Fix 'error 9 while decompressing xz file'

Once we've started writing data to a Sink, we can't restart a download
request, because then we end up writing duplicate data to the
Sink. Therefore we shouldn't handle retries in Downloader but at a
higher level (in particular, in copyStorePath()).

Fixes #2952.

(cherry picked from commit a67cf5a3585c41dd9f219a2c7aa9cf67fa69520b)

1 files changed, 9 insertions, 2 deletions

diff --git a/src/libstore/download.hh b/src/libstore/download.hh
index aa8c34be2d..752f918575 100644
--- a/src/libstore/download.hh
+++ b/src/libstore/download.hh
@@ -62,11 +62,13 @@ struct Downloader
 
     std::future<DownloadResult> enqueueDownload(const DownloadRequest & request);
 
-    /* Synchronously download a file. */
+    /* Synchronously download a file. The request will be retried in
+       case of transient failures. */
     DownloadResult download(const DownloadRequest & request);
 
     /* Download a file, writing its data to a sink. The sink will be
-       invoked on the thread of the caller. */
+       invoked on the thread of the caller. The request will not be
+       retried in case of transient failures. */
     void download(DownloadRequest && request, Sink & sink);
 
     /* Check if the specified file is already in ~/.cache/nix/tarballs
@@ -95,6 +97,11 @@ public:
     DownloadError(Downloader::Error error, const FormatOrString & fs)
         : Error(fs), error(error)
     { }
+
+    bool isTransient() override
+    {
+        return error == Downloader::Error::Transient;
+    }
 };
 
 bool isUri(const string & s);