Drain ManualExecutor on destruction

Summary:
For use in tests where the executor has a nontrivial lifetime but any
queued jobs should be completed, have ManualExecutor drain itself
before it's destroyed.

Reviewed By: yfeldblum

Differential Revision: D8845457

fbshipit-source-id: 7c2aa65aa27a7850ff73a93cfbe34c2248b62d26

folly/executors/ManualExecutor.cpp

@@ -22,6 +22,10 @@
 
 namespace folly {
 
+ManualExecutor::~ManualExecutor() {
+  drain();
+}
+
 void ManualExecutor::add(Func callback) {
   std::lock_guard<std::mutex> lock(lock_);
   funcs_.emplace(std::move(callback));

folly/executors/ManualExecutor.h

@@ -39,6 +39,8 @@ class ManualExecutor : public DrivableExecutor,
                        public ScheduledExecutor,
                        public SequencedExecutor {
  public:
+  ~ManualExecutor();
+
   void add(Func) override;
 
   /// Do work. Returns the number of functions that were executed (maybe 0).

folly/executors/test/ExecutorTest.cpp

@@ -37,6 +37,10 @@ TEST(ManualExecutor, runIsStable) {
   x.add(f2);
   x.run();
   EXPECT_EQ(count, 0);
+
+  // ManualExecutor's destructor drains, so explicitly clear the two added by
+  // f2.
+  EXPECT_EQ(2, x.clear());
 }
 
 TEST(ManualExecutor, drainIsNotStable) {
@@ -197,6 +201,15 @@ TEST(ManualExecutor, clear) {
   EXPECT_EQ(0, count);
 }
 
+TEST(ManualExecutor, drainsOnDestruction) {
+  size_t count = 0;
+  {
+    ManualExecutor x;
+    x.add([&] { ++count; });
+  }
+  EXPECT_EQ(1, count);
+}
+
 TEST(Executor, InlineExecutor) {
   InlineExecutor x;
   size_t counter = 0;

folly/futures/test/ViaTest.cpp

@@ -540,6 +540,7 @@ TEST(Via, viaExecutorDiscardsTaskFutureSetValueFirst) {
     ManualExecutor x;
     future = makeFuture().via(&x).then(
         [c = std::move(captured_promise)] { return 42; });
+    x.clear();
   }
 
   EXPECT_THROW(std::move(*future).get(std::chrono::seconds(5)), BrokenPromise);
@@ -564,6 +565,7 @@ TEST(Via, viaExecutorDiscardsTaskFutureSetCallbackFirst) {
     future = trigger.getFuture().via(&x).then(
         [c = std::move(captured_promise)] { return 42; });
     trigger.setValue();
+    x.clear();
   }
 
   EXPECT_THROW(std::move(*future).get(std::chrono::seconds(5)), BrokenPromise);