Add a sample RPC application, and fix some bugs / add some missing features needed by said app.

.gitignore

@@ -16,6 +16,7 @@
 /c++/src/os
 /c++/src/protobuf
 /c++/src/snappy
+/c++/src/samples
 
 # Ekam build artifacts.
 /c++/tmp/

c++/samples/calculator-server.c++

+#include "calculator.capnp.h"
+#include <kj/debug.h>
+#include <capnp/ez-rpc.h>
+#include <capnp/capability-context.h>  // for LocalMessage
+#include <iostream>
+
+typedef unsigned int uint;
+
+kj::Promise<double> readValue(Calculator::Value::Client value) {
+  // Helper function to asynchronously call read() on a Calculator::Value and
+  // return a promise for the result.  (In the future, the generated code might
+  // include something like this automatically.)
+
+  return value.readRequest().send()
+      .then([](capnp::Response<Calculator::Value::ReadResults> result) {
+    return result.getValue();
+  });
+}
+
+kj::Promise<double> evaluateImpl(
+    Calculator::Expression::Reader expression,
+    capnp::List<double>::Reader params = capnp::List<double>::Reader()) {
+  // Implementation of CalculatorImpl::evaluate(), also shared by
+  // FunctionImpl::call().  In the latter case, `params` are the parameter
+  // values passed to the function; in the former case, `params` is just an
+  // empty list.
+
+  switch (expression.which()) {
+    case Calculator::Expression::LITERAL:
+      return expression.getLiteral();
+
+    case Calculator::Expression::PREVIOUS_RESULT:
+      return readValue(expression.getPreviousResult());
+
+    case Calculator::Expression::PARAMETER: {
+      KJ_REQUIRE(expression.getParameter() < params.size(),
+                 "Parameter index out-of-range.");
+      return params[expression.getParameter()];
+    }
+
+    case Calculator::Expression::CALL: {
+      auto call = expression.getCall();
+      auto func = call.getFunction();
+
+      // Evaluate each parameter.
+      kj::Array<kj::Promise<double>> paramPromises =
+          KJ_MAP(param, call.getParams()) {
+            return evaluateImpl(param, params);
+          };
+
+      // Join the array of promises into a promise for an array.
+      kj::Promise<kj::Array<double>> joinedParams =
+          kj::joinPromises(kj::mv(paramPromises));
+
+      // When the parameters are complete, call the function.
+      return joinedParams.then([func](kj::Array<double>&& paramValues) mutable {
+        auto request = func.callRequest();
+        request.setParams(paramValues);
+        return request.send().then(
+            [](capnp::Response<Calculator::Function::CallResults>&& result) {
+          return result.getValue();
+        });
+      });
+    }
+
+    default:
+      // Throw an exception.
+      KJ_FAIL_REQUIRE("Unknown expression type.");
+  }
+}
+
+class ValueImpl final: public Calculator::Value::Server {
+  // Simple implementation of the Calculator.Value Cap'n Proto interface.
+
+public:
+  ValueImpl(double value): value(value) {}
+
+  kj::Promise<void> read(ReadContext context) {
+    context.getResults().setValue(value);
+    return kj::READY_NOW;
+  }
+
+private:
+  double value;
+};
+
+class FunctionImpl final: public Calculator::Function::Server {
+  // Implementation of the Calculator.Function Cap'n Proto interface, where the
+  // function is defined by a Calculator.Expression.
+
+public:
+  FunctionImpl(uint paramCount, Calculator::Expression::Reader body)
+      : paramCount(paramCount), body(body) {}
+
+  kj::Promise<void> call(CallContext context) {
+    auto params = context.getParams().getParams();
+    KJ_REQUIRE(params.size() == paramCount, "Wrong number of parameters.");
+
+    return evaluateImpl(body.getRoot().getAs<Calculator::Expression>(), params)
+        .then([context](double value) mutable {
+      context.getResults().setValue(value);
+    });
+  }
+
+private:
+  uint paramCount;
+  // The function's arity.
+
+  capnp::LocalMessage body;
+  // LocalMessage holds a message that might contain capabilities (interface
+  // references).  Here we're using it to hold a Calculator.Expression, which
+  // might contain Calculator.Function and/or Calculator.Value capabilities.
+};
+
+class OperatorImpl final: public Calculator::Function::Server {
+  // Implementation of the Calculator.Function Cap'n Proto interface, wrapping
+  // basic binary arithmetic operators.
+
+public:
+  OperatorImpl(Calculator::Operator op): op(op) {}
+
+  kj::Promise<void> call(CallContext context) {
+    auto params = context.getParams().getParams();
+    KJ_REQUIRE(params.size() == 2, "Wrong number of parameters.");
+
+    double result;
+    switch (op) {
+      case Calculator::Operator::ADD:     result = params[0] + params[1]; break;
+      case Calculator::Operator::SUBTRACT:result = params[0] - params[1]; break;
+      case Calculator::Operator::MULTIPLY:result = params[0] * params[1]; break;
+      case Calculator::Operator::DIVIDE:  result = params[0] / params[1]; break;
+      default:
+        KJ_FAIL_REQUIRE("Unknown operator.");
+    }
+
+    context.getResults().setValue(result);
+    return kj::READY_NOW;
+  }
+
+private:
+  Calculator::Operator op;
+};
+
+class CalculatorImpl final: public Calculator::Server {
+  // Implementation of the Calculator Cap'n Proto interface.
+
+public:
+  kj::Promise<void> evaluate(EvaluateContext context) override {
+    return evaluateImpl(context.getParams().getExpression())
+        .then([context](double value) mutable {
+      context.getResults().setValue(kj::heap<ValueImpl>(value));
+    });
+  }
+
+  kj::Promise<void> defFunction(DefFunctionContext context) override {
+    auto params = context.getParams();
+    context.getResults().setFunc(kj::heap<FunctionImpl>(
+        params.getParamCount(), params.getBody()));
+    return kj::READY_NOW;
+  }
+
+  kj::Promise<void> getOperator(GetOperatorContext context) override {
+    context.getResults().setFunc(kj::heap<OperatorImpl>(
+        context.getParams().getOp()));
+    return kj::READY_NOW;
+  }
+};
+
+int main(int argc, const char* argv[]) {
+  if (argc != 2) {
+    std::cerr << "usage: " << argv[0] << " ADDRESS[:PORT]\n"
+        "Runs the server bound to the given address/port.\n"
+        "ADDRESS may be '*' to bind to all local addresses.\n"
+        ":PORT may be omitted to choose a port automatically." << std::endl;
+    return 1;
+  }
+
+  // Set up a server.
+  capnp::EzRpcServer server(argv[1]);
+  server.exportCap("calculator", kj::heap<CalculatorImpl>());
+
+  // Write the port number to stdout, in case it was chosen automatically.
+  auto& waitScope = server.getWaitScope();
+  uint port = server.getPort().wait(waitScope);
+  if (port == 0) {
+    // The address format "unix:/path/to/socket" opens a unix domain socket,
+    // in which case the port will be zero.
+    std::cout << "Listening on Unix socket..." << std::endl;
+  } else {
+    std::cout << "Listening on port " << port << "..." << std::endl;
+  }
+
+  // Run forever, accepting connections and handling requests.
+  kj::NEVER_DONE.wait(waitScope);
+}

c++/samples/calculator.capnp

+@0x85150b117366d14b;
+
+interface Calculator {
+  # A "simple" mathematical calculator, callable via RPC.
+  #
+  # But, to show off Cap'n Proto, we add some twists:
+  #
+  # - You can use the result from one call as the input to the next
+  #   without a network round trip.  To accomplish this, evaluate()
+  #   returns a `Value` object wrapping the actual numeric value.
+  #   This object may be used in a subsequent expression.  With
+  #   promise pipelining, the Value can actually be used before
+  #   the evaluate() call that creates it returns!
+  #
+  # - You can define new functions, and then call them.  This again
+  #   shows off pipelining, but it also gives the client the
+  #   opportunity to define a function on the client side and have
+  #   the server call back to it.
+  #
+  # - The basic arithmetic operators are exposed as Functions, and
+  #   you have to call getOperator() to obtain them from the server.
+  #   This again demonstrates pipelining -- using getOperator() to
+  #   get each operator and then using them in evaluate() still
+  #   only takes one network round trip.
+
+  evaluate @0 (expression: Expression) -> (value: Value);
+  # Evaluate the given expression and return the result.  The
+  # result is returned wrapped in a Value interface so that you
+  # may pass it back to the server in a pipelined request.  To
+  # actually get the numeric value, you must call read() on the
+  # Value -- but again, this can be pipelined so that it incurs
+  # no additional latency.
+
+  struct Expression {
+    # A numeric expression.
+
+    union {
+      literal @0 :Float64;
+      # A literal numeric value.
+
+      previousResult @1 :Value;
+      # A value that was (or, will be) returned by a previous
+      # evaluate().
+
+      parameter @2 :UInt32;
+      # A parameter to the function (only valid in function bodies;
+      # see defFunction).
+
+      call :group {
+        # Call a function on a list of parameters.
+        function @3 :Function;
+        params @4 :List(Expression);
+      }
+    }
+  }
+
+  interface Value {
+    # Wraps a numeric value in an RPC object.  This allows the value
+    # to be used in subsequent evaluate() requests without the client
+    # waiting for the evaluate() that returns the Value to finish.
+
+    read @0 () -> (value :Float64);
+    # Read back the raw numeric value.
+  }
+
+  defFunction @1 (paramCount :Int32, body :Expression)
+              -> (func :Function);
+  # Define a function that takes `paramCount` parameters and returns the
+  # evaluation of `body` after substituting these parameters.
+
+  interface Function {
+    # An algebraic function.  Can be called directly, or can be used inside
+    # an Expression.
+    #
+    # A client can create a Function that runs on the server side using
+    # `defFunction()` or `getOperator()`.  Alternatively, a client can
+    # implement a Function on the client side and the server will call back
+    # to it.  However, a function defined on the client side will require a
+    # network round trip whenever the server needs to call it, whereas
+    # functions defined on the server and then passed back to it are called
+    # locally.
+
+    call @0 (params :List(Float64)) -> (value: Float64);
+    # Call the function on the given parameters.
+  }
+
+  getOperator @2 (op: Operator) -> (func: Function);
+  # Get a Function representing an arithmetic operator, which can then be
+  # used in Expressions.
+
+  enum Operator {
+    add @0;
+    subtract @1;
+    multiply @2;
+    divide @3;
+  }
+}

c++/samples/test.sh

@@ -11,3 +11,15 @@ c++ -std=c++11 -Wall addressbook.c++ addressbook.capnp.c++ -lcapnp -lkj -pthread
 ./addressbook dwrite | ./addressbook dread
 rm addressbook addressbook.capnp.c++ addressbook.capnp.h
 
+capnpc -oc++ calculator.capnp
+c++ -std=c++11 -Wall calculator-client.c++ calculator.capnp.c++ -lcapnp-rpc -lcapnp -lkj-async \
+    -lkj -pthread -o calculator-client
+c++ -std=c++11 -Wall calculator-server.c++ calculator.capnp.c++ -lcapnp-rpc -lcapnp -lkj-async \
+    -lkj -pthread -o calculator-server
+rm -f /tmp/capnp-calculator-example-$$
+./calculator-server unix:/tmp/capnp-calculator-example-$$ &
+sleep 0.1
+./calculator-client unix:/tmp/capnp-calculator-example-$$
+kill %+
+wait %+ || true
+rm calculator-client calculator-server calculator.capnp.c++ calculator.capnp.h /tmp/capnp-calculator-example-$$

c++/src/capnp/capability-context.h

@@ -105,7 +105,12 @@ class LocalMessage final {
   // know how to properly serialize its capabilities.
 
 public:
-  LocalMessage(kj::Maybe<MessageSize> sizeHint = nullptr);
+  explicit LocalMessage(kj::Maybe<MessageSize> sizeHint = nullptr);
+  template <typename T, typename = FromReader<T>>
+  inline LocalMessage(T&& reader): LocalMessage(reader.totalSize()) {
+    // Create a LocalMessage that is a copy of a given reader.
+    getRoot().setAs<FromReader<T>>(kj::fwd<T>(reader));
+  }
 
   inline AnyPointer::Builder getRoot() { return root; }
   inline AnyPointer::Reader getRootReader() const { return root.asReader(); }

c++/src/capnp/capnpc.ekam-rule

@@ -51,7 +51,7 @@ if test "$INTERCEPTOR" = ""; then
   exit 1
 fi
 
-echo findProvider file:capnp
+echo findProvider file:compiler/capnp
 read CAPNP
 
 if test "$CAPNP" = ""; then

c++/src/capnp/compiler/capnpc-c++.c++

@@ -944,7 +944,7 @@ private:
             "  inline void set", titleCase, "(", type, "::Reader value);\n",
             kind == FieldKind::LIST && !isStructOrCapList
             ? kj::strTree(
-              "  inline void set", titleCase, "(std::initializer_list<", elementReaderType, "> value);\n")
+              "  inline void set", titleCase, "(::kj::ArrayPtr<const ", elementReaderType, "> value);\n")
             : kj::strTree(),
             kind == FieldKind::STRUCT
             ? kj::strTree(
@@ -994,7 +994,7 @@ private:
             "}\n",
             kind == FieldKind::LIST && !isStructOrCapList
             ? kj::strTree(
-              "inline void ", scope, "Builder::set", titleCase, "(std::initializer_list<", elementReaderType, "> value) {\n",
+              "inline void ", scope, "Builder::set", titleCase, "(::kj::ArrayPtr<const ", elementReaderType, "> value) {\n",
               unionDiscrim.set,
               "  ::capnp::_::PointerHelpers<", type, ">::set(\n"
               "      _builder.getPointerField(", offset, " * ::capnp::POINTERS), value);\n"

c++/src/capnp/dynamic.c++

@@ -1403,12 +1403,12 @@ DynamicValue::Reader::Reader(const Reader& other) {
     case ENUM:
     case STRUCT:
     case ANY_POINTER:
-      static_assert(__has_trivial_copy(Text::Reader) &&
-                    __has_trivial_copy(Data::Reader) &&
-                    __has_trivial_copy(DynamicList::Reader) &&
-                    __has_trivial_copy(DynamicEnum) &&
-                    __has_trivial_copy(DynamicStruct::Reader) &&
-                    __has_trivial_copy(AnyPointer::Reader),
+      static_assert(kj::canMemcpy<Text::Reader>() &&
+                    kj::canMemcpy<Data::Reader>() &&
+                    kj::canMemcpy<DynamicList::Reader>() &&
+                    kj::canMemcpy<DynamicEnum>() &&
+                    kj::canMemcpy<DynamicStruct::Reader>() &&
+                    kj::canMemcpy<AnyPointer::Reader>(),
                     "Assumptions here don't hold.");
       break;
 
@@ -1434,12 +1434,12 @@ DynamicValue::Reader::Reader(Reader&& other) noexcept {
     case ENUM:
     case STRUCT:
     case ANY_POINTER:
-      static_assert(__has_trivial_copy(Text::Reader) &&
-                    __has_trivial_copy(Data::Reader) &&
-                    __has_trivial_copy(DynamicList::Reader) &&
-                    __has_trivial_copy(DynamicEnum) &&
-                    __has_trivial_copy(DynamicStruct::Reader) &&
-                    __has_trivial_copy(AnyPointer::Reader),
+      static_assert(kj::canMemcpy<Text::Reader>() &&
+                    kj::canMemcpy<Data::Reader>() &&
+                    kj::canMemcpy<DynamicList::Reader>() &&
+                    kj::canMemcpy<DynamicEnum>() &&
+                    kj::canMemcpy<DynamicStruct::Reader>() &&
+                    kj::canMemcpy<AnyPointer::Reader>(),
                     "Assumptions here don't hold.");
       break;
 
@@ -1486,7 +1486,7 @@ DynamicValue::Builder::Builder(Builder& other) {
     case ENUM:
     case STRUCT:
     case ANY_POINTER:
-      // Unfortunately __has_trivial_copy doesn't work on these types due to the use of
+      // Unfortunately canMemcpy() doesn't work on these types due to the use of
       // DisallowConstCopy, but __has_trivial_destructor should detect if any of these types
       // become non-trivial.
       static_assert(__has_trivial_destructor(Text::Builder) &&

c++/src/capnp/layout.c++

@@ -1830,7 +1830,12 @@ struct WireHelpers {
       SegmentReader* segment, const WirePointer* ref, int nestingLimit)) {
     kj::Maybe<kj::Own<ClientHook>> maybeCap;
 
-    if (ref->isNull()) {
+    if (segment == nullptr) {
+      // No capability context for unchecked messages.
+      // TODO(now):  This means a capability read from an omitted (and therefore default-valued)
+      //   sub-message will throw a fatal exception.
+      maybeCap = nullptr;
+    } else if (ref->isNull()) {
       maybeCap = segment->getArena()->newBrokenCap("Calling null capability pointer.");
     } else if (!ref->isCapability()) {
       KJ_FAIL_REQUIRE(

c++/src/capnp/pointer-helpers.h

@@ -77,8 +77,7 @@ struct PointerHelpers<List<T>, Kind::LIST> {
   static inline void set(PointerBuilder builder, typename List<T>::Reader value) {
     builder.setList(value.reader);
   }
-  template <typename U>
-  static void set(PointerBuilder builder, std::initializer_list<U> value) {
+  static void set(PointerBuilder builder, kj::ArrayPtr<const ReaderFor<T>> value) {
     auto l = init(builder, value.size());
     uint i = 0;
     for (auto& element: value) {

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

@@ -723,9 +723,9 @@ private:
           fork(eventual.fork()),
           resolveSelfPromise(fork.addBranch().then(
               [this](kj::Own<ClientHook>&& resolution) {
-                resolve(kj::mv(resolution));
+                resolve(kj::mv(resolution), false);
               }, [this](kj::Exception&& exception) {
-                resolve(newBrokenCap(kj::mv(exception)));
+                resolve(newBrokenCap(kj::mv(exception)), true);
               }).eagerlyEvaluate([&](kj::Exception&& e) {
                 // Make any exceptions thrown from resolve() go to the connection's TaskSet which
                 // will cause the connection to be terminated.
@@ -810,8 +810,8 @@ private:
 
     bool receivedCall = false;
 
-    void resolve(kj::Own<ClientHook> replacement) {
-      if (replacement->getBrand() != connectionState.get() && receivedCall) {
+    void resolve(kj::Own<ClientHook> replacement, bool isError) {
+      if (replacement->getBrand() != connectionState.get() && receivedCall && !isError) {
         // The new capability is hosted locally, not on the remote machine.  And, we had made calls
         // to the promise.  We need to make sure those calls echo back to us before we allow new
         // calls to go directly to the local capability, so we need to set a local embargo and send

c++/src/kj/array.h

@@ -570,8 +570,7 @@ T* HeapArrayDisposer::allocateUninitialized(size_t count) {
   return Allocate_<T, true, true>::allocate(0, count);
 }
 
-template <typename Element, typename Iterator,
-          bool trivial = __has_trivial_copy(Element) && __has_trivial_assign(Element)>
+template <typename Element, typename Iterator, bool = canMemcpy<Element>()>
 struct CopyConstructArray_;
 
 template <typename T>

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

@@ -172,6 +172,9 @@ protected:
 
 class ImmediatePromiseNodeBase: public PromiseNode {
 public:
+  ImmediatePromiseNodeBase();
+  ~ImmediatePromiseNodeBase() noexcept(false);
+
   void onReady(Event& event) noexcept override;
 };
 
@@ -480,6 +483,70 @@ private:
 
 // -------------------------------------------------------------------
 
+class ArrayJoinPromiseNodeBase: public PromiseNode {
+public:
+  ArrayJoinPromiseNodeBase(Array<Own<PromiseNode>> promises,
+                           ExceptionOrValue* resultParts, size_t partSize);
+  ~ArrayJoinPromiseNodeBase() noexcept(false);
+
+  void onReady(Event& event) noexcept override final;
+  void get(ExceptionOrValue& output) noexcept override final;
+  PromiseNode* getInnerForTrace() override final;
+
+protected:
+  virtual void getNoError(ExceptionOrValue& output) noexcept = 0;
+  // Called to compile the result only in the case where there were no errors.
+
+private:
+  uint countLeft;
+  OnReadyEvent onReadyEvent;
+
+  class Branch: public Event {
+  public:
+    Branch(ArrayJoinPromiseNodeBase& joinNode, Own<PromiseNode> dependency,
+           ExceptionOrValue& output);
+    ~Branch() noexcept(false);
+
+    Maybe<Own<Event>> fire() override;
+    _::PromiseNode* getInnerForTrace() override;
+
+    Maybe<Exception> getPart();
+    // Calls dependency->get(output).  If there was an exception, return it.
+
+  private:
+    ArrayJoinPromiseNodeBase& joinNode;
+    Own<PromiseNode> dependency;
+    ExceptionOrValue& output;
+  };
+
+  Array<Branch> branches;
+};
+
+template <typename T>
+class ArrayJoinPromiseNode final: public ArrayJoinPromiseNodeBase {
+public:
+  ArrayJoinPromiseNode(Array<Own<PromiseNode>> promises,
+                       Array<ExceptionOr<T>> resultParts)
+      : ArrayJoinPromiseNodeBase(kj::mv(promises), resultParts.begin(), sizeof(ExceptionOr<T>)),
+        resultParts(kj::mv(resultParts)) {}
+
+protected:
+  void getNoError(ExceptionOrValue& output) noexcept override {
+    auto builder = heapArrayBuilder<T>(resultParts.size());
+    for (auto& part: resultParts) {
+      KJ_IASSERT(part.value != nullptr,
+                 "Bug in KJ promise framework:  Promise result had neither value no exception.");
+      builder.add(kj::mv(*_::readMaybe(part.value)));
+    }
+    output.as<Array<T>>() = builder.finish();
+  }
+
+private:
+  Array<ExceptionOr<T>> resultParts;
+};
+
+// -------------------------------------------------------------------
+
 class EagerPromiseNodeBase: public PromiseNode, protected Event {
   // A PromiseNode that eagerly evaluates its dependency even if its dependent does not eagerly
   // evaluate it.
@@ -682,6 +749,13 @@ void Promise<void>::detach(ErrorFunc&& errorHandler) {
   return _::detach(then([]() {}, kj::fwd<ErrorFunc>(errorHandler)));
 }
 
+template <typename T>
+Promise<Array<T>> joinPromises(Array<Promise<T>>&& promises) {
+  return Promise<Array<T>>(false, kj::heap<_::ArrayJoinPromiseNode<T>>(
+      KJ_MAP(p, promises) { return kj::mv(p.node); },
+      heapArray<_::ExceptionOr<T>>(promises.size())));
+}
+
 // =======================================================================================
 
 namespace _ {  // private

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

@@ -34,6 +34,7 @@
 #include <sys/socket.h>
 #include <sys/un.h>
 #include <netinet/in.h>
+#include <netinet/tcp.h>
 #include <stddef.h>
 #include <stdlib.h>
 #include <arpa/inet.h>
@@ -294,11 +295,26 @@ public:
   }
 
   int socket(int type) const {
+    bool isStream = type == SOCK_STREAM;
+
     int result;
 #if __linux__
     type |= SOCK_NONBLOCK | SOCK_CLOEXEC;
 #endif
     KJ_SYSCALL(result = ::socket(addr.generic.sa_family, type, 0));
+
+    if (isStream && (addr.generic.sa_family == AF_INET ||
+                     addr.generic.sa_family == AF_INET6)) {
+      // TODO(0.5):  As a hack for the 0.4 release we are always setting
+      //   TCP_NODELAY because Nagle's algorithm pretty much kills Cap'n Proto's
+      //   RPC protocol.  Later, we should extend the interface to provide more
+      //   control over this.  Perhaps write() should have a flag which
+      //   specifies whether to pass MSG_MORE.
+      int one = 1;
+      KJ_SYSCALL(setsockopt(
+          result, IPPROTO_TCP, TCP_NODELAY, (char*)&one, sizeof(one)));
+    }
+
     return result;
   }
 
@@ -642,7 +658,7 @@ Promise<Array<SocketAddress>> SocketAddress::lookupHost(
           addr.addrlen = cur->ai_addrlen;
           memcpy(&addr.addr.generic, cur->ai_addr, cur->ai_addrlen);
         }
-        static_assert(__has_trivial_copy(SocketAddress), "Can't write() SocketAddress...");
+        static_assert(canMemcpy<SocketAddress>(), "Can't write() SocketAddress...");
         output.write(&addr, sizeof(addr));
         cur = cur->ai_next;
       }

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

@@ -36,6 +36,9 @@ template <typename T>
 class Promise;
 class WaitScope;
 
+template <typename T>
+Promise<Array<T>> joinPromises(Array<Promise<T>>&& promises);
+
 namespace _ {  // private
 
 template <typename T> struct JoinPromises_ { typedef T Type; };
@@ -44,6 +47,9 @@ template <typename T> struct JoinPromises_<Promise<T>> { typedef T Type; };
 template <typename T>
 using JoinPromises = typename JoinPromises_<T>::Type;
 // If T is Promise<U>, resolves to U, otherwise resolves to T.
+//
+// TODO(cleanup):  Rename to avoid confusion with joinPromises() call which is completely
+//   unrelated.
 
 class PropagateException {
   // A functor which accepts a kj::Exception as a parameter and returns a broken promise of
@@ -171,6 +177,8 @@ private:
   template <typename>
   friend class kj::Promise;
   friend class TaskSetImpl;
+  template <typename U>
+  friend Promise<Array<U>> kj::joinPromises(Array<Promise<U>>&& promises);
 };
 
 void detach(kj::Promise<void>&& promise);

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

@@ -514,6 +514,25 @@ TEST(Async, ExclusiveJoin) {
   }
 }
 
+TEST(Async, ArrayJoin) {
+  EventLoop loop;
+  WaitScope waitScope(loop);
+
+  auto builder = heapArrayBuilder<Promise<int>>(3);
+  builder.add(123);
+  builder.add(456);
+  builder.add(789);
+
+  Promise<Array<int>> promise = joinPromises(builder.finish());
+
+  auto result = promise.wait(waitScope);
+
+  ASSERT_EQ(3u, result.size());
+  EXPECT_EQ(123, result[0]);
+  EXPECT_EQ(456, result[1]);
+  EXPECT_EQ(789, result[2]);
+}
+
 class ErrorHandlerImpl: public TaskSet::ErrorHandler {
 public:
   uint exceptionCount = 0;

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

@@ -54,8 +54,6 @@ public:
   Promise<short> onFdEvent(int fd, short eventMask);
   // `eventMask` is a bitwise-OR of poll events (e.g. `POLLIN`, `POLLOUT`, etc.).  The next time
   // one or more of the given events occurs on `fd`, the set of events that occurred are returned.
-  //
-  // The result of waiting on the same FD twice at once is undefined.
 
   Promise<siginfo_t> onSignal(int signum);
   // When the given signal is delivered to this thread, return the corresponding siginfo_t.

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

@@ -508,6 +508,9 @@ void PromiseNode::OnReadyEvent::arm() {
 
 // -------------------------------------------------------------------
 
+ImmediatePromiseNodeBase::ImmediatePromiseNodeBase() {}
+ImmediatePromiseNodeBase::~ImmediatePromiseNodeBase() noexcept(false) {}
+
 void ImmediatePromiseNodeBase::onReady(Event& event) noexcept {
   event.armBreadthFirst();
 }
@@ -807,6 +810,73 @@ PromiseNode* ExclusiveJoinPromiseNode::Branch::getInnerForTrace() {
 
 // -------------------------------------------------------------------
 
+ArrayJoinPromiseNodeBase::ArrayJoinPromiseNodeBase(
+    Array<Own<PromiseNode>> promises, ExceptionOrValue* resultParts, size_t partSize)
+    : countLeft(promises.size()) {
+  // Make the branches.
+  auto builder = heapArrayBuilder<Branch>(promises.size());
+  for (uint i: indices(promises)) {
+    ExceptionOrValue& output = *reinterpret_cast<ExceptionOrValue*>(
+        reinterpret_cast<byte*>(resultParts) + i * partSize);
+    builder.add(*this, kj::mv(promises[i]), output);
+  }
+  branches = builder.finish();
+
+  if (branches.size() == 0) {
+    onReadyEvent.arm();
+  }
+}
+ArrayJoinPromiseNodeBase::~ArrayJoinPromiseNodeBase() noexcept(false) {}
+
+void ArrayJoinPromiseNodeBase::onReady(Event& event) noexcept {
+  onReadyEvent.init(event);
+}
+
+void ArrayJoinPromiseNodeBase::get(ExceptionOrValue& output) noexcept {
+  // If any of the elements threw exceptions, propagate them.
+  for (auto& branch: branches) {
+    KJ_IF_MAYBE(exception, branch.getPart()) {
+      output.addException(kj::mv(*exception));
+    }
+  }
+
+  if (output.exception == nullptr) {
+    // No errors.  The template subclass will need to fill in the result.
+    getNoError(output);
+  }
+}
+
+PromiseNode* ArrayJoinPromiseNodeBase::getInnerForTrace() {
+  return branches.size() == 0 ? nullptr : branches[0].getInnerForTrace();
+}
+
+ArrayJoinPromiseNodeBase::Branch::Branch(
+    ArrayJoinPromiseNodeBase& joinNode, Own<PromiseNode> dependencyParam, ExceptionOrValue& output)
+    : joinNode(joinNode), dependency(kj::mv(dependencyParam)), output(output) {
+  dependency->setSelfPointer(&dependency);
+  dependency->onReady(*this);
+}
+
+ArrayJoinPromiseNodeBase::Branch::~Branch() noexcept(false) {}
+
+Maybe<Own<Event>> ArrayJoinPromiseNodeBase::Branch::fire() {
+  if (--joinNode.countLeft == 0) {
+    joinNode.onReadyEvent.arm();
+  }
+  return nullptr;
+}
+
+_::PromiseNode* ArrayJoinPromiseNodeBase::Branch::getInnerForTrace() {
+  return dependency->getInnerForTrace();
+}
+
+Maybe<Exception> ArrayJoinPromiseNodeBase::Branch::getPart() {
+  dependency->get(output);
+  return kj::mv(output.exception);
+}
+
+// -------------------------------------------------------------------
+
 EagerPromiseNodeBase::EagerPromiseNodeBase(
     Own<PromiseNode>&& dependencyParam, ExceptionOrValue& resultRef)
     : dependency(kj::mv(dependencyParam)), resultRef(resultRef) {

c++/src/kj/async.h

@@ -290,6 +290,8 @@ private:
   friend class _::TaskSetImpl;
   friend Promise<void> _::yield();
   friend class _::NeverDone;
+  template <typename U>
+  friend Promise<Array<U>> joinPromises(Array<Promise<U>>&& promises);
 };
 
 template <typename T>
@@ -339,6 +341,10 @@ PromiseForResult<Func, void> evalLater(Func&& func);
 // If you schedule several evaluations with `evalLater` during the same callback, they are
 // guaranteed to be executed in order.
 
+template <typename T>
+Promise<Array<T>> joinPromises(Array<Promise<T>>&& promises);
+// Join an array of promises into a promise for an array.
+
 // =======================================================================================
 // Hack for creating a lambda that holds an owned pointer.
 

c++/src/kj/common.h

@@ -25,8 +25,6 @@
 //
 // This defines very simple utilities that are widely applicable.
 
-#include <stddef.h>
-
 #ifndef KJ_COMMON_H_
 #define KJ_COMMON_H_
 
@@ -69,6 +67,9 @@
 #endif
 #endif
 
+#include <stddef.h>
+#include <initializer_list>
+
 // =======================================================================================
 
 namespace kj {
@@ -344,6 +345,29 @@ constexpr bool canConvert() {
   return sizeof(CanConvert_<U>::sfinae(instance<T>())) == sizeof(int);
 }
 
+#if __clang__
+template <typename T>
+constexpr bool canMemcpy() {
+  // Returns true if T can be copied using memcpy instead of using the copy constructor or
+  // assignment operator.
+
+  // Clang unhelpfully defines __has_trivial_{copy,assign}(T) to be true if the copy constructor /
+  // assign operator are deleted, on the basis that a strict reading of the definition of "trivial"
+  // according to the standard says that deleted functions are in fact trivial.  Meanwhile Clang
+  // provides these admittedly-better intrinsics, but GCC does not.
+  return __is_trivially_constructible(T, const T&) && __is_trivially_assignable(T, const T&);
+}
+#else
+template <typename T>
+constexpr bool canMemcpy() {
+  // Returns true if T can be copied using memcpy instead of using the copy constructor or
+  // assignment operator.
+
+  // GCC defines these to mean what we want them to mean.
+  return __has_trivial_copy(T) && __has_trivial_assign(T);
+}
+#endif
+
 // =======================================================================================
 // Equivalents to std::move() and std::forward(), since these are very commonly needed and the
 // std header <utility> pulls in lots of other stuff.
@@ -917,6 +941,8 @@ public:
   inline constexpr ArrayPtr(decltype(nullptr)): ptr(nullptr), size_(0) {}
   inline constexpr ArrayPtr(T* ptr, size_t size): ptr(ptr), size_(size) {}
   inline constexpr ArrayPtr(T* begin, T* end): ptr(begin), size_(end - begin) {}
+  inline constexpr ArrayPtr(std::initializer_list<RemoveConstOrDisable<T>> init)
+      : ptr(init.begin()), size_(init.size()) {}
 
   template <size_t size>
   inline constexpr ArrayPtr(T (&native)[size]): ptr(native), size_(size) {}

mega-test.cfg

-linux-gcc-4.7     14763 ./super-test.sh tmpdir capnp-gcc-4.7
-linux-gcc-4.8     13108 ./super-test.sh tmpdir capnp-gcc-4.8 gcc-4.8
-linux-clang       14891 ./super-test.sh tmpdir capnp-clang clang
+linux-gcc-4.7     14779 ./super-test.sh tmpdir capnp-gcc-4.7
+linux-gcc-4.8     13122 ./super-test.sh tmpdir capnp-gcc-4.8 gcc-4.8
+linux-clang       14907 ./super-test.sh tmpdir capnp-clang clang
 mac                5740 ./super-test.sh remote beat caffeinate
 cygwin             6762 ./super-test.sh remote Kenton@flashman