Make LocalBinaryCacheStore::narFromPath() run in constant memory

This reduces memory consumption of

  nix copy --from file://... --to ~/my-nix /nix/store/95cwv4q54dc6giaqv6q6p4r02ia2km35-blender-2.79

from 514 MiB to 18 MiB for an uncompressed binary cache, and from 192
MiB to 53 MiB for a bzipped binary cache. It may also be faster
because fetching can happen concurrently with decompression/writing.

Continuation of 48662d151bdf4a38670897beacea9d1bd750376a.

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

1 files changed, 5 insertions, 6 deletions

diff --git a/src/libstore/local-binary-cache-store.cc b/src/libstore/local-binary-cache-store.cc
index ae0ffa6a56ef..b7001795be4d 100644
--- a/src/libstore/local-binary-cache-store.cc
+++ b/src/libstore/local-binary-cache-store.cc
@@ -34,15 +34,14 @@ protected:
         const std::string & data,
         const std::string & mimeType) override;
 
-    void getFile(const std::string & path,
-        Callback<std::shared_ptr<std::string>> callback) override
+    void getFile(const std::string & path, Sink & sink) override
     {
         try {
-            // FIXME: O(n) space
-            callback(std::make_shared<std::string>(readFile(binaryCacheDir + "/" + path)));
+            readFile(binaryCacheDir + "/" + path, sink);
         } catch (SysError & e) {
-            if (e.errNo == ENOENT) callback(nullptr); else callback.rethrow();
-        } catch (...) { callback.rethrow(); }
+            if (e.errNo == ENOENT)
+                throw NoSuchBinaryCacheFile("file '%s' does not exist in binary cache", path);
+        }
     }
 
     PathSet queryAllValidPaths() override