Singleton: remove dependency on Future

Summary: Singletons requires Futures, and Futures indirectly (via HHWheelTimeKeeper) require Singletons.  This breaks the dependency.  It removes a Future-using API which will be replaced with a better alternative.

Reviewed By: @luciang

Differential Revision: D2517807

fb-gh-sync-id: 93df371a74d0f80dc8c55fb0eadd0b688b27b525

folly/Singleton.cpp

@@ -128,7 +128,7 @@ void SingletonVault::doEagerInit() {
   }
 }
 
-Future<Unit> SingletonVault::doEagerInitVia(Executor* exe) {
+void SingletonVault::doEagerInitVia(Executor* exe) {
   std::unordered_set<detail::SingletonHolderBase*> singletonSet;
   {
     RWSpinLock::ReadHolder rh(&stateMutex_);
@@ -139,16 +139,13 @@ Future<Unit> SingletonVault::doEagerInitVia(Executor* exe) {
     singletonSet = eagerInitSingletons_; // copy set of pointers
   }
 
-  std::vector<Future<Unit>> resultFutures;
   for (auto* single : singletonSet) {
-    resultFutures.emplace_back(via(exe).then([single] {
+    exe->add([single] {
       if (!single->creationStarted()) {
         single->createInstance();
       }
-    }));
+    });
   }
-
-  return collectAll(resultFutures).via(exe).then();
 }
 
 void SingletonVault::destroyInstances() {

folly/Singleton.h

@@ -109,7 +109,6 @@
 #include <folly/RWSpinLock.h>
 #include <folly/Demangle.h>
 #include <folly/Executor.h>
-#include <folly/futures/Future.h>
 #include <folly/io/async/Request.h>
 
 #include <algorithm>
@@ -341,10 +340,8 @@ class SingletonVault {
 
   /**
    * Schedule eager singletons' initializations through the given executor.
-   * Return a future which is fulfilled after all the initialization functions
-   * complete.
    */
-  Future<Unit> doEagerInitVia(Executor* exe);
+  void doEagerInitVia(Executor* exe);
 
   // Destroy all singletons; when complete, the vault can't create
   // singletons once again until reenableInstances() is called.

folly/test/SingletonTest.cpp

@@ -459,9 +459,8 @@ TEST(Singleton, SingletonEagerInitAsync) {
   folly::EventBase eb;
   vault.registrationComplete();
   EXPECT_FALSE(didEagerInit);
-  auto result = vault.doEagerInitVia(&eb); // a Future<Unit> is returned
+  vault.doEagerInitVia(&eb);
   eb.loop();
-  result.get(); // ensure this completed successfully and didn't hang forever
   EXPECT_TRUE(didEagerInit);
   sing.get_weak();  // (avoid compile error complaining about unused var 'sing')
 }
@@ -538,7 +537,7 @@ TEST(Singleton, SingletonEagerInitParallel) {
       for (size_t j = 0; j < kThreads; j++) {
         threads.push_back(std::make_shared<std::thread>([&] {
           barrier.wait();
-          vault.doEagerInitVia(&exe).get();
+          vault.doEagerInitVia(&exe);
         }));
       }