add type info to broken promise what()

Summary: when a promise is broken, add type info to the message. This will allow for some small improvements to debugging errors from broken promises, namely giving developers a slightly more refined target to grep for.

Reviewed By: fugalh

Differential Revision: D2725575

fb-gh-sync-id: b92edbde874e63eaeab97395da7bb52f76968800

folly/futures/FutureException.h

@@ -48,8 +48,9 @@ public:
 
 class BrokenPromise : public FutureException {
   public:
-    explicit BrokenPromise() :
-      FutureException("Broken promise") { }
+    explicit BrokenPromise(std::string type) :
+      FutureException(
+          (std::string("Broken promise for type name `") + type) + '`') { }
 };
 
 class NoState : public FutureException {

folly/futures/detail/Core.h

@@ -216,7 +216,7 @@ class Core {
     // detachPromise() and setResult() should never be called in parallel
     // so we don't need to protect this.
     if (UNLIKELY(!result_)) {
-      setResult(Try<T>(exception_wrapper(BrokenPromise())));
+      setResult(Try<T>(exception_wrapper(BrokenPromise(typeid(T).name()))));
     }
     detachOne();
   }

folly/futures/test/PromiseTest.cpp

@@ -126,3 +126,34 @@ TEST(Promise, isFulfilledWithFuture) {
   p.setValue(42); // after here
   EXPECT_TRUE(p.isFulfilled());
 }
+
+TEST(Promise, brokenOnDelete) {
+  auto p = folly::make_unique<Promise<int>>();
+  auto f = p->getFuture();
+
+  EXPECT_FALSE(f.isReady());
+
+  p.reset();
+
+  EXPECT_TRUE(f.isReady());
+
+  auto t = f.getTry();
+
+  EXPECT_TRUE(t.hasException<BrokenPromise>());
+}
+
+TEST(Promise, brokenPromiseHasTypeInfo) {
+  auto pInt = folly::make_unique<Promise<int>>();
+  auto fInt = pInt->getFuture();
+
+  auto pFloat = folly::make_unique<Promise<float>>();
+  auto fFloat = pFloat->getFuture();
+
+  pInt.reset();
+  pFloat.reset();
+
+  auto whatInt = fInt.getTry().exception().what();
+  auto whatFloat = fFloat.getTry().exception().what();
+
+  EXPECT_NE(whatInt, whatFloat);
+}