libstore: split callback into metadata and finished parts

this will let us return metadata from FileTransfer::download, which in
turn is necessary to remove enqueueDownload. it also opens avenues for
streaming downloads that keep download metadata instead of dropping it

Change-Id: If0fc6af5eb2aeb689fc866c345c9d7bce4d59f2d

src/libstore/filetransfer.cc

@@ -60,7 +60,8 @@ struct curlFileTransfer : public FileTransfer
             /// transfer complete, result or failure reported
             transferComplete,
         } phase = initialSetup;
-        std::packaged_task<FileTransferResult(std::exception_ptr, FileTransferResult)> callback;
+        std::promise<FileTransferResult> metadataPromise;
+        std::packaged_task<void(std::exception_ptr)> doneCallback;
         std::function<void(std::string_view data)> dataCallback;
         CURL * req; // must never be nullptr
         std::string statusMsg;
@@ -100,7 +101,7 @@ struct curlFileTransfer : public FileTransfer
             const std::string & uri,
             const Headers & headers,
             ActivityId parentAct,
-            std::invocable<std::exception_ptr> auto callback,
+            std::invocable<std::exception_ptr> auto doneCallback,
             std::function<void(std::string_view data)> dataCallback,
             std::optional<std::string> uploadData,
             bool noBody
@@ -112,9 +113,8 @@ struct curlFileTransfer : public FileTransfer
                 {uri}, parentAct)
             , uploadData(std::move(uploadData))
             , noBody(noBody)
-            , callback([cb{std::move(callback)}] (std::exception_ptr ex, FileTransferResult r) {
+            , doneCallback([cb{std::move(doneCallback)}] (std::exception_ptr ex) {
                 cb(ex);
-                return r;
             })
             , dataCallback(std::move(dataCallback))
             , req(curl_easy_init())
@@ -143,8 +143,11 @@ struct curlFileTransfer : public FileTransfer
         void failEx(std::exception_ptr ex)
         {
             assert(phase != transferComplete);
+            if (phase == initialSetup) {
+                metadataPromise.set_exception(ex);
+            }
             phase = transferComplete;
-            callback(ex, std::move(result));
+            doneCallback(ex);
         }
 
         template<class T>
@@ -178,6 +181,9 @@ struct curlFileTransfer : public FileTransfer
 
             result.cached = getHTTPStatus() == 304;
 
+            if (phase == initialSetup) {
+                metadataPromise.set_value(result);
+            }
             phase = transferring;
         }
 
@@ -399,7 +405,7 @@ struct curlFileTransfer : public FileTransfer
             {
                 act.progress(bodySize, bodySize);
                 phase = transferComplete;
-                callback(nullptr, std::move(result));
+                doneCallback(nullptr);
             }
 
             else {
@@ -718,7 +724,7 @@ struct curlFileTransfer : public FileTransfer
 
         auto _state = std::make_shared<Sync<State>>();
 
-        auto transfer = enqueueFileTransfer(
+        auto [meta, done] = enqueueFileTransfer(
             uri,
             headers,
             [](std::exception_ptr ex) {
@@ -733,17 +739,21 @@ struct curlFileTransfer : public FileTransfer
             noBody
         );
 
-        return std::async(std::launch::deferred, [_state, transfer{std::move(transfer)}]() mutable {
+        return std::async(
-            auto result = transfer.get();
+            std::launch::deferred,
+            [_state, _meta{std::move(meta)}, done{std::move(done)}]() mutable {
+                auto meta = _meta.get();
+                done.get();
                 auto state(_state->lock());
-            return std::pair(std::move(result), std::move(state->data));
+                return std::pair(std::move(meta), std::move(state->data));
-        });
+            }
+        );
     }
 
-    std::future<FileTransferResult> enqueueFileTransfer(
+    std::pair<std::future<FileTransferResult>, std::future<void>> enqueueFileTransfer(
         const std::string & uri,
         const Headers & headers,
-        std::invocable<std::exception_ptr> auto callback,
+        std::invocable<std::exception_ptr> auto doneCallback,
         std::function<void(std::string_view data)> dataCallback,
         std::optional<std::string> data,
         bool noBody
@@ -752,7 +762,8 @@ struct curlFileTransfer : public FileTransfer
         /* Ugly hack to support s3:// URIs. */
         if (uri.starts_with("s3://")) {
             // FIXME: do this on a worker thread
-            return std::async(
+            return {
+                std::async(
                     std::launch::deferred,
                     [uri, dataCallback] {
 #if ENABLE_S3
@@ -778,20 +789,22 @@ struct curlFileTransfer : public FileTransfer
                         );
 #endif
                     }
-            );
+                ),
+                std::async(std::launch::deferred, []{}),
+            };
         }
 
-        return enqueueItem(std::make_shared<TransferItem>(
+        auto item = enqueueItem(std::make_shared<TransferItem>(
                                *this,
                                uri,
                                headers,
                                getCurActivity(),
-                               std::move(callback),
+                               std::move(doneCallback),
                                std::move(dataCallback),
                                std::move(data),
                                noBody
-                           ))
+                           ));
-            ->callback.get_future();
+        return {item->metadataPromise.get_future(), item->doneCallback.get_future()};
     }
 
     bool exists(const std::string & uri, const Headers & headers) override
@@ -819,7 +832,7 @@ struct curlFileTransfer : public FileTransfer
 
         auto _state = std::make_shared<Sync<State>>();
 
-        enqueueFileTransfer(
+        auto transfer = enqueueFileTransfer(
             uri,
             headers,
             [_state](std::exception_ptr ex) {