Bug 1626278: Implement MozPromise::AllSettled, based on JS Promise API. r=jya

Differential Revision: https://phabricator.services.mozilla.com/D91467

xpcom/threads/MozPromise.h

@@ -291,6 +291,9 @@ class MozPromise : public MozPromiseBase {
                      IsExclusive>
       AllPromiseType;
 
+  typedef MozPromise<CopyableTArray<ResolveOrRejectValue>, bool, IsExclusive>
+      AllSettledPromiseType;
+
  private:
   class AllPromiseHolder : public MozPromiseRefcountable {
    public:
@@ -340,6 +343,50 @@ class MozPromise : public MozPromiseBase {
     size_t mOutstandingPromises;
   };
 
+  // Trying to pass ResolveOrRejectValue by value fails static analysis checks,
+  // so we need to use either a const& or an rvalue reference, depending on
+  // whether IsExclusive is true or not.
+  typedef std::conditional_t<IsExclusive, ResolveOrRejectValue&&,
+                             const ResolveOrRejectValue&>
+      ResolveOrRejectValueParam;
+
+  class AllSettledPromiseHolder : public MozPromiseRefcountable {
+   public:
+    explicit AllSettledPromiseHolder(size_t aDependentPromises)
+        : mPromise(new typename AllSettledPromiseType::Private(__func__)),
+          mOutstandingPromises(aDependentPromises) {
+      MOZ_ASSERT(aDependentPromises > 0);
+      mValues.SetLength(aDependentPromises);
+    }
+
+    void Settle(size_t aIndex, ResolveOrRejectValueParam aValue) {
+      if (!mPromise) {
+        // Already rejected.
+        return;
+      }
+
+      mValues[aIndex].emplace(MaybeMove(aValue));
+      if (--mOutstandingPromises == 0) {
+        nsTArray<ResolveOrRejectValue> values;
+        values.SetCapacity(mValues.Length());
+        for (auto&& value : mValues) {
+          values.AppendElement(std::move(value.ref()));
+        }
+
+        mPromise->Resolve(std::move(values), __func__);
+        mPromise = nullptr;
+        mValues.Clear();
+      }
+    }
+
+    AllSettledPromiseType* Promise() { return mPromise; }
+
+   private:
+    nsTArray<Maybe<ResolveOrRejectValue>> mValues;
+    RefPtr<typename AllSettledPromiseType::Private> mPromise;
+    size_t mOutstandingPromises;
+  };
+
  public:
   [[nodiscard]] static RefPtr<AllPromiseType> All(
       nsISerialEventTarget* aProcessingTarget,
@@ -364,6 +411,26 @@ class MozPromise : public MozPromiseBase {
     return promise;
   }
 
+  [[nodiscard]] static RefPtr<AllSettledPromiseType> AllSettled(
+      nsISerialEventTarget* aProcessingTarget,
+      nsTArray<RefPtr<MozPromise>>& aPromises) {
+    if (aPromises.Length() == 0) {
+      return AllSettledPromiseType::CreateAndResolve(
+          CopyableTArray<ResolveOrRejectValue>(), __func__);
+    }
+
+    RefPtr<AllSettledPromiseHolder> holder =
+        new AllSettledPromiseHolder(aPromises.Length());
+    RefPtr<AllSettledPromiseType> promise = holder->Promise();
+    for (size_t i = 0; i < aPromises.Length(); ++i) {
+      aPromises[i]->Then(aProcessingTarget, __func__,
+                         [holder, i](ResolveOrRejectValueParam aValue) -> void {
+                           holder->Settle(i, MaybeMove(aValue));
+                         });
+    }
+    return promise;
+  }
+
   class Request : public MozPromiseRefcountable {
    public:
     virtual void Disconnect() = 0;