Make computeFSClosure() single-threaded again

The fact that queryPathInfo() is synchronous meant that we needed a
thread for every concurrent binary cache lookup, even though they end
up being handled by the same download thread. Requiring hundreds of
threads is not a good idea. So now there is an asynchronous version of
queryPathInfo() that takes a callback function to process the
result. Similarly, enqueueDownload() now takes a callback rather than
returning a future.

Thus, a command like

  nix path-info --store https://cache.nixos.org/ -r /nix/store/slljrzwmpygy1daay14kjszsr9xix063-nixos-16.09beta231.dccf8c5

that returns 4941 paths now takes 1.87s using only 2 threads (the main
thread and the downloader thread). (This is with a prewarmed
CloudFront.)

1 files changed, 6 insertions, 4 deletions

diff --git a/src/libstore/download.hh b/src/libstore/download.hh
index 6b90ff20273a..82b5d641fde9 100644
--- a/src/libstore/download.hh
+++ b/src/libstore/download.hh
@@ -23,8 +23,6 @@ struct DownloadRequest
 
 struct DownloadResult
 {
-    enum Status { Success, NotFound, Forbidden, Misc, Transient };
-    Status status;
     bool cached;
     std::string etag;
     std::string effectiveUrl;
@@ -38,7 +36,11 @@ struct Downloader
     /* Enqueue a download request, returning a future to the result of
        the download. The future may throw a DownloadError
        exception. */
-    virtual std::future<DownloadResult> enqueueDownload(const DownloadRequest & request) = 0;
+    virtual void enqueueDownload(const DownloadRequest & request,
+        std::function<void(const DownloadResult &)> success,
+        std::function<void(std::exception_ptr exc)> failure) = 0;
+
+    std::future<DownloadResult> enqueueDownload(const DownloadRequest & request);
 
     /* Synchronously download a file. */
     DownloadResult download(const DownloadRequest & request);
@@ -50,7 +52,7 @@ struct Downloader
     Path downloadCached(ref<Store> store, const string & uri, bool unpack, string name = "",
         const Hash & expectedHash = Hash(), string * effectiveUri = nullptr);
 
-    enum Error { NotFound, Forbidden, Misc, Transient };
+    enum Error { NotFound, Forbidden, Misc, Transient, Interrupted };
 };
 
 /* Return a shared Downloader object. Using this object is preferred