1 files changed, 120 insertions, 0 deletions
diff --git a/widget/PrintBackgroundTask.h b/widget/PrintBackgroundTask.h
new file mode 100644
index 0000000000..d8b4d8a4e5
--- /dev/null
+++ b/widget/PrintBackgroundTask.h
@@ -0,0 +1,120 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_PrintBackgroundTask_h_
+#define mozilla_PrintBackgroundTask_h_
+
+#include "mozilla/dom/Promise.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/Telemetry.h"
+#include <utility>
+#include <tuple>
+
+// A helper to resolve a DOM Promise with the result of a const method, executed
+// in another thread.
+//
+// Once in the main thread, the caller can turn the result of the method into a
+// JSValue by specializing ResolveOrReject.
+namespace mozilla {
+
+template <typename T, typename Result>
+void ResolveOrReject(dom::Promise& aPromise, T&, Result& aResult) {
+  aPromise.MaybeResolve(std::forward<Result>(aResult));
+}
+
+template <typename T, typename Result, typename... Args>
+using PrintBackgroundTask = Result (T::*)(Args...) const;
+
+template <typename T, typename Result, typename... Args>
+void SpawnPrintBackgroundTask(
+    T& aReceiver, dom::Promise& aPromise, const nsCString& aTelemetryKey,
+    PrintBackgroundTask<T, Result, Args...> aBackgroundTask, Args... aArgs) {
+  auto promiseHolder = MakeRefPtr<nsMainThreadPtrHolder<dom::Promise>>(
+      "nsPrinterBase::SpawnBackgroundTaskPromise", &aPromise);
+  // We actually want to allow to access the printer data from the callback, so
+  // disable strict checking. They should of course only access immutable
+  // members.
+  auto holder = MakeRefPtr<nsMainThreadPtrHolder<T>>(
+      "nsPrinterBase::SpawnBackgroundTaskPrinter", &aReceiver,
+      /* strict = */ false);
+  // See
+  // https://stackoverflow.com/questions/47496358/c-lambdas-how-to-capture-variadic-parameter-pack-from-the-upper-scope
+  // about the tuple shenanigans. It could be improved with C++20
+  NS_DispatchBackgroundTask(
+      NS_NewRunnableFunction(
+          "SpawnPrintBackgroundTask",
+          [holder = std::move(holder), promiseHolder = std::move(promiseHolder),
+           aTelemetryKey, startRoundTrip = TimeStamp::Now(),
+           backgroundTask = aBackgroundTask,
+           aArgs = std::make_tuple(std::forward<Args>(aArgs)...)] {
+            auto start = TimeStamp::Now();
+            Result result = std::apply(
+                [&](auto&&... args) {
+                  return (holder->get()->*backgroundTask)(args...);
+                },
+                std::move(aArgs));
+            Telemetry::AccumulateTimeDelta(
+                Telemetry::PRINT_BACKGROUND_TASK_TIME_MS, aTelemetryKey, start);
+            NS_DispatchToMainThread(NS_NewRunnableFunction(
+                "SpawnPrintBackgroundTaskResolution",
+                [holder = std::move(holder),
+                 promiseHolder = std::move(promiseHolder),
+                 telemetryKey = std::move(aTelemetryKey), startRoundTrip,
+                 result = std::move(result)] {
+                  Telemetry::AccumulateTimeDelta(
+                      Telemetry::PRINT_BACKGROUND_TASK_ROUND_TRIP_TIME_MS,
+                      telemetryKey, startRoundTrip);
+                  ResolveOrReject(*promiseHolder->get(), *holder->get(),
+                                  result);
+                }));
+          }),
+      NS_DISPATCH_EVENT_MAY_BLOCK);
+}
+
+// Gets a fresh promise into aResultPromise, that resolves whenever the print
+// background task finishes.
+template <typename T, typename Result, typename... Args>
+nsresult PrintBackgroundTaskPromise(
+    T& aReceiver, JSContext* aCx, dom::Promise** aResultPromise,
+    const nsCString& aTelemetryKey,
+    PrintBackgroundTask<T, Result, Args...> aTask, Args... aArgs) {
+  ErrorResult rv;
+  RefPtr<dom::Promise> promise =
+      dom::Promise::Create(xpc::CurrentNativeGlobal(aCx), rv);
+  if (MOZ_UNLIKELY(rv.Failed())) {
+    return rv.StealNSResult();
+  }
+
+  SpawnPrintBackgroundTask(aReceiver, *promise, aTelemetryKey, aTask,
+                           std::forward<Args>(aArgs)...);
+
+  promise.forget(aResultPromise);
+  return NS_OK;
+}
+
+// Resolves an async attribute via a background task, creating and storing a
+// promise as needed in aPromiseSlot.
+template <typename T, typename Result, typename... Args>
+nsresult AsyncPromiseAttributeGetter(
+    T& aReceiver, RefPtr<dom::Promise>& aPromiseSlot, JSContext* aCx,
+    dom::Promise** aResultPromise, const nsCString& aTelemetryKey,
+    PrintBackgroundTask<T, Result, Args...> aTask, Args... aArgs) {
+  if (RefPtr<dom::Promise> existing = aPromiseSlot) {
+    existing.forget(aResultPromise);
+    return NS_OK;
+  }
+
+  nsresult rv =
+      PrintBackgroundTaskPromise(aReceiver, aCx, aResultPromise, aTelemetryKey,
+                                 aTask, std::forward<Args>(aArgs)...);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  aPromiseSlot = *aResultPromise;
+  return NS_OK;
+}
+
+}  // namespace mozilla
+
+#endif