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.

src/libutil/retry.hh

@@ -0,0 +1,38 @@
+#pragma once
+
+#include "logging.hh"
+
+#include <functional>
+#include <cmath>
+#include <random>
+#include <thread>
+
+namespace nix {
+
+inline unsigned int retrySleepTime(unsigned int attempt)
+{
+    std::random_device rd;
+    std::mt19937 mt19937;
+    return 250.0 * std::pow(2.0f,
+        attempt - 1 + std::uniform_real_distribution<>(0.0, 0.5)(mt19937));
+}
+
+template<typename C>
+C retry(unsigned int attempts, std::function<C()> && f)
+{
+    unsigned int attempt = 0;
+    while (true) {
+        try {
+            return f();
+        } catch (BaseError & e) {
+            ++attempt;
+            if (attempt >= attempts || !e.isTransient())
+                throw;
+            auto ms = retrySleepTime(attempt);
+            warn("%s; retrying in %d ms", e.what(), ms);
+            std::this_thread::sleep_for(std::chrono::milliseconds(ms));
+        }
+    }
+}
+
+}

src/libutil/types.hh

@@ -109,6 +109,8 @@ public:
     const string & msg() const { return err; }
     const string & prefix() const { return prefix_; }
     BaseError & addPrefix(const FormatOrString & fs);
+
+    virtual bool isTransient() { return false; }
 };
 
 #define MakeError(newClass, superClass) \