diff --git a/src/libstore/filetransfer.cc b/src/libstore/filetransfer.cc
index 9dc742220..b4887d9e8 100644
--- a/src/libstore/filetransfer.cc
+++ b/src/libstore/filetransfer.cc
@@ -696,6 +696,105 @@ struct curlFileTransfer : public FileTransfer
 
         enqueueItem(std::make_shared<TransferItem>(*this, request, std::move(callback)));
     }
+
+    void download(FileTransferRequest && request, Sink & sink) override
+    {
+        /* Note: we can't call 'sink' via request.dataCallback, because
+           that would cause the sink to execute on the fileTransfer
+           thread. If 'sink' is a coroutine, this will fail. Also, if the
+           sink is expensive (e.g. one that does decompression and writing
+           to the Nix store), it would stall the download thread too much.
+           Therefore we use a buffer to communicate data between the
+           download thread and the calling thread. */
+
+        struct State {
+            bool quit = false;
+            std::exception_ptr exc;
+            std::string data;
+            std::condition_variable avail, request;
+        };
+
+        auto _state = std::make_shared<Sync<State>>();
+
+        /* In case of an exception, wake up the download thread. FIXME:
+           abort the download request. */
+        Finally finally([&]() {
+            auto state(_state->lock());
+            state->quit = true;
+            state->request.notify_one();
+        });
+
+        request.dataCallback = [_state](std::string_view data) {
+
+            auto state(_state->lock());
+
+            if (state->quit) return;
+
+            /* If the buffer is full, then go to sleep until the calling
+               thread wakes us up (i.e. when it has removed data from the
+               buffer). We don't wait forever to prevent stalling the
+               download thread. (Hopefully sleeping will throttle the
+               sender.) */
+            if (state->data.size() > 1024 * 1024) {
+                debug("download buffer is full; going to sleep");
+                state.wait_for(state->request, std::chrono::seconds(10));
+            }
+
+            /* Append data to the buffer and wake up the calling
+               thread. */
+            state->data.append(data);
+            state->avail.notify_one();
+        };
+
+        enqueueFileTransfer(request,
+            {[_state](std::future<FileTransferResult> fut) {
+                auto state(_state->lock());
+                state->quit = true;
+                try {
+                    fut.get();
+                } catch (...) {
+                    state->exc = std::current_exception();
+                }
+                state->avail.notify_one();
+                state->request.notify_one();
+            }});
+
+        while (true) {
+            checkInterrupt();
+
+            std::string chunk;
+
+            /* Grab data if available, otherwise wait for the download
+               thread to wake us up. */
+            {
+                auto state(_state->lock());
+
+                if (state->data.empty()) {
+
+                    if (state->quit) {
+                        if (state->exc) std::rethrow_exception(state->exc);
+                        return;
+                    }
+
+                    state.wait(state->avail);
+
+                    if (state->data.empty()) continue;
+                }
+
+                chunk = std::move(state->data);
+                /* Reset state->data after the move, since we check data.empty() */
+                state->data = "";
+
+                state->request.notify_one();
+            }
+
+            /* Flush the data to the sink and wake up the download thread
+               if it's blocked on a full buffer. We don't hold the state
+               lock while doing this to prevent blocking the download
+               thread if sink() takes a long time. */
+            sink(chunk);
+        }
+    }
 };
 
 ref<curlFileTransfer> makeCurlFileTransfer()
@@ -743,105 +842,6 @@ FileTransferResult FileTransfer::upload(const FileTransferRequest & request)
     return enqueueFileTransfer(request).get();
 }
 
-void FileTransfer::download(FileTransferRequest && request, Sink & sink)
-{
-    /* Note: we can't call 'sink' via request.dataCallback, because
-       that would cause the sink to execute on the fileTransfer
-       thread. If 'sink' is a coroutine, this will fail. Also, if the
-       sink is expensive (e.g. one that does decompression and writing
-       to the Nix store), it would stall the download thread too much.
-       Therefore we use a buffer to communicate data between the
-       download thread and the calling thread. */
-
-    struct State {
-        bool quit = false;
-        std::exception_ptr exc;
-        std::string data;
-        std::condition_variable avail, request;
-    };
-
-    auto _state = std::make_shared<Sync<State>>();
-
-    /* In case of an exception, wake up the download thread. FIXME:
-       abort the download request. */
-    Finally finally([&]() {
-        auto state(_state->lock());
-        state->quit = true;
-        state->request.notify_one();
-    });
-
-    request.dataCallback = [_state](std::string_view data) {
-
-        auto state(_state->lock());
-
-        if (state->quit) return;
-
-        /* If the buffer is full, then go to sleep until the calling
-           thread wakes us up (i.e. when it has removed data from the
-           buffer). We don't wait forever to prevent stalling the
-           download thread. (Hopefully sleeping will throttle the
-           sender.) */
-        if (state->data.size() > 1024 * 1024) {
-            debug("download buffer is full; going to sleep");
-            state.wait_for(state->request, std::chrono::seconds(10));
-        }
-
-        /* Append data to the buffer and wake up the calling
-           thread. */
-        state->data.append(data);
-        state->avail.notify_one();
-    };
-
-    enqueueFileTransfer(request,
-        {[_state](std::future<FileTransferResult> fut) {
-            auto state(_state->lock());
-            state->quit = true;
-            try {
-                fut.get();
-            } catch (...) {
-                state->exc = std::current_exception();
-            }
-            state->avail.notify_one();
-            state->request.notify_one();
-        }});
-
-    while (true) {
-        checkInterrupt();
-
-        std::string chunk;
-
-        /* Grab data if available, otherwise wait for the download
-           thread to wake us up. */
-        {
-            auto state(_state->lock());
-
-            if (state->data.empty()) {
-
-                if (state->quit) {
-                    if (state->exc) std::rethrow_exception(state->exc);
-                    return;
-                }
-
-                state.wait(state->avail);
-
-                if (state->data.empty()) continue;
-            }
-
-            chunk = std::move(state->data);
-            /* Reset state->data after the move, since we check data.empty() */
-            state->data = "";
-
-            state->request.notify_one();
-        }
-
-        /* Flush the data to the sink and wake up the download thread
-           if it's blocked on a full buffer. We don't hold the state
-           lock while doing this to prevent blocking the download
-           thread if sink() takes a long time. */
-        sink(chunk);
-    }
-}
-
 template<typename... Args>
 FileTransferError::FileTransferError(FileTransfer::Error error, std::optional<std::string> response, const Args & ... args)
     : Error(args...), error(error), response(response)
diff --git a/src/libstore/filetransfer.hh b/src/libstore/filetransfer.hh
index 6c11c14ee..c724005d7 100644
--- a/src/libstore/filetransfer.hh
+++ b/src/libstore/filetransfer.hh
@@ -115,7 +115,7 @@ struct FileTransfer
      * Download a file, writing its data to a sink. The sink will be
      * invoked on the thread of the caller.
      */
-    void download(FileTransferRequest && request, Sink & sink);
+    virtual void download(FileTransferRequest && request, Sink & sink) = 0;
 
     enum Error { NotFound, Forbidden, Misc, Transient, Interrupted };
 };