widget/PrintBackgroundTask.h

   1 /* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* This Source Code Form is subject to the terms of the Mozilla Public
   3  * License, v. 2.0. If a copy of the MPL was not distributed with this
   4  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   5
   6 #ifndef mozilla_PrintBackgroundTask_h_
   7 #define mozilla_PrintBackgroundTask_h_
   8
   9 #include "mozilla/dom/Promise.h"
  10 #include "mozilla/ErrorResult.h"
  11 #include "mozilla/Telemetry.h"
  12 #include <utility>
  13 #include <tuple>
  14
  15 // A helper to resolve a DOM Promise with the result of a const method, executed
  16 // in another thread.
  17 //
  18 // Once in the main thread, the caller can turn the result of the method into a
  19 // JSValue by specializing ResolveOrReject.
  20 namespace mozilla {
  21
  22 template <typename T, typename Result>
  23 void ResolveOrReject(dom::Promise& aPromise, T&, Result& aResult) {
  24   aPromise.MaybeResolve(std::forward<Result>(aResult));
  25 }
  26
  27 template <typename T, typename Result, typename... Args>
  28 using PrintBackgroundTask = Result (T::*)(Args...) const;
  29
  30 template <typename T, typename Result, typename... Args>
  31 void SpawnPrintBackgroundTask(
  32     T& aReceiver, dom::Promise& aPromise, const nsCString& aTelemetryKey,
  33     PrintBackgroundTask<T, Result, Args...> aBackgroundTask, Args... aArgs) {
  34   auto promiseHolder = MakeRefPtr<nsMainThreadPtrHolder<dom::Promise>>(
  35       "nsPrinterBase::SpawnBackgroundTaskPromise", &aPromise);
  36   // We actually want to allow to access the printer data from the callback, so
  37   // disable strict checking. They should of course only access immutable
  38   // members.
  39   auto holder = MakeRefPtr<nsMainThreadPtrHolder<T>>(
  40       "nsPrinterBase::SpawnBackgroundTaskPrinter", &aReceiver,
  41       /* strict = */ false);
  42   // See
  43   // https://stackoverflow.com/questions/47496358/c-lambdas-how-to-capture-variadic-parameter-pack-from-the-upper-scope
  44   // about the tuple shenanigans. It could be improved with C++20
  45   NS_DispatchBackgroundTask(
  46       NS_NewRunnableFunction(
  47           "SpawnPrintBackgroundTask",
  48           [holder = std::move(holder), promiseHolder = std::move(promiseHolder),
  49            aTelemetryKey, startRoundTrip = TimeStamp::Now(),
  50            backgroundTask = aBackgroundTask,
  51            aArgs = std::make_tuple(std::forward<Args>(aArgs)...)] {
  52             auto start = TimeStamp::Now();
  53             Result result = std::apply(
  54                 [&](auto&&... args) {
  55                   return (holder->get()->*backgroundTask)(args...);
  56                 },
  57                 std::move(aArgs));
  58             Telemetry::AccumulateTimeDelta(
  59                 Telemetry::PRINT_BACKGROUND_TASK_TIME_MS, aTelemetryKey, start);
  60             NS_DispatchToMainThread(NS_NewRunnableFunction(
  61                 "SpawnPrintBackgroundTaskResolution",
  62                 [holder = std::move(holder),
  63                  promiseHolder = std::move(promiseHolder),
  64                  telemetryKey = std::move(aTelemetryKey), startRoundTrip,
  65                  result = std::move(result)] {
  66                   Telemetry::AccumulateTimeDelta(
  67                       Telemetry::PRINT_BACKGROUND_TASK_ROUND_TRIP_TIME_MS,
  68                       telemetryKey, startRoundTrip);
  69                   ResolveOrReject(*promiseHolder->get(), *holder->get(),
  70                                   result);
  71                 }));
  72           }),
  73       NS_DISPATCH_EVENT_MAY_BLOCK);
  74 }
  75
  76 // Gets a fresh promise into aResultPromise, that resolves whenever the print
  77 // background task finishes.
  78 template <typename T, typename Result, typename... Args>
  79 nsresult PrintBackgroundTaskPromise(
  80     T& aReceiver, JSContext* aCx, dom::Promise** aResultPromise,
  81     const nsCString& aTelemetryKey,
  82     PrintBackgroundTask<T, Result, Args...> aTask, Args... aArgs) {
  83   ErrorResult rv;
  84   RefPtr<dom::Promise> promise =
  85       dom::Promise::Create(xpc::CurrentNativeGlobal(aCx), rv);
  86   if (MOZ_UNLIKELY(rv.Failed())) {
  87     return rv.StealNSResult();
  88   }
  89
  90   SpawnPrintBackgroundTask(aReceiver, *promise, aTelemetryKey, aTask,
  91                            std::forward<Args>(aArgs)...);
  92
  93   promise.forget(aResultPromise);
  94   return NS_OK;
  95 }
  96
  97 // Resolves an async attribute via a background task, creating and storing a
  98 // promise as needed in aPromiseSlot.
  99 template <typename T, typename Result, typename... Args>
 100 nsresult AsyncPromiseAttributeGetter(
 101     T& aReceiver, RefPtr<dom::Promise>& aPromiseSlot, JSContext* aCx,
 102     dom::Promise** aResultPromise, const nsCString& aTelemetryKey,
 103     PrintBackgroundTask<T, Result, Args...> aTask, Args... aArgs) {
 104   if (RefPtr<dom::Promise> existing = aPromiseSlot) {
 105     existing.forget(aResultPromise);
 106     return NS_OK;
 107   }
 108
 109   nsresult rv =
 110       PrintBackgroundTaskPromise(aReceiver, aCx, aResultPromise, aTelemetryKey,
 111                                  aTask, std::forward<Args>(aArgs)...);
 112   NS_ENSURE_SUCCESS(rv, rv);
 113
 114   aPromiseSlot = *aResultPromise;
 115   return NS_OK;
 116 }
 117
 118 }  // namespace mozilla
 119
 120 #endif