filetransfer: correctly abort empty transfers

returning 0 from the callback for errors signals successful transfer if
the source returned no data even though the exception we've just caught
clearly disagrees. while this is not all that important (since the only
viable cause of such errors will be dataCallback, and the sole instance
of it being used already takes care of exceptions) we can just do this.

Change-Id: I2bb150eff447121d82e8e3aa4e00057c40523ac6

1 files changed, 3 insertions, 2 deletions

diff --git a/src/libstore/filetransfer.cc b/src/libstore/filetransfer.cc
index a0e3fd586..67b9fef81 100644
--- a/src/libstore/filetransfer.cc
+++ b/src/libstore/filetransfer.cc
@@ -149,6 +149,8 @@ struct curlFileTransfer : public FileTransfer
 
         size_t writeCallback(void * contents, size_t size, size_t nmemb)
         {
+            const size_t realSize = size * nmemb;
+
             try {
                 if (!headersProcessed) {
                     if (auto h = getHeader("content-encoding")) {
@@ -161,7 +163,6 @@ struct curlFileTransfer : public FileTransfer
                     headersProcessed = true;
                 }
 
-                size_t realSize = size * nmemb;
                 result.bodySize += realSize;
 
                 if (successfulStatuses.count(getHTTPStatus()) && this->dataCallback) {
@@ -174,7 +175,7 @@ struct curlFileTransfer : public FileTransfer
                 return realSize;
             } catch (...) {
                 writeException = std::current_exception();
-                return 0;
+                return CURL_WRITEFUNC_ERROR;
             }
         }