId(const Aws::String& versionId) { std::lock_guard<std::mutex> locker(m_getterSetterLock); m_versionId = versionId; }
 
+            /**
+             * (Download only) ETAG of the object to retrieve.
+            */
+            const Aws::String GetEtag() const { std::lock_guard<std::mutex> locker(m_getterSetterLock); return m_etag; }
+            void SetEtag(const Aws::String& etag) { std::lock_guard<std::mutex> locker(m_getterSetterLock); m_etag = etag; }
+
             /**
              * Upload or Download?
              */
@@ -405,6 +411,7 @@ namespace Aws
             Aws::String m_fileName;
             Aws::String m_contentType;
             Aws::String m_versionId;
+            Aws::String m_etag;
             Aws::Map<Aws::String, Aws::String> m_metadata;
             TransferStatus m_status;
             Aws::Client::AWSError<Aws::S3::S3Errors> m_lastError;

Diff for: src/aws-cpp-sdk-transfer/source/transfer/TransferManager.cpp

@@ -931,6 +931,7 @@ namespace Aws
                 handle->SetBytesTotalSize(downloadSize);
                 handle->SetContentType(headObjectOutcome.GetResult().GetContentType());
                 handle->SetMetadata(headObjectOutcome.GetResult().GetMetadata());
+                handle->SetEtag(headObjectOutcome.GetResult().GetETag());
                 /* When bucket versioning is suspended, head object will return "null" for unversioned object.
                  * Send following GetObject with "null" as versionId will result in 403 access denied error if your IAM role or policy
                  * doesn't have GetObjectVersion permission.
@@ -1012,6 +1013,7 @@ namespace Aws
                     getObjectRangeRequest.WithKey(handle->GetKey());
                     getObjectRangeRequest.SetRange(FormatRangeSpecifier(rangeStart, rangeEnd));
                     getObjectRangeRequest.SetResponseStreamFactory(responseStreamFunction);
+                    getObjectRangeRequest.SetIfMatch(handle->GetEtag());
                     if(handle->GetVersionId().size() > 0)
                     {
                         getObjectRangeRequest.SetVersionId(handle->GetVersionId());