filetransfer: make two-arg download abstract

this lets each implementation of FileTransfer (of which currently only
the one exists at all) implement appropriate handling for its internal
behaviours that are not otherwise exposed. in curl this lets us switch
the buffer-full handling method from "block the entire curl thread" to
"pause just the one transfer", move the non-libcurl body decompression
out of the actual curl wrapper (which will let us eventually morph the
curl wrapper intto an actual source of Sources), and some other things

Change-Id: Id6d3593cde6b4915aab3e90a43b175c103cc3f18

src/libstore/filetransfer.cc

@@ -696,55 +696,9 @@ struct curlFileTransfer : public FileTransfer
 
         enqueueItem(std::make_shared<TransferItem>(*this, request, std::move(callback)));
     }
-};
 
-ref<curlFileTransfer> makeCurlFileTransfer()
-{
-    return make_ref<curlFileTransfer>();
-}
-
-ref<FileTransfer> getFileTransfer()
-{
-    static ref<curlFileTransfer> fileTransfer = makeCurlFileTransfer();
-
-    if (fileTransfer->state_.lock()->quit)
-        fileTransfer = makeCurlFileTransfer();
-
-    return fileTransfer;
-}
-
-ref<FileTransfer> makeFileTransfer()
-{
-    return makeCurlFileTransfer();
-}
-
-std::future<FileTransferResult> FileTransfer::enqueueFileTransfer(const FileTransferRequest & request)
-{
-    auto promise = std::make_shared<std::promise<FileTransferResult>>();
-    enqueueFileTransfer(request,
-        {[promise](std::future<FileTransferResult> fut) {
-            try {
-                promise->set_value(fut.get());
-            } catch (...) {
-                promise->set_exception(std::current_exception());
-            }
-        }});
-    return promise->get_future();
-}
-
-FileTransferResult FileTransfer::download(const FileTransferRequest & request)
-{
-    return enqueueFileTransfer(request).get();
-}
-
-FileTransferResult FileTransfer::upload(const FileTransferRequest & request)
-{
-    /* Note: this method is the same as download, but helps in readability */
-    return enqueueFileTransfer(request).get();
-}
-
-void FileTransfer::download(FileTransferRequest && request, Sink & sink)
-{
+    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
@@ -840,6 +794,52 @@ void FileTransfer::download(FileTransferRequest && request, Sink & sink)
                thread if sink() takes a long time. */
             sink(chunk);
         }
+    }
+};
+
+ref<curlFileTransfer> makeCurlFileTransfer()
+{
+    return make_ref<curlFileTransfer>();
+}
+
+ref<FileTransfer> getFileTransfer()
+{
+    static ref<curlFileTransfer> fileTransfer = makeCurlFileTransfer();
+
+    if (fileTransfer->state_.lock()->quit)
+        fileTransfer = makeCurlFileTransfer();
+
+    return fileTransfer;
+}
+
+ref<FileTransfer> makeFileTransfer()
+{
+    return makeCurlFileTransfer();
+}
+
+std::future<FileTransferResult> FileTransfer::enqueueFileTransfer(const FileTransferRequest & request)
+{
+    auto promise = std::make_shared<std::promise<FileTransferResult>>();
+    enqueueFileTransfer(request,
+        {[promise](std::future<FileTransferResult> fut) {
+            try {
+                promise->set_value(fut.get());
+            } catch (...) {
+                promise->set_exception(std::current_exception());
+            }
+        }});
+    return promise->get_future();
+}
+
+FileTransferResult FileTransfer::download(const FileTransferRequest & request)
+{
+    return enqueueFileTransfer(request).get();
+}
+
+FileTransferResult FileTransfer::upload(const FileTransferRequest & request)
+{
+    /* Note: this method is the same as download, but helps in readability */
+    return enqueueFileTransfer(request).get();
 }
 
 template<typename... Args>

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 };
 };