Make all Promise methods consistently consume the promise (returning a new…

Make all Promise methods consistently consume the promise (returning a new promise when it makes sense), rename daemonize -> detach, and make eagerlyEvaluate() require an error handler (this caught several places where I forgot to use one).

c++/src/capnp/capability-test.c++

@@ -196,8 +196,7 @@ TEST(Capability, AsyncCancelation) {
 
   auto paf = kj::newPromiseAndFulfiller<void>();
   bool destroyed = false;
-  auto destructionPromise = paf.promise.then([&]() { destroyed = true; });
-  destructionPromise.eagerlyEvaluate();
+  auto destructionPromise = paf.promise.then([&]() { destroyed = true; }).eagerlyEvaluate(nullptr);
 
   int callCount = 0;
 
@@ -212,8 +211,7 @@ TEST(Capability, AsyncCancelation) {
     promise = request.send().then(
         [&](Response<test::TestMoreStuff::ExpectAsyncCancelResults>&& response) {
       returned = true;
-    });
-    promise.eagerlyEvaluate();
+    }).eagerlyEvaluate(nullptr);
   }
   kj::evalLater([]() {}).wait(waitScope);
   kj::evalLater([]() {}).wait(waitScope);
@@ -249,8 +247,7 @@ TEST(Capability, SyncCancelation) {
     promise = request.send().then(
         [&](Response<test::TestMoreStuff::ExpectSyncCancelResults>&& response) {
       returned = true;
-    });
-    promise.eagerlyEvaluate();
+    }).eagerlyEvaluate(nullptr);
   }
   kj::evalLater([]() {}).wait(waitScope);
   kj::evalLater([]() {}).wait(waitScope);

c++/src/capnp/capability.c++

@@ -203,11 +203,10 @@ public:
 
     // We daemonize one branch, but only after joining it with the promise that fires if
     // cancellation is allowed.
-    auto daemonPromise = forked.addBranch();
-    daemonPromise.attach(kj::addRef(*context));
-    daemonPromise.exclusiveJoin(kj::mv(cancelPaf.promise));
-    // Daemonize, ignoring exceptions.
-    daemonPromise.daemonize([](kj::Exception&&) {});
+    forked.addBranch()
+        .attach(kj::addRef(*context))
+        .exclusiveJoin(kj::mv(cancelPaf.promise))
+        .detach([](kj::Exception&&) {});  // ignore exceptions
 
     // Now the other branch returns the response from the context.
     auto contextPtr = context.get();
@@ -218,7 +217,7 @@ public:
 
     // We also want to notify the context that cancellation was requested if this branch is
     // destroyed.
-    promise.attach(LocalCallContext::Canceler(kj::mv(context)));
+    promise = promise.attach(LocalCallContext::Canceler(kj::mv(context)));
 
     // We return the other branch.
     return RemotePromise<ObjectPointer>(
@@ -250,12 +249,11 @@ class QueuedPipeline final: public PipelineHook, public kj::Refcounted {
 public:
   QueuedPipeline(kj::Promise<kj::Own<PipelineHook>>&& promiseParam)
       : promise(promiseParam.fork()),
-        selfResolutionOp(promise.addBranch().then(
-            [this](kj::Own<PipelineHook>&& inner) {
+        selfResolutionOp(promise.addBranch().then([this](kj::Own<PipelineHook>&& inner) {
           redirect = kj::mv(inner);
-            })) {
-    selfResolutionOp.eagerlyEvaluate();
-  }
+        }, [this](kj::Exception&& exception) {
+          redirect = newBrokenPipeline(kj::mv(exception));
+        }).eagerlyEvaluate(nullptr)) {}
 
   kj::Own<PipelineHook> addRef() override {
     return kj::addRef(*this);
@@ -288,14 +286,13 @@ class QueuedClient final: public ClientHook, public kj::Refcounted {
 public:
   QueuedClient(kj::Promise<kj::Own<ClientHook>>&& promiseParam)
       : promise(promiseParam.fork()),
-        selfResolutionOp(promise.addBranch().then(
-            [this](kj::Own<ClientHook>&& inner) {
+        selfResolutionOp(promise.addBranch().then([this](kj::Own<ClientHook>&& inner) {
           redirect = kj::mv(inner);
-            })),
+        }, [this](kj::Exception&& exception) {
+          redirect = newBrokenCap(kj::mv(exception));
+        }).eagerlyEvaluate(nullptr)),
         promiseForCallForwarding(promise.addBranch().fork()),
-        promiseForClientResolution(promise.addBranch().fork()) {
-    selfResolutionOp.eagerlyEvaluate();
-  }
+        promiseForClientResolution(promise.addBranch().fork()) {}
 
   Request<ObjectPointer, ObjectPointer> newCall(
       uint64_t interfaceId, uint16_t methodId, uint firstSegmentWordSize) override {
@@ -470,10 +467,7 @@ public:
     auto promise = kj::evalLater([this,interfaceId,methodId,contextPtr]() {
       return server->dispatchCall(interfaceId, methodId,
                                   CallContext<ObjectPointer, ObjectPointer>(*contextPtr));
-    });
-
-    // Make sure that this client cannot be destroyed until the promise completes.
-    promise.attach(kj::addRef(*this));
+    }).attach(kj::addRef(*this));
 
     // We have to fork this promise for the pipeline to receive a copy of the answer.
     auto forked = promise.fork();
@@ -488,10 +482,9 @@ public:
       return kj::mv(pipeline.hook);
     });
 
-    pipelinePromise.exclusiveJoin(kj::mv(tailPipelinePromise));
+    pipelinePromise = pipelinePromise.exclusiveJoin(kj::mv(tailPipelinePromise));
 
-    auto completionPromise = forked.addBranch();
-    completionPromise.attach(kj::mv(context));
+    auto completionPromise = forked.addBranch().attach(kj::mv(context));
 
     return VoidPromiseAndPipeline { kj::mv(completionPromise),
         kj::refcounted<QueuedPipeline>(kj::mv(pipelinePromise)) };

c++/src/capnp/ez-rpc.c++

@@ -240,9 +240,7 @@ struct EzRpcServer::Impl final: public SturdyRefRestorer<Text>, public kj::TaskS
 
       // Arrange to destroy the server context when all references are gone, or when the
       // EzRpcServer is destroyed (which will destroy the TaskSet).
-      auto promise = server->network.onDrained();
-      promise.attach(kj::mv(server));
-      tasks.add(kj::mv(promise));
+      tasks.add(server->network.onDrained().attach(kj::mv(server)));
     })));
   }
 

c++/src/capnp/rpc-test.c++

@@ -575,8 +575,7 @@ TEST(Rpc, AsyncCancelation) {
 
   auto paf = kj::newPromiseAndFulfiller<void>();
   bool destroyed = false;
-  auto destructionPromise = paf.promise.then([&]() { destroyed = true; });
-  destructionPromise.eagerlyEvaluate();
+  auto destructionPromise = paf.promise.then([&]() { destroyed = true; }).eagerlyEvaluate(nullptr);
 
   auto client = context.connect(test::TestSturdyRefObjectId::Tag::TEST_MORE_STUFF)
       .castAs<test::TestMoreStuff>();
@@ -590,8 +589,7 @@ TEST(Rpc, AsyncCancelation) {
     promise = request.send().then(
         [&](Response<test::TestMoreStuff::ExpectAsyncCancelResults>&& response) {
       returned = true;
-    });
-    promise.eagerlyEvaluate();
+    }).eagerlyEvaluate(nullptr);
   }
   kj::evalLater([]() {}).wait(context.waitScope);
   kj::evalLater([]() {}).wait(context.waitScope);
@@ -630,8 +628,7 @@ TEST(Rpc, SyncCancelation) {
     promise = request.send().then(
         [&](Response<test::TestMoreStuff::ExpectSyncCancelResults>&& response) {
       returned = true;
-    });
-    promise.eagerlyEvaluate();
+    }).eagerlyEvaluate(nullptr);
   }
   kj::evalLater([]() {}).wait(context.waitScope);
   kj::evalLater([]() {}).wait(context.waitScope);
@@ -698,8 +695,7 @@ TEST(Rpc, RetainAndRelease) {
 
   auto paf = kj::newPromiseAndFulfiller<void>();
   bool destroyed = false;
-  auto destructionPromise = paf.promise.then([&]() { destroyed = true; });
-  destructionPromise.eagerlyEvaluate();
+  auto destructionPromise = paf.promise.then([&]() { destroyed = true; }).eagerlyEvaluate(nullptr);
 
   {
     auto client = context.connect(test::TestSturdyRefObjectId::Tag::TEST_MORE_STUFF)
@@ -765,8 +761,7 @@ TEST(Rpc, Cancel) {
 
   auto paf = kj::newPromiseAndFulfiller<void>();
   bool destroyed = false;
-  auto destructionPromise = paf.promise.then([&]() { destroyed = true; });
-  destructionPromise.eagerlyEvaluate();
+  auto destructionPromise = paf.promise.then([&]() { destroyed = true; }).eagerlyEvaluate(nullptr);
 
   {
     auto request = client.neverReturnRequest();
@@ -797,8 +792,7 @@ TEST(Rpc, SendTwice) {
 
   auto paf = kj::newPromiseAndFulfiller<void>();
   bool destroyed = false;
-  auto destructionPromise = paf.promise.then([&]() { destroyed = true; });
-  destructionPromise.eagerlyEvaluate();
+  auto destructionPromise = paf.promise.then([&]() { destroyed = true; }).eagerlyEvaluate(nullptr);
 
   auto cap = test::TestInterface::Client(kj::heap<TestCapDestructor>(kj::mv(paf.fulfiller)));
 

c++/src/capnp/rpc-twoparty.c++

@@ -83,11 +83,9 @@ public:
       }, [&](kj::Exception&& exception) {
         // Exception during write!
         network.disconnectFulfiller->fulfill();
-      });
-      promise.eagerlyEvaluate();
+      }).eagerlyEvaluate(nullptr);
       return kj::mv(promise);
-    });
-    network.previousWrite.attach(kj::addRef(*this));
+    }).attach(kj::addRef(*this));
   }
 
 private:

c++/src/capnp/rpc.c++

@@ -273,7 +273,7 @@ public:
     auto questionRef = kj::refcounted<QuestionRef>(*this, questionId, kj::mv(paf.fulfiller));
     question.selfRef = *questionRef;
 
-    paf.promise.attach(kj::addRef(*questionRef));
+    paf.promise = paf.promise.attach(kj::addRef(*questionRef));
 
     {
       auto message = connection->newOutgoingMessage(
@@ -865,6 +865,10 @@ private:
                 resolve(kj::mv(resolution));
               }, [this](kj::Exception&& exception) {
                 resolve(newBrokenCap(kj::mv(exception)));
+              }).eagerlyEvaluate([&](kj::Exception&& e) {
+                // Make any exceptions thrown from resolve() go to the connection's TaskSet which
+                // will cause the connection to be terminated.
+                connectionState.tasks.add(kj::mv(e));
               })) {
       // Create a client that starts out forwarding all calls to `initial` but, once `eventual`
       // resolves, will forward there instead.  In addition, `whenMoreResolved()` will return a fork
@@ -872,13 +876,6 @@ private:
       // the `PromiseClient` is destroyed; `eventual` must therefore make sure to hold references to
       // anything that needs to stay alive in order to resolve it correctly (such as making sure the
       // import ID is not released).
-
-      // Make any exceptions thrown from resolveSelfPromise go to the connection's TaskSet which
-      // will cause the connection to be terminated.
-      resolveSelfPromise = resolveSelfPromise.then(
-          []() {}, [&](kj::Exception&& e) { connectionState.tasks.add(kj::mv(e)); });
-
-      resolveSelfPromise.eagerlyEvaluate();
     }
 
     ~PromiseClient() noexcept(false) {
@@ -1080,7 +1077,7 @@ private:
     // to eventually resolve to the ClientHook produced by `promise`.  This method waits for that
     // resolve to happen and then sends the appropriate `Resolve` message to the peer.
 
-    auto result = promise.then(
+    return promise.then(
         [this,exportId](kj::Own<ClientHook>&& resolution) -> kj::Promise<void> {
       // Successful resolution.
 
@@ -1136,9 +1133,10 @@ private:
       resolve.setPromiseId(exportId);
       fromException(exception, resolve.initException());
       message->send();
+    }).eagerlyEvaluate([this](kj::Exception&& exception) {
+      // Put the exception on the TaskSet which will cause the connection to be terminated.
+      tasks.add(kj::mv(exception));
     });
-    result.eagerlyEvaluate();
-    return kj::mv(result);
   }
 
   // =====================================================================================
@@ -1296,7 +1294,7 @@ private:
 
             auto paf = kj::newPromiseAndFulfiller<kj::Own<ClientHook>>();
             import.promiseFulfiller = kj::mv(paf.fulfiller);
-            paf.promise.attach(kj::addRef(*importClient));
+            paf.promise = paf.promise.attach(kj::addRef(*importClient));
             result = kj::refcounted<PromiseClient>(
                 connectionState, kj::mv(importClient), kj::mv(paf.promise), importId);
           } else {
@@ -1683,8 +1681,7 @@ private:
 
       message->send();
 
-      result.promise = kj::mv(paf.promise);
-      result.promise.attach(kj::addRef(*result.questionRef));
+      result.promise = paf.promise.attach(kj::addRef(*result.questionRef));
 
       return kj::mv(result);
     }
@@ -1701,10 +1698,13 @@ private:
                 resolve(kj::mv(response));
               }, [this](kj::Exception&& exception) {
                 resolve(kj::mv(exception));
+              }).eagerlyEvaluate([&](kj::Exception&& e) {
+                // Make any exceptions thrown from resolve() go to the connection's TaskSet which
+                // will cause the connection to be terminated.
+                connectionState.tasks.add(kj::mv(e));
               })) {
       // Construct a new RpcPipeline.
 
-      resolveSelfPromise.eagerlyEvaluate();
       state.init<Waiting>(kj::mv(questionRef));
     }
 
@@ -2347,15 +2347,15 @@ private:
         auto forked = resultsPromise.fork();
         answer.redirectedResults = forked.addBranch();
 
-        auto promise = kj::mv(cancelPaf.promise);
-        promise.exclusiveJoin(forked.addBranch().then([](kj::Own<RpcResponse>&&){}));
-        promise.daemonize([](kj::Exception&&) {});
+        cancelPaf.promise
+            .exclusiveJoin(forked.addBranch().then([](kj::Own<RpcResponse>&&){}))
+            .detach([](kj::Exception&&) {});
       } else {
         // Hack:  Both the success and error continuations need to use the context.  We could
         //   refcount, but both will be destroyed at the same time anyway.
         RpcCallContext* contextPtr = context;
 
-        auto promise = promiseAndPipeline.promise.then(
+        promiseAndPipeline.promise.then(
             [contextPtr]() {
               contextPtr->sendReturn();
             }, [contextPtr](kj::Exception&& exception) {
@@ -2363,10 +2363,9 @@ private:
             }).then([]() {}, [&](kj::Exception&& exception) {
               // Handle exceptions that occur in sendReturn()/sendErrorReturn().
               taskFailed(kj::mv(exception));
-            });
-        promise.attach(kj::mv(context));
-        promise.exclusiveJoin(kj::mv(cancelPaf.promise));
-        promise.daemonize([](kj::Exception&&) {});
+            }).attach(kj::mv(context))
+            .exclusiveJoin(kj::mv(cancelPaf.promise))
+            .detach([](kj::Exception&&) {});
       }
     }
   }

c++/src/capnp/test-util.c++

@@ -1015,7 +1015,7 @@ kj::Promise<void> TestMoreStuffImpl::neverReturnAdvanced(
   neverFulfill = kj::mv(paf.fulfiller);
 
   // Attach `cap` to the promise to make sure it is released.
-  paf.promise.attach(context.getParams().getCap());
+  paf.promise = paf.promise.attach(context.getParams().getCap());
 
   // Also attach `cap` to the result struct to make sure that is released.
   context.getResults().setCapCopy(context.getParams().getCap());

c++/src/kj/async-inl.h

@@ -617,20 +617,34 @@ Promise<T> ForkedPromise<T>::addBranch() {
 }
 
 template <typename T>
-void Promise<T>::exclusiveJoin(Promise<T>&& other) {
-  node = heap<_::ExclusiveJoinPromiseNode>(kj::mv(node), kj::mv(other.node));
+Promise<T> Promise<T>::exclusiveJoin(Promise<T>&& other) {
+  return Promise(false, heap<_::ExclusiveJoinPromiseNode>(kj::mv(node), kj::mv(other.node)));
 }
 
 template <typename T>
 template <typename... Attachments>
-void Promise<T>::attach(Attachments&&... attachments) {
-  node = kj::heap<_::AttachmentPromiseNode<Tuple<Attachments...>>>(
-      kj::mv(node), kj::tuple(kj::fwd<Attachments>(attachments)...));
+Promise<T> Promise<T>::attach(Attachments&&... attachments) {
+  return Promise(false, kj::heap<_::AttachmentPromiseNode<Tuple<Attachments...>>>(
+      kj::mv(node), kj::tuple(kj::fwd<Attachments>(attachments)...)));
 }
 
 template <typename T>
-void Promise<T>::eagerlyEvaluate() {
-  node = _::spark<_::FixVoid<T>>(kj::mv(node));
+template <typename ErrorFunc>
+Promise<T> Promise<T>::eagerlyEvaluate(ErrorFunc&& errorHandler) {
+  return Promise(false, _::spark<_::FixVoid<T>>(
+      then([](T&& value) -> T { return kj::mv(value); }, kj::fwd<ErrorFunc>(errorHandler)).node));
+}
+
+template <>
+template <typename ErrorFunc>
+Promise<void> Promise<void>::eagerlyEvaluate(ErrorFunc&& errorHandler) {
+  return Promise(false, _::spark<_::Void>(
+      then([]() {}, kj::fwd<ErrorFunc>(errorHandler)).node));
+}
+
+template <typename T>
+Promise<T> Promise<T>::eagerlyEvaluate(decltype(nullptr)) {
+  return Promise(false, _::spark<_::FixVoid<T>>(kj::mv(node)));
 }
 
 template <typename T>
@@ -645,14 +659,14 @@ inline PromiseForResult<Func, void> evalLater(Func&& func) {
 
 template <typename T>
 template <typename ErrorFunc>
-void Promise<T>::daemonize(ErrorFunc&& errorHandler) {
-  return _::daemonize(then([](T&&) {}, kj::fwd<ErrorFunc>(errorHandler)));
+void Promise<T>::detach(ErrorFunc&& errorHandler) {
+  return _::detach(then([](T&&) {}, kj::fwd<ErrorFunc>(errorHandler)));
 }
 
 template <>
 template <typename ErrorFunc>
-void Promise<void>::daemonize(ErrorFunc&& errorHandler) {
-  return _::daemonize(then([]() {}, kj::fwd<ErrorFunc>(errorHandler)));
+void Promise<void>::detach(ErrorFunc&& errorHandler) {
+  return _::detach(then([]() {}, kj::fwd<ErrorFunc>(errorHandler)));
 }
 
 // =======================================================================================

c++/src/kj/async-io-test.c++

@@ -48,7 +48,7 @@ TEST(AsyncIo, SimpleNetwork) {
   }).then([&](Own<AsyncIoStream>&& result) {
     client = kj::mv(result);
     return client->write("foo", 3);
-  }).daemonize([](kj::Exception&& exception) {
+  }).detach([](kj::Exception&& exception) {
     ADD_FAILURE() << kj::str(exception).cStr();
   });
 
@@ -102,7 +102,7 @@ TEST(AsyncIo, OneWayPipe) {
   auto pipe = ioContext.provider->newOneWayPipe();
   char receiveBuffer[4];
 
-  pipe.out->write("foo", 3).daemonize([](kj::Exception&& exception) {
+  pipe.out->write("foo", 3).detach([](kj::Exception&& exception) {
     ADD_FAILURE() << kj::str(exception).cStr();
   });
 

c++/src/kj/async-io.c++

@@ -649,9 +649,7 @@ Promise<Array<SocketAddress>> SocketAddress::lookupHost(
   }));
 
   auto reader = heap<LookupReader>(kj::mv(thread), kj::mv(input));
-  auto result = reader->read();
-  result.attach(kj::mv(reader));
-  return kj::mv(result);
+  return reader->read().attach(kj::mv(reader));
 }
 
 // =======================================================================================

c++/src/kj/async-prelude.h

@@ -173,7 +173,7 @@ private:
   friend class TaskSetImpl;
 };
 
-void daemonize(kj::Promise<void>&& promise);
+void detach(kj::Promise<void>&& promise);
 void waitImpl(Own<_::PromiseNode>&& node, _::ExceptionOrValue& result, WaitScope& waitScope);
 Promise<void> yield();
 

c++/src/kj/async-test.c++

@@ -259,8 +259,7 @@ TEST(Async, Ordering) {
       auto paf = kj::newPromiseAndFulfiller<void>();
       promises[2] = paf.promise.then([&]() {
         EXPECT_EQ(1, counter++);
-      });
-      promises[2].eagerlyEvaluate();
+      }).eagerlyEvaluate(nullptr);
       paf.fulfiller->fulfill();
     }
 
@@ -270,25 +269,21 @@ TEST(Async, Ordering) {
       EXPECT_EQ(4, counter++);
     }).then([&]() {
       EXPECT_EQ(5, counter++);
-    });
-    promises[3].eagerlyEvaluate();
+    }).eagerlyEvaluate(nullptr);
 
     {
       auto paf = kj::newPromiseAndFulfiller<void>();
       promises[4] = paf.promise.then([&]() {
         EXPECT_EQ(2, counter++);
-      });
-      promises[4].eagerlyEvaluate();
+      }).eagerlyEvaluate(nullptr);
       paf.fulfiller->fulfill();
     }
 
     // evalLater() is like READY_NOW.then().
     promises[5] = evalLater([&]() {
       EXPECT_EQ(6, counter++);
-    });
-    promises[5].eagerlyEvaluate();
-  });
-  promises[1].eagerlyEvaluate();
+    }).eagerlyEvaluate(nullptr);
+  }).eagerlyEvaluate(nullptr);
 
   promises[0] = evalLater([&]() {
     EXPECT_EQ(3, counter++);
@@ -296,8 +291,7 @@ TEST(Async, Ordering) {
     // Making this a chain should NOT cause it to preempt promises[1].  (This was a problem at one
     // point.)
     return Promise<void>(READY_NOW);
-  });
-  promises[0].eagerlyEvaluate();
+  }).eagerlyEvaluate(nullptr);
 
   for (auto i: indices(promises)) {
     kj::mv(promises[i]).wait(waitScope);
@@ -372,9 +366,7 @@ TEST(Async, ExclusiveJoin) {
     auto left = evalLater([&]() { return 123; });
     auto right = newPromiseAndFulfiller<int>();  // never fulfilled
 
-    left.exclusiveJoin(kj::mv(right.promise));
-
-    EXPECT_EQ(123, left.wait(waitScope));
+    EXPECT_EQ(123, left.exclusiveJoin(kj::mv(right.promise)).wait(waitScope));
   }
 
   {
@@ -384,9 +376,7 @@ TEST(Async, ExclusiveJoin) {
     auto left = newPromiseAndFulfiller<int>();  // never fulfilled
     auto right = evalLater([&]() { return 123; });
 
-    left.promise.exclusiveJoin(kj::mv(right));
-
-    EXPECT_EQ(123, left.promise.wait(waitScope));
+    EXPECT_EQ(123, left.promise.exclusiveJoin(kj::mv(right)).wait(waitScope));
   }
 
   {
@@ -396,9 +386,7 @@ TEST(Async, ExclusiveJoin) {
     auto left = evalLater([&]() { return 123; });
     auto right = evalLater([&]() { return 456; });
 
-    left.exclusiveJoin(kj::mv(right));
-
-    EXPECT_EQ(123, left.wait(waitScope));
+    EXPECT_EQ(123, left.exclusiveJoin(kj::mv(right)).wait(waitScope));
   }
 
   {
@@ -406,13 +394,9 @@ TEST(Async, ExclusiveJoin) {
     WaitScope waitScope(loop);
 
     auto left = evalLater([&]() { return 123; });
-    auto right = evalLater([&]() { return 456; });
-
-    right.eagerlyEvaluate();
+    auto right = evalLater([&]() { return 456; }).eagerlyEvaluate(nullptr);
 
-    left.exclusiveJoin(kj::mv(right));
-
-    EXPECT_EQ(456, left.wait(waitScope));
+    EXPECT_EQ(456, left.exclusiveJoin(kj::mv(right)).wait(waitScope));
   }
 }
 
@@ -474,9 +458,7 @@ TEST(Async, Attach) {
   Promise<int> promise = evalLater([&]() {
     EXPECT_FALSE(destroyed);
     return 123;
-  });
-
-  promise.attach(kj::heap<DestructorDetector>(destroyed));
+  }).attach(kj::heap<DestructorDetector>(destroyed));
 
   promise = promise.then([&](int i) {
     EXPECT_TRUE(destroyed);
@@ -501,14 +483,14 @@ TEST(Async, EagerlyEvaluate) {
 
   EXPECT_FALSE(called);
 
-  promise.eagerlyEvaluate();
+  promise = promise.eagerlyEvaluate(nullptr);
 
   evalLater([]() {}).wait(waitScope);
 
   EXPECT_TRUE(called);
 }
 
-TEST(Async, Daemonize) {
+TEST(Async, Detach) {
   EventLoop loop;
   WaitScope waitScope(loop);
 
@@ -517,8 +499,8 @@ TEST(Async, Daemonize) {
   bool ran3 = false;
 
   evalLater([&]() { ran1 = true; });
-  evalLater([&]() { ran2 = true; }).daemonize([](kj::Exception&&) { ADD_FAILURE(); });
-  evalLater([]() { KJ_FAIL_ASSERT("foo"); }).daemonize([&](kj::Exception&& e) { ran3 = true; });
+  evalLater([&]() { ran2 = true; }).detach([](kj::Exception&&) { ADD_FAILURE(); });
+  evalLater([]() { KJ_FAIL_ASSERT("foo"); }).detach([&](kj::Exception&& e) { ran3 = true; });
 
   EXPECT_FALSE(ran1);
   EXPECT_FALSE(ran2);
@@ -553,8 +535,7 @@ TEST(Async, SetRunnable) {
   EXPECT_EQ(0, port.callCount);
 
   {
-    auto promise = evalLater([]() {});
-    promise.eagerlyEvaluate();
+    auto promise = evalLater([]() {}).eagerlyEvaluate(nullptr);
 
     EXPECT_TRUE(port.runnable);
     loop.run(1);
@@ -568,12 +549,10 @@ TEST(Async, SetRunnable) {
 
   {
     auto paf = newPromiseAndFulfiller<void>();
-    auto promise = paf.promise.then([]() {});
-    promise.eagerlyEvaluate();
+    auto promise = paf.promise.then([]() {}).eagerlyEvaluate(nullptr);
     EXPECT_FALSE(port.runnable);
 
-    auto promise2 = evalLater([]() {});
-    promise2.eagerlyEvaluate();
+    auto promise2 = evalLater([]() {}).eagerlyEvaluate(nullptr);
     paf.fulfiller->fulfill();
 
     EXPECT_TRUE(port.runnable);

c++/src/kj/async-unix-test.c++

@@ -92,7 +92,7 @@ TEST_F(AsyncUnixTest, SignalsMultiListen) {
 
   port.onSignal(SIGIO).then([](siginfo_t&&) {
     ADD_FAILURE() << "Received wrong signal.";
-  }).daemonize([](kj::Exception&& exception) {
+  }).detach([](kj::Exception&& exception) {
     ADD_FAILURE() << kj::str(exception).cStr();
   });
 
@@ -152,12 +152,12 @@ TEST_F(AsyncUnixTest, SignalsNoWait) {
     receivedSigusr2 = true;
     EXPECT_EQ(SIGUSR2, info.si_signo);
     EXPECT_SI_CODE(SI_USER, info.si_code);
-  }).daemonize([](Exception&& e) { ADD_FAILURE() << str(e).cStr(); });
+  }).detach([](Exception&& e) { ADD_FAILURE() << str(e).cStr(); });
   port.onSignal(SIGIO).then([&](siginfo_t&& info) {
     receivedSigio = true;
     EXPECT_EQ(SIGIO, info.si_signo);
     EXPECT_SI_CODE(SI_USER, info.si_code);
-  }).daemonize([](Exception&& e) { ADD_FAILURE() << str(e).cStr(); });
+  }).detach([](Exception&& e) { ADD_FAILURE() << str(e).cStr(); });
 
   kill(getpid(), SIGUSR2);
   kill(getpid(), SIGIO);
@@ -205,7 +205,7 @@ TEST_F(AsyncUnixTest, PollMultiListen) {
 
   port.onFdEvent(bogusPipefds[0], POLLIN | POLLPRI).then([](short s) {
     ADD_FAILURE() << "Received wrong poll.";
-  }).daemonize([](kj::Exception&& exception) {
+  }).detach([](kj::Exception&& exception) {
     ADD_FAILURE() << kj::str(exception).cStr();
   });
 
@@ -273,11 +273,11 @@ TEST_F(AsyncUnixTest, PollNoWait) {
   port.onFdEvent(pipefds[0], POLLIN | POLLPRI).then([&](short&& events) {
     receivedCount++;
     EXPECT_EQ(POLLIN, events);
-  }).daemonize([](Exception&& e) { ADD_FAILURE() << str(e).cStr(); });
+  }).detach([](Exception&& e) { ADD_FAILURE() << str(e).cStr(); });
   port.onFdEvent(pipefds2[0], POLLIN | POLLPRI).then([&](short&& events) {
     receivedCount++;
     EXPECT_EQ(POLLIN, events);
-  }).daemonize([](Exception&& e) { ADD_FAILURE() << str(e).cStr(); });
+  }).detach([](Exception&& e) { ADD_FAILURE() << str(e).cStr(); });
 
   KJ_SYSCALL(write(pipefds[1], "foo", 3));
   KJ_SYSCALL(write(pipefds2[1], "bar", 3));

c++/src/kj/async.c++

@@ -331,7 +331,7 @@ Promise<void> yield() {
   return Promise<void>(false, kj::heap<YieldPromiseNode>());
 }
 
-void daemonize(kj::Promise<void>&& promise) {
+void detach(kj::Promise<void>&& promise) {
   EventLoop& loop = currentEventLoop();
   KJ_REQUIRE(loop.daemons.get() != nullptr, "EventLoop is shutting down.") { return; }
   loop.daemons->add(kj::mv(promise));

c++/src/kj/async.h

@@ -66,7 +66,11 @@ class Promise: protected _::PromiseBase {
   //
   // Promises are linear types -- they are moveable but not copyable.  If a Promise is destroyed
   // or goes out of scope (without being moved elsewhere), any ongoing asynchronous operations
-  // meant to fulfill the promise will be canceled if possible.
+  // meant to fulfill the promise will be canceled if possible.  All methods of `Promise` (unless
+  // otherwise noted) actually consume the promise in the sense of move semantics.  (Arguably they
+  // should be rvalue-qualified, but at the time this interface was created compilers didn't widely
+  // support that yet and anyway it would be pretty ugly typing kj::mv(promise).whatever().)  If
+  // you want to use one Promise in two different places, you must fork it with `fork()`.
   //
   // To use the result of a Promise, you must call `then()` and supply a callback function to
   // call with the result.  `then()` returns another promise, for the result of the callback.
@@ -161,8 +165,9 @@ public:
   // If the returned promise is destroyed before the callback runs, the callback will be canceled
   // (it will never run).
   //
-  // Note that `then()` consumes the promise on which it is called, in the sense of move semantics.
-  // After returning, the original promise is no longer valid, but `then()` returns a new promise.
+  // Note that `then()` -- like all other Promise methods -- consumes the promise on which it is
+  // called, in the sense of move semantics.  After returning, the original promise is no longer
+  // valid, but `then()` returns a new promise.
   //
   // *Advanced implementation tips:*  Most users will never need to worry about the below, but
   // it is good to be aware of.
@@ -216,9 +221,6 @@ public:
   //   around them in arbitrary ways.  Therefore, callers really need to know if a function they
   //   are calling might wait(), and the `WaitScope&` parameter makes this clear.
   //
-  // Note that `wait()` consumes the promise on which it is called, in the sense of move semantics.
-  // After returning, the original promise is no longer valid.
-  //
   // TODO(someday):  Implement fibers, and let them call wait() even when they are handling an
   //   event.
 
@@ -227,31 +229,36 @@ public:
   // `T` must be copy-constructable for this to work.  Or, in the special case where `T` is
   // `Own<U>`, `U` must have a method `Own<U> addRef()` which returns a new reference to the same
   // (or an equivalent) object (probably implemented via reference counting).
-  //
-  // Note that `fork()` consumes the promise on which it is called, in the sense of move semantics.
-  // After returning, the original promise is no longer valid.
 
-  void exclusiveJoin(Promise<T>&& other);
-  // Replace this promise with one that resolves when either the original promise resolves or
-  // `other` resolves (whichever comes first).  The promise that didn't resolve first is canceled.
+  Promise<T> exclusiveJoin(Promise<T>&& other) KJ_WARN_UNUSED_RESULT;
+  // Return a new promise that resolves when either the original promise resolves or `other`
+  // resolves (whichever comes first).  The promise that didn't resolve first is canceled.
 
   // TODO(someday): inclusiveJoin(), or perhaps just join(), which waits for both completions
   //   and produces a tuple?
 
   template <typename... Attachments>
-  void attach(Attachments&&... attachments);
+  Promise<T> attach(Attachments&&... attachments) KJ_WARN_UNUSED_RESULT;
   // "Attaches" one or more movable objects (often, Own<T>s) to the promise, such that they will
   // be destroyed when the promise resolves.  This is useful when a promise's callback contains
   // pointers into some object and you want to make sure the object still exists when the callback
   // runs -- after calling then(), use attach() to add necessary objects to the result.
 
-  void eagerlyEvaluate();
+  template <typename ErrorFunc>
+  Promise<T> eagerlyEvaluate(ErrorFunc&& errorHandler) KJ_WARN_UNUSED_RESULT;
+  Promise<T> eagerlyEvaluate(decltype(nullptr)) KJ_WARN_UNUSED_RESULT;
   // Force eager evaluation of this promise.  Use this if you are going to hold on to the promise
   // for awhile without consuming the result, but you want to make sure that the system actually
   // processes it.
+  //
+  // `errorHandler` is a function that takes `kj::Exception&&`, like the second parameter to
+  // `then()`, except that it must return void.  We make you specify this because otherwise it's
+  // easy to forget to handle errors in a promise that you never use.  You may specify nullptr for
+  // the error handler if you are sure that ignoring errors is fine, or if you know that you'll
+  // eventually wait on the promise somewhere.
 
   template <typename ErrorFunc>
-  void daemonize(ErrorFunc&& errorHandler);
+  void detach(ErrorFunc&& errorHandler);
   // Allows the promise to continue running in the background until it completes or the
   // `EventLoop` is destroyed.  Be careful when using this: since you can no longer cancel this
   // promise, you need to make sure that the promise owns all the objects it touches or make sure
@@ -262,12 +269,10 @@ public:
   //
   // This function exists mainly to implement the Cap'n Proto requirement that RPC calls cannot be
   // canceled unless the callee explicitly permits it.
-  //
-  // Note that `daemonize()` consumes the promise on which it is called, in the sense of move
-  // semantics.  After returning, the original promise is no longer valid.
 
   kj::String trace();
   // Returns a dump of debug info about this promise.  Not for production use.  Requires RTTI.
+  // This method does NOT consume the promise as other methods do.
 
 private:
   Promise(bool, Own<_::PromiseNode>&& node): PromiseBase(kj::mv(node)) {}
@@ -462,7 +467,7 @@ class TaskSet {
   //
   // This is useful for "daemon" objects that perform background tasks which aren't intended to
   // fulfill any particular external promise, but which may need to be canceled (and thus can't
-  // use `Promise::daemonize()`).  The daemon object holds a TaskSet to collect these tasks it is
+  // use `Promise::detach()`).  The daemon object holds a TaskSet to collect these tasks it is
   // working on.  This way, if the daemon itself is destroyed, the TaskSet is detroyed as well,
   // and everything the daemon is doing is canceled.
 
@@ -593,7 +598,7 @@ private:
   void enterScope();
   void leaveScope();
 
-  friend void _::daemonize(kj::Promise<void>&& promise);
+  friend void _::detach(kj::Promise<void>&& promise);
   friend void _::waitImpl(Own<_::PromiseNode>&& node, _::ExceptionOrValue& result,
                           WaitScope& waitScope);
   friend class _::Event;

mega-test-quick.cfg

-linux-gcc-4.7      1771 ./super-test.sh tmpdir capnp-gcc-4.7 quick
-linux-gcc-4.8      1774 ./super-test.sh tmpdir capnp-gcc-4.8 quick gcc-4.8
-linux-clang        1794 ./super-test.sh tmpdir capnp-clang quick clang
+linux-gcc-4.7      1773 ./super-test.sh tmpdir capnp-gcc-4.7 quick
+linux-gcc-4.8      1776 ./super-test.sh tmpdir capnp-gcc-4.8 quick gcc-4.8
+linux-clang        1796 ./super-test.sh tmpdir capnp-clang quick clang
 mac                 807 ./super-test.sh remote beat caffeinate quick
 cygwin              812 ./super-test.sh remote Kenton@flashman quick