Support BufferRequests in HttpClient based backends

Duplicati/Library/Backend/OAuthHelper/OAuthHttpClient.cs

@@ -60,10 +60,6 @@ namespace Duplicati.Library
                 this.Timeout = HttpContextSettings.OperationTimeout;
             }
 
-            // We would also set AllowReadStreamBuffering = HttpContextSettings.BufferRequests, except HttpClient doesn't appear to expose this.
-            // However, starting in .NET 4.5, it looks like HttpClient doesn't buffer by default.
-            // https://www.strathweb.com/2012/09/dealing-with-large-files-in-asp-net-web-api/
-
             // Set the default user agent
             this.DefaultRequestHeaders.UserAgent.Add(new ProductInfoHeaderValue("Duplicati", USER_AGENT_VERSION));
         }
@@ -82,13 +78,19 @@ namespace Duplicati.Library
                 this.PreventAuthentication(request);
             }
 
+            // The HttpCompletionOptions are a nice way to emulate the BufferRequests behavior.
+            // When set to ResponseContentRead, async call will not complete until the response has been
+            // read and is cached in memory (somehow).
+            // When set to ResponseHeadersRead, it looks like both Mono and .NET don't buffer the result.
+            HttpCompletionOption httpCompletionOption = HttpContextSettings.BufferRequests ? HttpCompletionOption.ResponseContentRead : HttpCompletionOption.ResponseHeadersRead;
+
             // The HttpClient.SendAsync method throws an OperationCanceledException when the timeout is exceeded.
             // In order to provide a more informative exception, we will detect this case and throw a TimeoutException
             // instead.  This will also allow the BackendUploader to differentiate between cancellations requested by
             // the user and those generated by timeouts.
             try
             {
-                return await this.SendAsync(request, cancellationToken).ConfigureAwait(false);
+                return await this.SendAsync(request, httpCompletionOption, cancellationToken).ConfigureAwait(false);
             }
             catch (OperationCanceledException) when (!cancellationToken.IsCancellationRequested)
             {