Let window work with functions returning SemiFuture

Summary: [Folly] Let `window` work with functions returning `SemiFuture`.

Reviewed By: LeeHowes

Differential Revision: D8505836

fbshipit-source-id: 51c14b131760528eb6c367298cb23c4deec30ab3

folly/futures/Future-inl.h

@@ -1639,8 +1639,8 @@ window(Executor* executor, Collection input, F func, size_t n) {
     static void spawn(std::shared_ptr<WindowContext> ctx) {
       size_t i = ctx->i++;
       if (i < ctx->input.size()) {
-        auto fut =
-            makeFutureWith([&] { return ctx->func(std::move(ctx->input[i])); });
+        auto fut = makeSemiFutureWith(
+            [&] { return ctx->func(std::move(ctx->input[i])); });
         fut.setCallback_([ctx = std::move(ctx), i](Try<Result>&& t) mutable {
           const auto executor_ = ctx->executor;
           executor_->add([ctx = std::move(ctx), i, t = std::move(t)]() mutable {

folly/futures/test/WindowTest.cpp

@@ -82,6 +82,20 @@ TEST(Window, basic) {
       }).get();
     EXPECT_EQ(6, res);
   }
+  {
+    // string -> return SemiFuture<int>
+    auto res = reduce(
+                   window(
+                       std::vector<std::string>{"1", "2", "3"},
+                       [](std::string s) {
+                         return makeSemiFuture<int>(folly::to<int>(s));
+                       },
+                       2),
+                   0,
+                   [](int sum, const Try<int>& b) { return sum + *b; })
+                   .get();
+    EXPECT_EQ(6, res);
+  }
   {
     SCOPED_TRACE("repeat same fn");
     auto res = reduce(