libstore: return transfer metadata from download

as promised earlier. nothing uses it yet, but just you wait.

Change-Id: I77d185578d96c2134b756d20f2fcf1c02de0da6f

src/libstore/builtins/fetchurl.cc

@@ -36,7 +36,7 @@ void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData,
 
     auto fetch = [&](const std::string & url) {
 
-        auto raw = fileTransfer->download(url);
+        auto raw = fileTransfer->download(url).second;
         auto decompressor = makeDecompressionSource(
             unpack && mainUrl.ends_with(".xz") ? "xz" : "none", *raw);
 

src/libstore/filetransfer.cc

@@ -821,7 +821,8 @@ struct curlFileTransfer : public FileTransfer
         }
     }
 
-    box_ptr<Source> download(const std::string & uri, const Headers & headers) override
+    std::pair<FileTransferResult, box_ptr<Source>>
+    download(const std::string & uri, const Headers & headers) override
     {
         struct State {
             bool done = false, failed = false;
@@ -832,7 +833,7 @@ struct curlFileTransfer : public FileTransfer
 
         auto _state = std::make_shared<Sync<State>>();
 
-        auto transfer = enqueueFileTransfer(
+        auto [metadataFuture, _done] = enqueueFileTransfer(
             uri,
             headers,
             [_state](std::exception_ptr ex) {
@@ -939,10 +940,11 @@ struct curlFileTransfer : public FileTransfer
             }
         };
 
+        auto metadata = metadataFuture.get();
         auto source = make_box_ptr<DownloadSource>(_state);
         auto lock(_state->lock());
         source->awaitData(lock);
-        return source;
+        return {std::move(metadata), std::move(source)};
     }
 };
 

src/libstore/filetransfer.hh

@@ -101,7 +101,8 @@ struct FileTransfer
      * thrown by the returned source. The source will only throw errors detected
      * during the transfer itself (decompression errors, connection drops, etc).
      */
-    virtual box_ptr<Source> download(const std::string & uri, const Headers & headers = {}) = 0;
+    virtual std::pair<FileTransferResult, box_ptr<Source>>
+    download(const std::string & uri, const Headers & headers = {}) = 0;
 
     enum Error { NotFound, Forbidden, Misc, Transient, Interrupted };
 };

src/libstore/http-binary-cache-store.cc

@@ -147,7 +147,7 @@ protected:
     {
         checkEnabled();
         try {
-            return getFileTransfer()->download(makeURI(path));
+            return getFileTransfer()->download(makeURI(path)).second;
         } catch (FileTransferError & e) {
             if (e.error == FileTransfer::NotFound || e.error == FileTransfer::Forbidden)
                 throw NoSuchBinaryCacheFile("file '%s' does not exist in binary cache '%s'", path, getUri());

src/nix/prefetch.cc

@@ -98,7 +98,7 @@ std::tuple<StorePath, Hash> prefetchFile(
 
             FdSink sink(fd.get());
 
-            getFileTransfer()->download(url)->drainInto(sink);
+            getFileTransfer()->download(url).second->drainInto(sink);
         }
 
         /* Optionally unpack the file. */