TransportHandler.ProcessMessageAsync overwrites Authorization provided by a custom RequestHandler

Describe the bug When using ResourceTokens that may expire, I have a custom RequestHandler that will pull an updated token and write it to the RequestMessage.Headers. The TransportHandler.ProcessMessageAsync does not check for an existing header before overwriting it with the expired value.

        internal Task<DocumentServiceResponse> ProcessMessageAsync(
            RequestMessage request,
            CancellationToken cancellationToken)
        {
            if (request == null)
            {
                throw new ArgumentNullException(nameof(request));
            }

            DocumentServiceRequest serviceRequest = request.ToDocumentServiceRequest();

-            //TODO: extrace auth into a separate handler
-            string authorization = ((IAuthorizationTokenProvider)this.client.DocumentClient).GetUserAuthorizationToken(
-                serviceRequest.ResourceAddress,
-                PathsHelper.GetResourcePath(request.ResourceType),
-                request.Method.ToString(),
-                serviceRequest.Headers,
-                AuthorizationTokenType.PrimaryMasterKey,
-                payload: out _);
-
-            serviceRequest.Headers[HttpConstants.HttpHeaders.Authorization] = authorization;
+            if (string.IsNullOrEmpty(serviceRequest.Headers[HttpConstants.HttpHeaders.Authorization]))
+            {
+                //TODO: extrace auth into a separate handler
+                string authorization = ((IAuthorizationTokenProvider)this.client.DocumentClient).GetUserAuthorizationToken(
+                    serviceRequest.ResourceAddress,
+                    PathsHelper.GetResourcePath(request.ResourceType),
+                    request.Method.ToString(),
+                    serviceRequest.Headers,
+                    AuthorizationTokenType.PrimaryMasterKey,
+                    payload: out _);
+
+                serviceRequest.Headers[HttpConstants.HttpHeaders.Authorization] = authorization;
+            }

            IStoreModel storeProxy = this.client.DocumentClient.GetStoreProxy(serviceRequest);
            if (request.OperationType == OperationType.Upsert)
            {
                return this.ProcessUpsertAsync(storeProxy, serviceRequest, cancellationToken);
            }
            else
            {
                return storeProxy.ProcessMessageAsync(serviceRequest, cancellationToken);
            }
        }

Environment summary SDK Version: Source OS Version (e.g. Windows, Linux, MacOSX)

Additional context Add any other context about the problem here (for example, complete stack traces or logs).

[j82w]

@ausfeldt will implementing a swap credentials be easier once the work is done for the auth fix you are working on?

I don’t see any reason to not support it in v3. It’s not a breaking change.

[ausfeldt]

I think it should be both v3 and v4.