Promises: Implement Promise<> handle with Then and Catch

Implements Then and Catch on top of AbstractPromise as well as
ManualPromiseResolver<> which is needed for testing at this
stage (PostTask doesn't yet return a promise). It's also
necessary for promises to interface with old style callback
based libraries which are extremly common in Chromium.

Design: https://docs.google.com/document/d/1l12PAJgEtlrqTXKiw6mk2cR2jP7FAfCCDr-DGIdiC9w/edit

Bug: 906125
Change-Id: Id7206698f37c59cfc201f3970cbf66a2ef89ee4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1599620
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661045}

base/BUILD.gn

@@ -755,9 +755,14 @@ jumbo_component("base") {
     "task/promise/abstract_promise.h",
     "task/promise/dependent_list.cc",
     "task/promise/dependent_list.h",
+    "task/promise/helpers.h",
     "task/promise/no_op_promise_executor.cc",
     "task/promise/no_op_promise_executor.h",
+    "task/promise/promise.h",
+    "task/promise/promise_result.h",
     "task/promise/small_unique_object.h",
+    "task/promise/then_and_catch_executor.cc",
+    "task/promise/then_and_catch_executor.h",
     "task/scoped_set_task_priority_for_current_thread.cc",
     "task/scoped_set_task_priority_for_current_thread.h",
     "task/sequence_manager/associated_thread_id.cc",
@@ -2631,6 +2636,8 @@ test("base_unittests") {
     "task/post_task_unittest.cc",
     "task/promise/abstract_promise_unittest.cc",
     "task/promise/dependent_list_unittest.cc",
+    "task/promise/helpers_unittest.cc",
+    "task/promise/promise_unittest.cc",
     "task/scoped_set_task_priority_for_current_thread_unittest.cc",
     "task/sequence_manager/atomic_flag_set_unittest.cc",
     "task/sequence_manager/lazily_deallocated_deque_unittest.cc",
@@ -3034,6 +3041,7 @@ if (enable_nocompile_tests) {
       "observer_list_unittest.nc",
       "optional_unittest.nc",
       "strings/string16_unittest.nc",
+      "task/promise/promise_unittest.nc",
       "task/task_traits_extension_unittest.nc",
       "task/task_traits_unittest.nc",
       "thread_annotations_unittest.nc",

base/callback_internal.h

@@ -19,14 +19,15 @@ struct FakeBindState;
 
 namespace internal {
 
-class CallbackBase;
-class CallbackBaseCopyable;
-
+class BaseThenAndCatchExecutor;
 class BindStateBase;
 
 template <typename Functor, typename... BoundArgs>
 struct BindState;
 
+class CallbackBase;
+class CallbackBaseCopyable;
+
 struct BindStateBaseRefCountTraits {
   static void Destruct(const BindStateBase*);
 };
@@ -135,6 +136,8 @@ class BASE_EXPORT CallbackBase {
   void Reset();
 
  protected:
+  friend class BaseThenAndCatchExecutor;
+
   using InvokeFuncStorage = BindStateBase::InvokeFuncStorage;
 
   // Returns true if this callback equals |other|. |other| may be null.

base/task/promise/abstract_promise.cc

@@ -47,6 +47,11 @@ const AbstractPromise* AbstractPromise::FindNonCurriedAncestor() const {
 void AbstractPromise::AddAsDependentForAllPrerequisites() {
   DCHECK(prerequisites_);
 
+  // Note a curried promise will eventually get to all its children and pass
+  // them catch responsibility through AddAsDependentForAllPrerequisites,
+  // although that'll be done lazily (only once they resolve/reject, so there
+  // is a possibility the DCHECKs might be racy.
+
   for (AdjacencyListNode& node : prerequisites_->prerequisite_list) {
     node.dependent_node.dependent = this;
 
@@ -196,20 +201,6 @@ void AbstractPromise::MaybeInheritChecks(AbstractPromise* prerequisite) {
   prerequisite->passed_catch_responsibility_ = true;
 }
 
-void AbstractPromise::PassCatchResponsibilityOntoDependentsForCurriedPromise(
-    DependentList::Node* dependent_list) {
-  CheckedAutoLock lock(GetCheckedLock());
-  if (!dependent_list)
-    return;
-
-  if (IsResolvedWithPromise()) {
-    for (DependentList::Node* node = dependent_list; node;
-         node = node->next.load(std::memory_order_relaxed)) {
-      node->dependent->MaybeInheritChecks(this);
-    }
-  }
-}
-
 AbstractPromise::LocationRef::LocationRef(const Location& from_here)
     : from_here_(from_here) {}
 
@@ -353,10 +344,6 @@ void AbstractPromise::OnResolvePostReadyDependents() {
   DependentList::Node* dependent_list = dependents_.ConsumeOnceForResolve();
   dependent_list = NonThreadSafeReverseList(dependent_list);
 
-#if DCHECK_IS_ON()
-  PassCatchResponsibilityOntoDependentsForCurriedPromise(dependent_list);
-#endif
-
   // Propagate resolve to dependents.
   DependentList::Node* next;
   for (DependentList::Node* node = dependent_list; node; node = next) {
@@ -374,10 +361,6 @@ void AbstractPromise::OnRejectPostReadyDependents() {
   DependentList::Node* dependent_list = dependents_.ConsumeOnceForReject();
   dependent_list = NonThreadSafeReverseList(dependent_list);
 
-#if DCHECK_IS_ON()
-  PassCatchResponsibilityOntoDependentsForCurriedPromise(dependent_list);
-#endif
-
   // Propagate rejection to dependents. We always propagate rejection
   // immediately.
   DependentList::Node* next;
@@ -429,7 +412,8 @@ void AbstractPromise::OnCanceled() {
 
 void AbstractPromise::OnResolved() {
 #if DCHECK_IS_ON()
-  DCHECK(executor_can_resolve_) << from_here_.ToString();
+  DCHECK(executor_can_resolve_ || IsResolvedWithPromise())
+      << from_here_.ToString();
 #endif
   if (IsResolvedWithPromise()) {
     scoped_refptr<AbstractPromise> curried_promise =

base/task/promise/abstract_promise.h

@@ -46,7 +46,7 @@ enum class RejectPolicy {
 // resolved. This lets us disambiguate promises with the same resolve and reject
 // type.
 template <typename T>
-struct BASE_EXPORT Resolved {
+struct Resolved {
   using Type = T;
 
   static_assert(!std::is_same<T, NoReject>::value,
@@ -64,7 +64,7 @@ struct BASE_EXPORT Resolved {
 };
 
 template <>
-struct BASE_EXPORT Resolved<void> {
+struct Resolved<void> {
   using Type = void;
   Void value;
 };
@@ -73,7 +73,7 @@ struct BASE_EXPORT Resolved<void> {
 // rejected. This lets us disambiguate promises with the same resolve and reject
 // type.
 template <typename T>
-struct BASE_EXPORT Rejected {
+struct Rejected {
   using Type = T;
   T value;
 
@@ -93,7 +93,7 @@ struct BASE_EXPORT Rejected {
 };
 
 template <>
-struct BASE_EXPORT Rejected<void> {
+struct Rejected<void> {
   using Type = void;
   Void value;
 };
@@ -399,10 +399,6 @@ class BASE_EXPORT AbstractPromise
   void MaybeInheritChecks(AbstractPromise* source)
       EXCLUSIVE_LOCKS_REQUIRED(GetCheckedLock());
 
-  // Does nothing if this promise wasn't resolved by a promise.
-  void PassCatchResponsibilityOntoDependentsForCurriedPromise(
-      DependentList::Node* dependent_list);
-
   // Controls how we deal with unhandled rejection.
   const RejectPolicy reject_policy_;
 

base/task/promise/promise_result.h

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_TASK_PROMISE_PROMISE_RESULT_H_
+#define BASE_TASK_PROMISE_PROMISE_RESULT_H_
+
+#include <type_traits>
+
+#include "base/containers/unique_any.h"
+
+namespace base {
+
+// An optional return type from a promise callback, which allows the callback to
+// decide whether or not it should reject.  E.g.
+//
+// enum class Error { kReason };
+//
+// PromiseResult<int, Error> MyFn() {
+//   int result;
+//   ...
+//   if (error)
+//     return Error::kReason;
+//   return result;
+// }
+//
+// If ResolveType and RejectType are distinct PromiseResult's constructor will
+// accept them.  E.g.
+//
+// PromiseResult<int, std::string> pr(123);       // Resolve
+// PromiseResult<int, std::string> pr("whoops");  // Reject
+//
+// If ResolveType and RejectType are the same you need to use Resolved<> and
+// Rejected<> to disambiguate.  E.g.
+//
+// PromiseResult<int, int> pr(Resolved{123});     // Resolve
+// PromiseResult<int, int> pr(Rejected{123});     // Reject
+//
+// PromiseResult<ResolveType, RejectType> has a constructor that accepts
+// Promise<ResolveType, RejectType>.
+template <typename ResolveType, typename RejectType = NoReject>
+class PromiseResult {
+ public:
+  template <typename ResolveT = ResolveType,
+            typename RejectT = RejectType,
+            class Enable = std::enable_if<std::is_void<ResolveT>::value ^
+                                          std::is_void<RejectT>::value>>
+  PromiseResult() : PromiseResult(typename Analyze<void>::TagType()) {}
+
+  template <typename T>
+  PromiseResult(T&& t)
+      : PromiseResult(typename Analyze<std::decay_t<T>>::TagType(),
+                      std::forward<T>(t)) {}
+
+  PromiseResult(PromiseResult&& other) noexcept = default;
+
+  PromiseResult(const PromiseResult&) = delete;
+  PromiseResult& operator=(const PromiseResult&) = delete;
+
+  unique_any& value() { return value_; }
+
+ private:
+  struct IsWrapped {};
+  struct IsResolved {};
+  struct IsRejected {};
+  struct IsPromise {};
+
+  // Helper that assigns one of the above tags for |T|.
+  template <typename T>
+  struct Analyze {
+    using DecayedT = std::decay_t<T>;
+
+    static constexpr bool is_resolve =
+        std::is_convertible<DecayedT, std::decay_t<ResolveType>>::value;
+
+    static constexpr bool is_reject =
+        std::is_convertible<DecayedT, std::decay_t<RejectType>>::value;
+
+    static_assert(!is_reject || !std::is_same<DecayedT, NoReject>::value,
+                  "A NoReject promise can't reject");
+
+    static_assert(!std::is_same<std::decay_t<ResolveType>,
+                                std::decay_t<RejectType>>::value,
+                  "Ambiguous because ResolveType and RejectType are the same");
+
+    static_assert(is_resolve || is_reject,
+                  "Argument matches neither resolve nor reject type.");
+
+    static_assert(
+        is_resolve != is_reject && (is_resolve || is_reject),
+        "Ambiguous because argument matches both ResolveType and RejectType");
+
+    using TagType = std::conditional_t<is_resolve, IsResolved, IsRejected>;
+  };
+
+  template <typename T>
+  struct Analyze<Resolved<T>> {
+    using TagType = IsWrapped;
+    static_assert(std::is_same<ResolveType, T>::value,
+                  "T in Resolved<T> is not ResolveType");
+  };
+
+  template <typename T>
+  struct Analyze<Rejected<T>> {
+    static_assert(std::is_same<RejectType, T>::value,
+                  "T in Rejected<T> is not RejectType");
+
+    static_assert(!std::is_same<RejectType, NoReject>::value,
+                  "A NoReject promise can't reject");
+    using TagType = IsWrapped;
+  };
+
+  template <typename ResolveT, typename RejectT>
+  struct Analyze<Promise<ResolveT, RejectT>> {
+    using TagType = IsPromise;
+
+    static_assert(std::is_same<ResolveT, ResolveType>::value,
+                  "Promise resolve types don't match");
+
+    static_assert(std::is_same<RejectT, RejectType>::value,
+                  "Promise reject types don't match");
+  };
+
+  template <typename... Args>
+  PromiseResult(IsResolved, Args&&... args)
+      : value_(in_place_type_t<Resolved<ResolveType>>(),
+               std::forward<Args>(args)...) {}
+
+  template <typename... Args>
+  PromiseResult(IsRejected, Args&&... args)
+      : value_(in_place_type_t<Rejected<RejectType>>(),
+               std::forward<Args>(args)...) {}
+
+  PromiseResult(IsPromise, const Promise<ResolveType, RejectType>& promise)
+      : value_(promise.abstract_promise_) {}
+
+  template <typename T>
+  PromiseResult(IsWrapped, T&& t) : value_(std::forward<T>(t)) {}
+
+  unique_any value_;
+};
+
+}  // namespace base
+
+#endif  // BASE_TASK_PROMISE_PROMISE_RESULT_H_

base/task/promise/promise_unittest.nc

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+//
+// This is a "No Compile Test" suite.
+// http://dev.chromium.org/developers/testing/no-compile-tests
+
+#include "base/task/promise/promise.h"
+#include "base/task/promise/promise_result.h"
+
+namespace base {
+
+#if defined(NCTEST_METHOD_CANT_CATCH_NOREJECT_PROMISE) // [r"fatal error: static_assert failed .*\"Can't catch a NoReject promise\."]
+void WontCompile() {
+  Promise<int> p;
+  p.CatchOnCurrent(FROM_HERE, BindOnce([]() {}));
+}
+#elif defined(NCTEST_METHOD_CANT_CATCH_NOREJECT_PROMISE_TYPE_TWO) // [r"fatal error: static_assert failed .*\"Can't catch a NoReject promise\."]
+void WontCompile() {
+  Promise<int> p;
+  p.ThenOnCurrent(FROM_HERE, BindOnce([](int) {}), BindOnce([]() {}));
+}
+#elif defined(NCTEST_METHOD_RESOLVE_CALLBACK_TYPE_MISSMATCH) // [r"fatal error: static_assert failed .*\"|on_resolve| callback must accept Promise::ResolveType or void\."]
+void WontCompile() {
+  Promise<int, void> p;
+  p.ThenOnCurrent(FROM_HERE, BindOnce([](bool) { }));
+}
+#elif defined(NCTEST_METHOD_REJECT_CALLBACK_TYPE_MISSMATCH) // [r"fatal error: static_assert failed .*\"|on_reject| callback must accept Promise::ResolveType or void\."]
+void WontCompile() {
+  Promise<int, void> p;
+  p.CatchOnCurrent(FROM_HERE, BindOnce([](bool) { }));
+}
+#elif defined(NCTEST_METHOD_REJECT_CALLBACK_TYPE_MISSMATCH_TYPE_TWO) // [r"fatal error: static_assert failed .*\"|on_reject| callback must accept Promise::ResolveType or void\."]
+void WontCompile() {
+  Promise<int, void> p;
+  p.ThenOnCurrent(FROM_HERE, BindOnce([](int) { }), BindOnce([](bool) { }));
+}
+#elif defined(NCTEST_METHOD_INCOMPATIBLE_RETURN_TYPES) // [r"fatal error: static_assert failed .*\"|on_resolve| callback and |on_resolve| callback must return compatible types\."]
+void WontCompile() {
+  Promise<void> p;
+  p.ThenOnCurrent(
+      FROM_HERE,
+      BindOnce([]() -> PromiseResult<int, std::string> { return 123; }),
+      BindOnce([](int err) -> Rejected<bool> { return "123"; }));
+}
+#elif defined(NCTEST_METHOD_INCOMPATIBLE_RETURN_TYPES2) // [r"fatal error: static_assert failed .*\"|on_resolve| callback and |on_resolve| callback must return compatible types\."]
+void WontCompile() {
+  Promise<void> p;
+  p.ThenOnCurrent(
+      FROM_HERE,
+      BindOnce([]() -> PromiseResult<int, std::string> { return 123; }),
+      BindOnce([](int err) -> Resolved<std::string> { return "123"; }));
+}
+#elif defined(NCTEST_METHOD_INCOMPATIBLE_RETURN_TYPES3) // [r"fatal error: static_assert failed .*\"|on_resolve| callback and |on_resolve| callback must return compatible types\."]
+void WontCompile() {
+  Promise<int, void> p;
+  p.ThenOnCurrent(FROM_HERE, BindOnce([](int) { return true; }),
+                             BindOnce([](int) { return 123.0; }));
+}
+#elif defined(NCTEST_METHOD_AMBIGUOUS_CONSTRUCTOR) // [r"fatal error: static_assert failed .*\"Ambiguous because ResolveType and RejectType are the same"]
+void WontCompile() {
+  PromiseResult<void, void> pr;
+}
+#elif defined(NCTEST_METHOD_AMBIGUOUS_CONSTRUCTOR2) // [r"fatal error: static_assert failed .*\"Ambiguous because ResolveType and RejectType are the same"]
+void WontCompile() {
+  PromiseResult<int, int> pr(123);
+}
+#elif defined(NCTEST_METHOD_REJECTED_NOREJECT) // [r"fatal error: static_assert failed .*\"Can't have Rejected<NoReject>"]
+void WontCompile() {
+  Rejected<NoReject>();
+}
+#elif defined(NCTEST_METHOD_ARGUMENT_DOESNT_MATCH) // [r"fatal error: static_assert failed .*\"Argument matches neither resolve nor reject type\."]
+void WontCompile() {
+  PromiseResult<int, float> pr("invalid");
+}
+#elif defined(NCTEST_METHOD_ARGUMENT_DOESNT_MATCH2) // [r"fatal error: static_assert failed .*\"Promise resolve types don't match"]
+void WontCompile() {
+  Promise<void> p;
+  PromiseResult<int, float> pr(p);
+}
+#elif defined(NCTEST_METHOD_UNRELATED_RESOLVE) // [r"fatal error: static_assert failed .*\"T in Resolved<T> is not ResolveType"]
+void WontCompile() {
+  struct Unrelated{};
+  PromiseResult<int, void> pr(Resolved<Unrelated>{});
+}
+#elif defined(NCTEST_METHOD_UNRELATED_REJECT) // [r"fatal error: static_assert failed .*\"T in Rejected<T> is not RejectType"]
+void WontCompile() {
+  struct Unrelated{};
+  PromiseResult<int, void> pr(Rejected<Unrelated>{});
+}
+#elif defined(NCTEST_METHOD_AMBIGUOUS_REJECT_TYPE) // [r"fatal error: static_assert failed .*\"Ambiguous promise reject type"]
+void WontCompile() {
+  Promise<int, void> p1;
+  // If supported |p2| would have type Promise<NoReject, variant<void, bool>>.
+  auto p2 = p1.ThenOnCurrent(FROM_HERE, BindOnce([]() { return Rejected<bool>(true); }));
+}
+#elif defined(NCTEST_METHOD_AMBIGUOUS_RESOLVE_TYPE) // [r"fatal error: static_assert failed .*\"Ambiguous promise resolve type"]
+void WontCompile() {
+  Promise<int, void> p1;
+  // If supported |p2| would have type Promise<variant<int, bool>, NoReject>.
+  auto p2 = p1.CatchOnCurrent(FROM_HERE, BindOnce([](int) { return Resolved<bool>(true); }));
+}
+#endif
+
+}  // namespace base

base/task/promise/then_and_catch_executor.cc

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/task/promise/then_and_catch_executor.h"
+
+namespace base {
+namespace internal {
+
+BaseThenAndCatchExecutor::BaseThenAndCatchExecutor(
+    internal::CallbackBase&& resolve_executor,
+    internal::CallbackBase&& reject_executor)
+    : resolve_callback_(std::move(resolve_executor)),
+      reject_callback_(std::move(reject_executor)) {}
+
+BaseThenAndCatchExecutor::~BaseThenAndCatchExecutor() = default;
+
+bool BaseThenAndCatchExecutor::IsCancelled() const {
+  if (!resolve_callback_.is_null()) {
+    // If there is both a resolve and a reject executor they must be canceled
+    // at the same time.
+    DCHECK(reject_callback_.is_null() ||
+           reject_callback_.IsCancelled() == resolve_callback_.IsCancelled());
+    return resolve_callback_.IsCancelled();
+  }
+  return reject_callback_.IsCancelled();
+}
+
+AbstractPromise::Executor::PrerequisitePolicy
+BaseThenAndCatchExecutor::GetPrerequisitePolicy() {
+  return PrerequisitePolicy::kAll;
+}
+
+void BaseThenAndCatchExecutor::Execute(AbstractPromise* promise) {
+  AbstractPromise* prerequisite =
+      promise->GetOnlyPrerequisite()->FindNonCurriedAncestor();
+  if (prerequisite->IsResolved()) {
+    if (ProcessNullCallback(resolve_callback_, prerequisite, promise)) {
+      promise->OnResolved();
+      return;
+    }
+
+    ExecuteThen(prerequisite, promise);
+  } else {
+    DCHECK(prerequisite->IsRejected());
+    if (ProcessNullCallback(reject_callback_, prerequisite, promise)) {
+      promise->OnResolved();
+      return;
+    }
+
+    ExecuteCatch(prerequisite, promise);
+  }
+}
+
+// static
+bool BaseThenAndCatchExecutor::ProcessNullCallback(const CallbackBase& callback,
+                                                   AbstractPromise* arg,
+                                                   AbstractPromise* result) {
+  if (callback.is_null()) {
+    // A curried promise is used to forward the result through null callbacks.
+    result->emplace(scoped_refptr<AbstractPromise>(arg));
+    DCHECK(result->IsResolvedWithPromise());
+    return true;
+  }
+
+  return false;
+}
+
+}  // namespace internal
+}  // namespace base

base/task/promise/then_and_catch_executor.h

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_TASK_PROMISE_THEN_AND_CATCH_EXECUTOR_H_
+#define BASE_TASK_PROMISE_THEN_AND_CATCH_EXECUTOR_H_
+
+#include <type_traits>
+
+#include "base/callback.h"
+#include "base/task/promise/abstract_promise.h"
+#include "base/task/promise/helpers.h"
+
+namespace base {
+namespace internal {
+
+// Exists to reduce template bloat.
+class BASE_EXPORT BaseThenAndCatchExecutor : public AbstractPromise::Executor {
+ public:
+  BaseThenAndCatchExecutor(CallbackBase&& resolve_callback,
+                           CallbackBase&& reject_callback);
+
+  ~BaseThenAndCatchExecutor() override;
+
+  // AbstractPromise::Executor:
+  bool IsCancelled() const override;
+  PrerequisitePolicy GetPrerequisitePolicy() override;
+  void Execute(AbstractPromise* promise) override;
+
+ protected:
+  virtual void ExecuteThen(AbstractPromise* prerequisite,
+                           AbstractPromise* promise) = 0;
+
+  virtual void ExecuteCatch(AbstractPromise* prerequisite,
+                            AbstractPromise* promise) = 0;
+
+  // If |executor| is null then the value of |arg| is moved or copied into
+  // |result| and true is returned. Otherwise false is returned.
+  static bool ProcessNullCallback(const CallbackBase& executor,
+                                  AbstractPromise* arg,
+                                  AbstractPromise* result);
+
+  CallbackBase resolve_callback_;
+  CallbackBase reject_callback_;
+};
+
+// Tag signals no callback which is used to eliminate dead code.
+struct NoCallback {};
+
+struct CouldResolveOrReject {};
+struct CanOnlyResolve {};
+struct CanOnlyReject {};
+
+template <bool can_resolve, bool can_reject>
+struct CheckResultHelper;
+
+template <>
+struct CheckResultHelper<true, false> {
+  using TagType = CanOnlyResolve;
+};
+
+template <>
+struct CheckResultHelper<true, true> {
+  using TagType = CouldResolveOrReject;
+};
+
+template <>
+struct CheckResultHelper<false, true> {
+  using TagType = CanOnlyReject;
+};
+
+template <typename ResolveOnceCallback,
+          typename RejectOnceCallback,
+          typename ArgResolve,
+          typename ArgReject,
+          typename ResolveStorage,
+          typename RejectStorage>
+class ThenAndCatchExecutor : public BaseThenAndCatchExecutor {
+ public:
+  using ResolveReturnT =
+      typename CallbackTraits<ResolveOnceCallback>::ReturnType;
+  using RejectReturnT = typename CallbackTraits<RejectOnceCallback>::ReturnType;
+  using PrerequisiteCouldResolve =
+      std::integral_constant<bool,
+                             !std::is_same<ArgResolve, NoCallback>::value>;
+  using PrerequisiteCouldReject =
+      std::integral_constant<bool, !std::is_same<ArgReject, NoCallback>::value>;
+
+  ThenAndCatchExecutor(ResolveOnceCallback&& resolve_callback,
+                       RejectOnceCallback&& reject_callback)
+      : BaseThenAndCatchExecutor(std::move(resolve_callback),
+                                 std::move(reject_callback)) {
+    static_assert(sizeof(CallbackBase) == sizeof(ResolveOnceCallback),
+                  "We assume it's possible to cast from CallbackBase to "
+                  "ResolveOnceCallback");
+    static_assert(sizeof(CallbackBase) == sizeof(RejectOnceCallback),
+                  "We assume it's possible to cast from CallbackBase to "
+                  "RejectOnceCallback");
+  }
+
+  ~ThenAndCatchExecutor() override = default;
+
+  void ExecuteThen(AbstractPromise* prerequisite,
+                   AbstractPromise* promise) override {
+    ExecuteThenInternal(prerequisite, promise, PrerequisiteCouldResolve());
+  }
+
+  void ExecuteCatch(AbstractPromise* prerequisite,
+                    AbstractPromise* promise) override {
+    ExecuteCatchInternal(prerequisite, promise, PrerequisiteCouldReject());
+  }
+
+#if DCHECK_IS_ON()
+  ArgumentPassingType ResolveArgumentPassingType() const override {
+    return resolve_callback_.is_null()
+               ? AbstractPromise::Executor::ArgumentPassingType::kNoCallback
+               : CallbackTraits<ResolveOnceCallback>::argument_passing_type;
+  }
+
+  ArgumentPassingType RejectArgumentPassingType() const override {
+    return reject_callback_.is_null()
+               ? AbstractPromise::Executor::ArgumentPassingType::kNoCallback
+               : CallbackTraits<RejectOnceCallback>::argument_passing_type;
+  }
+
+  bool CanResolve() const override {
+    return (!resolve_callback_.is_null() &&
+            PromiseCallbackTraits<ResolveReturnT>::could_resolve) ||
+           (!reject_callback_.is_null() &&
+            PromiseCallbackTraits<RejectReturnT>::could_resolve);
+  }
+
+  bool CanReject() const override {
+    return (!resolve_callback_.is_null() &&
+            PromiseCallbackTraits<ResolveReturnT>::could_reject) ||
+           (!reject_callback_.is_null() &&
+            PromiseCallbackTraits<RejectReturnT>::could_reject);
+  }
+#endif
+
+ private:
+  void ExecuteThenInternal(AbstractPromise* prerequisite,
+                           AbstractPromise* promise,
+                           std::true_type can_resolve) {
+    ResolveOnceCallback* resolve_callback =
+        static_cast<ResolveOnceCallback*>(&resolve_callback_);
+    RunHelper<ResolveOnceCallback, Resolved<ArgResolve>, ResolveStorage,
+              RejectStorage>::Run(std::move(*resolve_callback), prerequisite,
+                                  promise);
+
+    using CheckResultTagType = typename CheckResultHelper<
+        PromiseCallbackTraits<ResolveReturnT>::could_resolve,
+        PromiseCallbackTraits<ResolveReturnT>::could_reject>::TagType;
+
+    CheckResultType(promise, CheckResultTagType());
+  }
+
+  void ExecuteThenInternal(AbstractPromise* prerequisite,
+                           AbstractPromise* promise,
+                           std::false_type can_resolve) {
+    // |prerequisite| can't resolve so don't generate dead code.
+  }
+
+  void ExecuteCatchInternal(AbstractPromise* prerequisite,
+                            AbstractPromise* promise,
+                            std::true_type can_reject) {
+    RejectOnceCallback* reject_callback =
+        static_cast<RejectOnceCallback*>(&reject_callback_);
+    RunHelper<RejectOnceCallback, Rejected<ArgReject>, ResolveStorage,
+              RejectStorage>::Run(std::move(*reject_callback), prerequisite,
+                                  promise);
+
+    using CheckResultTagType = typename CheckResultHelper<
+        PromiseCallbackTraits<RejectReturnT>::could_resolve,
+        PromiseCallbackTraits<RejectReturnT>::could_reject>::TagType;
+
+    CheckResultType(promise, CheckResultTagType());
+  }
+
+  void ExecuteCatchInternal(AbstractPromise* prerequisite,
+                            AbstractPromise* promise,
+                            std::false_type can_reject) {
+    // |prerequisite| can't reject so don't generate dead code.
+  }
+
+  void CheckResultType(AbstractPromise* promise, CouldResolveOrReject) {
+    if (promise->IsResolvedWithPromise() ||
+        promise->value().type() == TypeId::From<ResolveStorage>()) {
+      promise->OnResolved();
+    } else {
+      DCHECK_EQ(promise->value().type(), TypeId::From<RejectStorage>())
+          << " See " << promise->from_here().ToString();
+      promise->OnRejected();
+    }
+  }
+
+  void CheckResultType(AbstractPromise* promise, CanOnlyResolve) {
+    promise->OnResolved();
+  }
+
+  void CheckResultType(AbstractPromise* promise, CanOnlyReject) {
+    promise->OnRejected();
+  }
+};
+
+}  // namespace internal
+}  // namespace base
+
+#endif  // BASE_TASK_PROMISE_THEN_AND_CATCH_EXECUTOR_H_

base/task_runner.cc

@@ -40,8 +40,40 @@ bool PostTaskAndReplyTaskRunner::PostTask(const Location& from_here,
   return destination_->PostTask(from_here, std::move(task));
 }
 
+// TODO(alexclarke): Remove this when TaskRunner::PostPromiseInternal becomes
+// pure virtual.
+class PromiseHolder {
+ public:
+  explicit PromiseHolder(scoped_refptr<internal::AbstractPromise> promise)
+      : promise_(std::move(promise)) {}
+
+  ~PromiseHolder() {
+    // Detect if the promise was not executed and if so cancel to ensure memory
+    // is released.
+    if (promise_)
+      promise_->OnCanceled();
+  }
+
+  PromiseHolder(PromiseHolder&& other) : promise_(std::move(other.promise_)) {}
+
+  scoped_refptr<internal::AbstractPromise> Unwrap() const {
+    return std::move(promise_);
+  }
+
+ private:
+  mutable scoped_refptr<internal::AbstractPromise> promise_;
+};
+
 }  // namespace
 
+template <>
+struct BindUnwrapTraits<PromiseHolder> {
+  static scoped_refptr<internal::AbstractPromise> Unwrap(
+      const PromiseHolder& o) {
+    return o.Unwrap();
+  }
+};
+
 bool TaskRunner::PostTask(const Location& from_here, OnceClosure task) {
   return PostDelayedTask(from_here, std::move(task), base::TimeDelta());
 }
@@ -58,7 +90,8 @@ bool TaskRunner::PostPromiseInternal(
     base::TimeDelta delay) {
   return PostDelayedTask(
       promise->from_here(),
-      BindOnce(&internal::AbstractPromise::Execute, std::move(promise)), delay);
+      BindOnce(&internal::AbstractPromise::Execute, PromiseHolder(promise)),
+      delay);
 }
 
 TaskRunner::TaskRunner() = default;