libstore: reunify all file transfer methods again

with the api cleaned up we can suddenly reunify uploads, downloads, and
existence checks through curl in the same wrapper function. uploads and
existence checks simply don't use the result source, and given that all
transfers (or at least *most* transfers to date) go through the network
the few extra allocations do not hurt us at all. even for file:// calls
the overhead won't much matter as going to disk and back *is* expensive

Change-Id: I4f9ca6681a8fc303377b4cf4c63e3363ae32c18b

src/libstore/filetransfer.cc

@@ -702,64 +702,19 @@ struct curlFileTransfer : public FileTransfer
 
     void upload(const std::string & uri, std::string data, const Headers & headers) override
     {
-        enqueueFileTransfer(uri, headers, std::move(data), false).get();
+        enqueueFileTransfer(uri, headers, std::move(data), false);
     }
 
-    std::future<std::pair<FileTransferResult, std::string>> enqueueFileTransfer(
+    std::pair<FileTransferResult, box_ptr<Source>> enqueueFileTransfer(
         const std::string & uri,
         const Headers & headers,
         std::optional<std::string> data,
         bool noBody
     )
-    {
-        struct State {
-            std::string data;
-        };
-
-        auto _state = std::make_shared<Sync<State>>();
-
-        auto [meta, done] = enqueueFileTransfer(
-            uri,
-            headers,
-            [](std::exception_ptr ex) {
-                if (ex) {
-                    std::rethrow_exception(ex);
-                }
-            },
-            [_state](std::string_view data) {
-                _state->lock()->data.append(data);
-            },
-            std::move(data),
-            noBody
-        );
-
-        return std::async(
-            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(meta), std::move(state->data));
-            }
-        );
-    }
-
-    std::pair<std::future<FileTransferResult>, std::future<void>> enqueueFileTransfer(
-        const std::string & uri,
-        const Headers & headers,
-        std::invocable<std::exception_ptr> auto doneCallback,
-        std::function<void(std::string_view data)> dataCallback,
-        std::optional<std::string> data,
-        bool noBody
-    )
     {
         /* Ugly hack to support s3:// URIs. */
         if (uri.starts_with("s3://")) {
             // FIXME: do this on a worker thread
-            return {
-                std::async(
-                    std::launch::deferred,
-                    [uri, dataCallback] {
 #if ENABLE_S3
             auto [bucketName, key, params] = parseS3Uri(uri);
 
@@ -775,49 +730,14 @@ struct curlFileTransfer : public FileTransfer
             FileTransferResult res;
             if (!s3Res.data)
                 throw FileTransferError(NotFound, "S3 object '%s' does not exist", uri);
-                        dataCallback(*s3Res.data);
+            return {res, make_box_ptr<StringSource>(std::move(*s3Res.data))};
-                        return res;
 #else
             throw nix::Error(
                 "cannot download '%s' because Lix is not built with S3 support", uri
             );
 #endif
         }
-                ),
-                std::async(std::launch::deferred, []{}),
-            };
-        }
 
-        auto item = enqueueItem(std::make_shared<TransferItem>(
-                               *this,
-                               uri,
-                               headers,
-                               getCurActivity(),
-                               std::move(doneCallback),
-                               std::move(dataCallback),
-                               std::move(data),
-                               noBody
-                           ));
-        return {item->metadataPromise.get_future(), item->doneCallback.get_future()};
-    }
-
-    bool exists(const std::string & uri, const Headers & headers) override
-    {
-        try {
-            enqueueFileTransfer(uri, headers, std::nullopt, true).get();
-            return true;
-        } catch (FileTransferError & e) {
-            /* S3 buckets return 403 if a file doesn't exist and the
-                bucket is unlistable, so treat 403 as 404. */
-            if (e.error == FileTransfer::NotFound || e.error == FileTransfer::Forbidden)
-                return false;
-            throw;
-        }
-    }
-
-    std::pair<FileTransferResult, box_ptr<Source>>
-    download(const std::string & uri, const Headers & headers) override
-    {
         struct State {
             bool done = false, failed = false;
             std::exception_ptr exc;
@@ -827,9 +747,11 @@ struct curlFileTransfer : public FileTransfer
 
         auto _state = std::make_shared<Sync<State>>();
 
-        auto [metadataFuture, _done] = enqueueFileTransfer(
+        auto item = enqueueItem(std::make_shared<TransferItem>(
+            *this,
             uri,
             headers,
+            getCurActivity(),
             [_state](std::exception_ptr ex) {
                 auto state(_state->lock());
                 state->done = true;
@@ -860,19 +782,19 @@ struct curlFileTransfer : public FileTransfer
                 state->data.append(data);
                 state->avail.notify_one();
             },
-            std::nullopt,
+            std::move(data),
-            false
+            noBody
-        );
+        ));
 
-        struct DownloadSource : Source
+        struct TransferSource : Source
         {
             const std::shared_ptr<Sync<State>> _state;
             std::string chunk;
             std::string_view buffered;
 
-            explicit DownloadSource(const std::shared_ptr<Sync<State>> & state) : _state(state) {}
+            explicit TransferSource(const std::shared_ptr<Sync<State>> & state) : _state(state) {}
 
-            ~DownloadSource()
+            ~TransferSource()
             {
                 // wake up the download thread if it's still going and have it abort
                 auto state(_state->lock());
@@ -927,19 +849,39 @@ struct curlFileTransfer : public FileTransfer
                 }
 
                 if (total == 0) {
-                    throw EndOfFile("download finished");
+                    throw EndOfFile("transfer finished");
                 }
 
                 return total;
             }
         };
 
-        auto metadata = metadataFuture.get();
+        auto metadata = item->metadataPromise.get_future().get();
-        auto source = make_box_ptr<DownloadSource>(_state);
+        auto source = make_box_ptr<TransferSource>(_state);
         auto lock(_state->lock());
         source->awaitData(lock);
         return {std::move(metadata), std::move(source)};
     }
+
+    bool exists(const std::string & uri, const Headers & headers) override
+    {
+        try {
+            enqueueFileTransfer(uri, headers, std::nullopt, true);
+            return true;
+        } catch (FileTransferError & e) {
+            /* S3 buckets return 403 if a file doesn't exist and the
+                bucket is unlistable, so treat 403 as 404. */
+            if (e.error == FileTransfer::NotFound || e.error == FileTransfer::Forbidden)
+                return false;
+            throw;
+        }
+    }
+
+    std::pair<FileTransferResult, box_ptr<Source>>
+    download(const std::string & uri, const Headers & headers) override
+    {
+        return enqueueFileTransfer(uri, headers, std::nullopt, false);
+    }
 };
 
 ref<curlFileTransfer> makeCurlFileTransfer(std::optional<unsigned int> baseRetryTimeMs)