Make HttpBinaryCacheStore::narFromPath() run in constant memory

This reduces memory consumption of

  nix copy --from https://cache.nixos.org --to ~/my-nix /nix/store/95cwv4q54dc6giaqv6q6p4r02ia2km35-blender-2.79

from 176 MiB to 82 MiB. (The remaining memory is probably due to xz
decompression overhead.)

Issue https://github.com/NixOS/nix/issues/1681.
Issue https://github.com/NixOS/nix/issues/1969.

1 files changed, 20 insertions, 2 deletions

diff --git a/src/libstore/http-binary-cache-store.cc b/src/libstore/http-binary-cache-store.cc
index b8d670417..6fdae40e3 100644
--- a/src/libstore/http-binary-cache-store.cc
+++ b/src/libstore/http-binary-cache-store.cc
@@ -77,11 +77,29 @@ protected:
         }
     }
 
-    void getFile(const std::string & path,
-        Callback<std::shared_ptr<std::string>> callback) override
+    DownloadRequest makeRequest(const std::string & path)
     {
         DownloadRequest request(cacheUri + "/" + path);
         request.tries = 8;
+        return request;
+    }
+
+    void getFile(const std::string & path, Sink & sink) override
+    {
+        auto request(makeRequest(path));
+        try {
+            getDownloader()->download(std::move(request), sink);
+        } catch (DownloadError & e) {
+            if (e.error == Downloader::NotFound || e.error == Downloader::Forbidden)
+                throw NoSuchBinaryCacheFile("file '%s' does not exist in binary cache '%s'", path, getUri());
+            throw;
+        }
+    }
+
+    void getFile(const std::string & path,
+        Callback<std::shared_ptr<std::string>> callback) override
+    {
+        auto request(makeRequest(path));
 
         getDownloader()->enqueueDownload(request,
             {[callback](std::future<DownloadResult> result) {