Make folly::collectN return a SemiFuture

Summary: Start with the easy one.

Reviewed By: yfeldblum

Differential Revision: D7957655

fbshipit-source-id: 0c2227402673306e1e6ba3f463b1a93db2582b07

folly/futures/Future-inl.h

@@ -1362,9 +1362,8 @@ collectAnyWithoutException(InputIterator first, InputIterator last) {
 
 // collectN (iterator)
 
-// TODO(T26439406): Make return SemiFuture
 template <class InputIterator>
-Future<std::vector<std::pair<
+SemiFuture<std::vector<std::pair<
     size_t,
     Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>>
 collectN(InputIterator first, InputIterator last, size_t n) {
@@ -1427,7 +1426,7 @@ collectN(InputIterator first, InputIterator last, size_t n) {
     });
   }
 
-  return ctx->p.getSemiFuture().via(&folly::InlineExecutor::instance());
+  return ctx->p.getSemiFuture();
 }
 
 // reduce (iterator)

folly/futures/helpers.h

@@ -396,7 +396,7 @@ auto collectAnyWithoutException(Collection&& c)
   Not thread safe.
   */
 template <class InputIterator>
-Future<std::vector<std::pair<
+SemiFuture<std::vector<std::pair<
     size_t,
     Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>>
 collectN(InputIterator first, InputIterator last, size_t n);

folly/futures/test/CollectTest.cpp

@@ -577,6 +577,7 @@ TEST(Collect, collectN) {
   bool flag = false;
   size_t n = 3;
   collectN(futures, n)
+      .via(&folly::InlineExecutor::instance())
       .then([&](std::vector<std::pair<size_t, Try<Unit>>> v) {
         flag = true;
         EXPECT_EQ(n, v.size());
@@ -603,7 +604,9 @@ TEST(Collect, collectNParallel) {
 
   bool flag = false;
   size_t n = 90;
-  collectN(futures, n).then([&](std::vector<std::pair<size_t, Try<Unit>>> v) {
+  collectN(futures, n)
+      .via(&folly::InlineExecutor::instance())
+      .then([&](std::vector<std::pair<size_t, Try<Unit>>> v) {
         flag = true;
         EXPECT_EQ(n, v.size());
         for (auto& tt : v) {