libstore/filetransfer: Return an opaque handle from enqueueFileTransfer

This is necessary to make pausing/unpausing possible in a follow-up commit.

src/libstore/filetransfer.cc

@@ -48,7 +48,7 @@ struct curlFileTransfer : public FileTransfer
     std::random_device rd;
     std::mt19937 mt19937;
 
-    struct TransferItem : public std::enable_shared_from_this<TransferItem>
+    struct TransferItem : public std::enable_shared_from_this<TransferItem>, public FileTransfer::Item
     {
         curlFileTransfer & fileTransfer;
         FileTransferRequest request;
@@ -849,7 +849,7 @@ struct curlFileTransfer : public FileTransfer
         }
     }
 
-    void enqueueItem(ref<TransferItem> item)
+    ItemHandle enqueueItem(ref<TransferItem> item)
     {
         if (item->request.data && item->request.uri.scheme() != "http" && item->request.uri.scheme() != "https"
             && item->request.uri.scheme() != "s3")
@@ -864,19 +864,20 @@ struct curlFileTransfer : public FileTransfer
 #ifndef _WIN32 // TODO need graceful async exit support on Windows?
         writeFull(wakeupPipe.writeSide.get(), " ");
 #endif
+
+        return ItemHandle(static_cast<Item &>(*item));
     }
 
-    void enqueueFileTransfer(const FileTransferRequest & request, Callback<FileTransferResult> callback) override
+    ItemHandle enqueueFileTransfer(const FileTransferRequest & request, Callback<FileTransferResult> callback) override
     {
         /* Handle s3:// URIs by converting to HTTPS and optionally adding auth */
         if (request.uri.scheme() == "s3") {
             auto modifiedRequest = request;
             modifiedRequest.setupForS3();
-            enqueueItem(make_ref<TransferItem>(*this, std::move(modifiedRequest), std::move(callback)));
+            return enqueueItem(make_ref<TransferItem>(*this, std::move(modifiedRequest), std::move(callback)));
-            return;
         }
 
-        enqueueItem(make_ref<TransferItem>(*this, request, std::move(callback)));
+        return enqueueItem(make_ref<TransferItem>(*this, request, std::move(callback)));
     }
 };
 

src/libstore/include/nix/store/filetransfer.hh

@@ -226,6 +226,26 @@ class Store;
 
 struct FileTransfer
 {
+protected:
+    class Item
+    {};
+
+public:
+    /**
+     * An opaque handle to the file transfer. Can be used to reference an in-flight transfer operations.
+     */
+    class ItemHandle
+    {
+        std::reference_wrapper<Item> item;
+        friend struct FileTransfer;
+
+    public:
+        ItemHandle(Item & item)
+            : item(item)
+        {
+        }
+    };
+
     virtual ~FileTransfer() {}
 
     /**
@@ -233,7 +253,8 @@ struct FileTransfer
      * the download. The future may throw a FileTransferError
      * exception.
      */
-    virtual void enqueueFileTransfer(const FileTransferRequest & request, Callback<FileTransferResult> callback) = 0;
+    virtual ItemHandle
+    enqueueFileTransfer(const FileTransferRequest & request, Callback<FileTransferResult> callback) = 0;
 
     std::future<FileTransferResult> enqueueFileTransfer(const FileTransferRequest & request);