Merge pull request #558 from capnproto/capability-stream

Add AsyncCapabilityStream, an abstraction of unix sockets SCM_RIGHTS FD passing.

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

@@ -192,6 +192,47 @@ TEST(AsyncIo, TwoWayPipe) {
   EXPECT_EQ("bar", result2);
 }
 
+#if !_WIN32
+TEST(AsyncIo, CapabilityPipe) {
+  auto ioContext = setupAsyncIo();
+
+  auto pipe = ioContext.provider->newCapabilityPipe();
+  auto pipe2 = ioContext.provider->newCapabilityPipe();
+  char receiveBuffer1[4];
+  char receiveBuffer2[4];
+
+  // Expect to receive a stream, then write "bar" to it, then receive "foo" from it.
+  Own<AsyncCapabilityStream> receivedStream;
+  auto promise = pipe2.ends[1]->receiveStream()
+      .then([&](Own<AsyncCapabilityStream> stream) {
+    receivedStream = kj::mv(stream);
+    return receivedStream->write("bar", 3);
+  }).then([&]() {
+    return receivedStream->tryRead(receiveBuffer2, 3, 4);
+  }).then([&](size_t n) {
+    EXPECT_EQ(3u, n);
+    return heapString(receiveBuffer2, n);
+  });
+
+  // Send a stream, then write "foo" to the other end of the sent stream, then receive "bar"
+  // from it.
+  kj::String result = pipe2.ends[0]->sendStream(kj::mv(pipe.ends[1]))
+      .then([&]() {
+    return pipe.ends[0]->write("foo", 3);
+  }).then([&]() {
+    return pipe.ends[0]->tryRead(receiveBuffer1, 3, 4);
+  }).then([&](size_t n) {
+    EXPECT_EQ(3u, n);
+    return heapString(receiveBuffer1, n);
+  }).wait(ioContext.waitScope);
+
+  kj::String result2 = promise.wait(ioContext.waitScope);
+
+  EXPECT_EQ("bar", result);
+  EXPECT_EQ("foo", result2);
+}
+#endif
+
 TEST(AsyncIo, PipeThread) {
   auto ioContext = setupAsyncIo();
 

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

@@ -45,25 +45,35 @@
 #include <set>
 #include <poll.h>
 #include <limits.h>
+#include <sys/ioctl.h>
 
 namespace kj {
 
 namespace {
 
 void setNonblocking(int fd) {
+#ifdef FIONBIO
+  int opt = 1;
+  KJ_SYSCALL(ioctl(fd, FIONBIO, &opt));
+#else
   int flags;
   KJ_SYSCALL(flags = fcntl(fd, F_GETFL));
   if ((flags & O_NONBLOCK) == 0) {
     KJ_SYSCALL(fcntl(fd, F_SETFL, flags | O_NONBLOCK));
   }
+#endif
 }
 
 void setCloseOnExec(int fd) {
+#ifdef FIOCLEX
+  KJ_SYSCALL(ioctl(fd, FIOCLEX));
+#else
   int flags;
   KJ_SYSCALL(flags = fcntl(fd, F_GETFD));
   if ((flags & FD_CLOEXEC) == 0) {
     KJ_SYSCALL(fcntl(fd, F_SETFD, flags | FD_CLOEXEC));
   }
+#endif
 }
 
 static constexpr uint NEW_FD_FLAGS =
@@ -112,10 +122,11 @@ private:
 
 // =======================================================================================
 
-class AsyncStreamFd: public OwnedFileDescriptor, public AsyncIoStream {
+class AsyncStreamFd: public OwnedFileDescriptor, public AsyncCapabilityStream {
 public:
   AsyncStreamFd(UnixEventPort& eventPort, int fd, uint flags)
       : OwnedFileDescriptor(fd, flags),
+        eventPort(eventPort),
         observer(eventPort, fd, UnixEventPort::FdObserver::OBSERVE_READ_WRITE) {}
   virtual ~AsyncStreamFd() noexcept(false) {}
 
@@ -198,6 +209,57 @@ public:
     *length = socklen;
   }
 
+  kj::Promise<Maybe<Own<AsyncCapabilityStream>>> tryReceiveStream() override {
+    return tryReceiveFdImpl<Own<AsyncCapabilityStream>>();
+  }
+
+  kj::Promise<void> sendStream(Own<AsyncCapabilityStream> stream) override {
+    auto downcasted = stream.downcast<AsyncStreamFd>();
+    auto promise = sendFd(downcasted->fd);
+    return promise.attach(kj::mv(downcasted));
+  }
+
+  kj::Promise<kj::Maybe<AutoCloseFd>> tryReceiveFd() override {
+    return tryReceiveFdImpl<AutoCloseFd>();
+  }
+
+  kj::Promise<void> sendFd(int fdToSend) override {
+    struct msghdr msg;
+    struct iovec iov;
+    union {
+      struct cmsghdr cmsg;
+      char cmsgSpace[CMSG_LEN(sizeof(int))];
+    };
+    memset(&msg, 0, sizeof(msg));
+    memset(&iov, 0, sizeof(iov));
+    memset(cmsgSpace, 0, sizeof(cmsgSpace));
+
+    char c = 0;
+    iov.iov_base = &c;
+    iov.iov_len = 1;
+    msg.msg_iov = &iov;
+    msg.msg_iovlen = 1;
+
+    msg.msg_control = &cmsg;
+    msg.msg_controllen = sizeof(cmsgSpace);
+
+    cmsg.cmsg_len = sizeof(cmsgSpace);
+    cmsg.cmsg_level = SOL_SOCKET;
+    cmsg.cmsg_type = SCM_RIGHTS;
+    *reinterpret_cast<int*>(CMSG_DATA(&cmsg)) = fdToSend;
+
+    ssize_t n;
+    KJ_NONBLOCKING_SYSCALL(n = sendmsg(fd, &msg, 0));
+    if (n < 0) {
+      return observer.whenBecomesWritable().then([this,fdToSend]() {
+        return sendFd(fdToSend);
+      });
+    } else {
+      KJ_ASSERT(n == 1);
+      return kj::READY_NOW;
+    }
+  }
+
   Promise<void> waitConnected() {
     // Wait until initial connection has completed. This actually just waits until it is writable.
 
@@ -222,6 +284,7 @@ public:
   }
 
 private:
+  UnixEventPort& eventPort;
   UnixEventPort::FdObserver observer;
 
   Promise<size_t> tryReadInternal(void* buffer, size_t minBytes, size_t maxBytes,
@@ -354,6 +417,71 @@ private:
       }
     }
   }
+
+  template <typename T>
+  kj::Promise<kj::Maybe<T>> tryReceiveFdImpl() {
+    struct msghdr msg;
+    memset(&msg, 0, sizeof(msg));
+
+    struct iovec iov;
+    memset(&iov, 0, sizeof(iov));
+    char c;
+    iov.iov_base = &c;
+    iov.iov_len = 1;
+    msg.msg_iov = &iov;
+    msg.msg_iovlen = 1;
+
+    // Allocate space to receive a cmsg.
+    union {
+      struct cmsghdr cmsg;
+      char cmsgSpace[CMSG_SPACE(sizeof(int))];
+    };
+    msg.msg_control = &cmsg;
+    msg.msg_controllen = sizeof(cmsgSpace);
+
+#ifdef MSG_CMSG_CLOEXEC
+    int recvmsgFlags = MSG_CMSG_CLOEXEC;
+#else
+    int recvmsgFlags = 0;
+#endif
+
+    ssize_t n;
+    KJ_NONBLOCKING_SYSCALL(n = recvmsg(fd, &msg, recvmsgFlags));
+    if (n < 0) {
+      return observer.whenBecomesReadable().then([this]() {
+        return tryReceiveFdImpl<T>();
+      });
+    } else if (n == 0) {
+      return kj::Maybe<T>(nullptr);
+    } else {
+      KJ_REQUIRE(msg.msg_controllen >= sizeof(cmsg),
+          "expected to receive FD over socket; received data instead");
+
+      // We expect an SCM_RIGHTS message with a single FD.
+      KJ_REQUIRE(cmsg.cmsg_level == SOL_SOCKET);
+      KJ_REQUIRE(cmsg.cmsg_type == SCM_RIGHTS);
+      KJ_REQUIRE(cmsg.cmsg_len == CMSG_LEN(sizeof(int)));
+
+      int receivedFd;
+      memcpy(&receivedFd, CMSG_DATA(&cmsg), sizeof(receivedFd));
+      return kj::Maybe<T>(wrapFd(receivedFd, (T*)nullptr));
+    }
+  }
+
+  AutoCloseFd wrapFd(int newFd, AutoCloseFd*) {
+    auto result = AutoCloseFd(newFd);
+#ifndef MSG_CMSG_CLOEXEC
+    setCloseOnExec(result);
+#endif
+    return result;
+  }
+  Own<AsyncCapabilityStream> wrapFd(int newFd, Own<AsyncCapabilityStream>*) {
+    return kj::heap<AsyncStreamFd>(eventPort, newFd,
+#ifdef MSG_CMSG_CLOEXEC
+        LowLevelAsyncIoProvider::ALREADY_CLOEXEC |
+#endif
+        LowLevelAsyncIoProvider::TAKE_OWNERSHIP);
+  }
 };
 
 // =======================================================================================
@@ -951,6 +1079,9 @@ public:
   Own<AsyncIoStream> wrapSocketFd(int fd, uint flags = 0) override {
     return heap<AsyncStreamFd>(eventPort, fd, flags);
   }
+  Own<AsyncCapabilityStream> wrapUnixSocketFd(Fd fd, uint flags = 0) override {
+    return heap<AsyncStreamFd>(eventPort, fd, flags);
+  }
   Promise<Own<AsyncIoStream>> wrapConnectingSocketFd(
       int fd, const struct sockaddr* addr, uint addrlen, uint flags = 0) override {
     // Unfortunately connect() doesn't fit the mold of KJ_NONBLOCKING_SYSCALL, since it indicates
@@ -1375,6 +1506,19 @@ public:
     } };
   }
 
+  CapabilityPipe newCapabilityPipe() override {
+    int fds[2];
+    int type = SOCK_STREAM;
+#if __linux__ && !__BIONIC__
+    type |= SOCK_NONBLOCK | SOCK_CLOEXEC;
+#endif
+    KJ_SYSCALL(socketpair(AF_UNIX, type, 0, fds));
+    return CapabilityPipe { {
+      lowLevel.wrapUnixSocketFd(fds[0], NEW_FD_FLAGS),
+      lowLevel.wrapUnixSocketFd(fds[1], NEW_FD_FLAGS)
+    } };
+  }
+
   Network& getNetwork() override {
     return network;
   }

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

@@ -29,6 +29,7 @@
 #include "async-io-internal.h"
 #include "debug.h"
 #include "vector.h"
+#include "io.h"
 
 #if _WIN32
 #include <winsock2.h>
@@ -181,6 +182,34 @@ Maybe<Promise<uint64_t>> AsyncOutputStream::tryPumpFrom(
   return nullptr;
 }
 
+Promise<Own<AsyncCapabilityStream>> AsyncCapabilityStream::receiveStream() {
+  return tryReceiveStream()
+      .then([](Maybe<Own<AsyncCapabilityStream>>&& result)
+            -> Promise<Own<AsyncCapabilityStream>> {
+    KJ_IF_MAYBE(r, result) {
+      return kj::mv(*r);
+    } else {
+      return KJ_EXCEPTION(FAILED, "EOF when expecting to receive capability");
+    }
+  });
+}
+
+Promise<AutoCloseFd> AsyncCapabilityStream::receiveFd() {
+  return tryReceiveFd().then([](Maybe<AutoCloseFd>&& result) -> Promise<AutoCloseFd> {
+    KJ_IF_MAYBE(r, result) {
+      return kj::mv(*r);
+    } else {
+      return KJ_EXCEPTION(FAILED, "EOF when expecting to receive capability");
+    }
+  });
+}
+Promise<Maybe<AutoCloseFd>> AsyncCapabilityStream::tryReceiveFd() {
+  return KJ_EXCEPTION(UNIMPLEMENTED, "this stream cannot receive file descriptors");
+}
+Promise<void> AsyncCapabilityStream::sendFd(int fd) {
+  return KJ_EXCEPTION(UNIMPLEMENTED, "this stream cannot send file descriptors");
+}
+
 void AsyncIoStream::getsockopt(int level, int option, void* value, uint* length) {
   KJ_UNIMPLEMENTED("Not a socket.");
 }
@@ -212,6 +241,43 @@ Own<DatagramPort> LowLevelAsyncIoProvider::wrapDatagramSocketFd(
     Fd fd, LowLevelAsyncIoProvider::NetworkFilter& filter, uint flags) {
   KJ_UNIMPLEMENTED("Datagram sockets not implemented.");
 }
+CapabilityPipe AsyncIoProvider::newCapabilityPipe() {
+  KJ_UNIMPLEMENTED("Capability pipes not implemented.");
+}
+
+Own<AsyncInputStream> LowLevelAsyncIoProvider::wrapInputFd(OwnFd fd, uint flags) {
+  return wrapInputFd(reinterpret_cast<Fd>(fd.release()), flags | TAKE_OWNERSHIP);
+}
+Own<AsyncOutputStream> LowLevelAsyncIoProvider::wrapOutputFd(OwnFd fd, uint flags) {
+  return wrapOutputFd(reinterpret_cast<Fd>(fd.release()), flags | TAKE_OWNERSHIP);
+}
+Own<AsyncIoStream> LowLevelAsyncIoProvider::wrapSocketFd(OwnFd fd, uint flags) {
+  return wrapSocketFd(reinterpret_cast<Fd>(fd.release()), flags | TAKE_OWNERSHIP);
+}
+#if !_WIN32
+Own<AsyncCapabilityStream> LowLevelAsyncIoProvider::wrapUnixSocketFd(OwnFd fd, uint flags) {
+  return wrapUnixSocketFd(reinterpret_cast<Fd>(fd.release()), flags | TAKE_OWNERSHIP);
+}
+#endif
+Promise<Own<AsyncIoStream>> LowLevelAsyncIoProvider::wrapConnectingSocketFd(
+    OwnFd fd, const struct sockaddr* addr, uint addrlen, uint flags) {
+  return wrapConnectingSocketFd(reinterpret_cast<Fd>(fd.release()), addr, addrlen,
+                                flags | TAKE_OWNERSHIP);
+}
+Own<ConnectionReceiver> LowLevelAsyncIoProvider::wrapListenSocketFd(
+    OwnFd fd, NetworkFilter& filter, uint flags) {
+  return wrapListenSocketFd(reinterpret_cast<Fd>(fd.release()), filter, flags | TAKE_OWNERSHIP);
+}
+Own<ConnectionReceiver> LowLevelAsyncIoProvider::wrapListenSocketFd(OwnFd fd, uint flags) {
+  return wrapListenSocketFd(reinterpret_cast<Fd>(fd.release()), flags | TAKE_OWNERSHIP);
+}
+Own<DatagramPort> LowLevelAsyncIoProvider::wrapDatagramSocketFd(
+    OwnFd fd, NetworkFilter& filter, uint flags) {
+  return wrapDatagramSocketFd(reinterpret_cast<Fd>(fd.release()), filter, flags | TAKE_OWNERSHIP);
+}
+Own<DatagramPort> LowLevelAsyncIoProvider::wrapDatagramSocketFd(OwnFd fd, uint flags) {
+  return wrapDatagramSocketFd(reinterpret_cast<Fd>(fd.release()), flags | TAKE_OWNERSHIP);
+}
 
 namespace {
 
@@ -227,6 +293,39 @@ LowLevelAsyncIoProvider::NetworkFilter& LowLevelAsyncIoProvider::NetworkFilter::
   return result;
 }
 
+// =======================================================================================
+// Convenience adapters.
+
+Promise<Own<AsyncIoStream>> CapabilityStreamConnectionReceiver::accept() {
+  return inner.receiveStream()
+      .then([](Own<AsyncCapabilityStream>&& stream) -> Own<AsyncIoStream> {
+    return kj::mv(stream);
+  });
+}
+
+uint CapabilityStreamConnectionReceiver::getPort() {
+  return 0;
+}
+
+Promise<Own<AsyncIoStream>> CapabilityStreamNetworkAddress::connect() {
+  auto pipe = provider.newCapabilityPipe();
+  auto result = kj::mv(pipe.ends[0]);
+  return inner.sendStream(kj::mv(pipe.ends[1]))
+      .then(kj::mvCapture(result, [](Own<AsyncIoStream>&& result) {
+    return kj::mv(result);
+  }));
+}
+Own<ConnectionReceiver> CapabilityStreamNetworkAddress::listen() {
+  return kj::heap<CapabilityStreamConnectionReceiver>(inner);
+}
+
+Own<NetworkAddress> CapabilityStreamNetworkAddress::clone() {
+  KJ_UNIMPLEMENTED("can't clone CapabilityStreamNetworkAddress");
+}
+String CapabilityStreamNetworkAddress::toString() {
+  return kj::str("<CapabilityStreamNetworkAddress>");
+}
+
 // =======================================================================================
 
 namespace _ {  // private

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

@@ -37,12 +37,15 @@ namespace kj {
 
 #if _WIN32
 class Win32EventPort;
+class AutoCloseHandle;
 #else
 class UnixEventPort;
 #endif
 
+class AutoCloseFd;
 class NetworkAddress;
 class AsyncOutputStream;
+class AsyncIoStream;
 
 // =======================================================================================
 // Streaming I/O
@@ -130,6 +133,42 @@ public:
   // ephemeral addresses for a single connection.
 };
 
+class AsyncCapabilityStream: public AsyncIoStream {
+  // An AsyncIoStream that also allows sending and receiving new connections or other kinds of
+  // capabilities, in addition to simple data.
+  //
+  // For correct functioning, a protocol must be designed such that the receiver knows when to
+  // expect a capability transfer. The receiver must not read() when a capability is expected, and
+  // must not receiveStream() when data is expected -- if it does, an exception may be thrown or
+  // invalid data may be returned. This implies that data sent over an AsyncCapabilityStream must
+  // be framed such that the receiver knows exactly how many bytes to read before receiving a
+  // capability.
+  //
+  // On Unix, KJ provides an implementation based on Unix domain sockets and file descriptor
+  // passing via SCM_RIGHTS. Due to the nature of SCM_RIGHTS, if the application accidentally
+  // read()s when it should have called receiveStream(), it will observe a NUL byte in the data
+  // and the capability will be discarded. Of course, an application should not depend on this
+  // behavior; it should avoid read()ing through a capability.
+  //
+  // KJ does not provide any implementation of this type on Windows, as there's no obvious
+  // implementation there. Handle passing on Windows requires at least one of the processes
+  // involved to have permission to modify the other's handle table, which is effectively full
+  // control. Handle passing between mutually non-trusting processes would require a trusted
+  // broker process to facilitate. One could possibly implement this type in terms of such a
+  // broker, or in terms of direct handle passing if at least one process trusts the other.
+
+public:
+  Promise<Own<AsyncCapabilityStream>> receiveStream();
+  virtual Promise<Maybe<Own<AsyncCapabilityStream>>> tryReceiveStream() = 0;
+  virtual Promise<void> sendStream(Own<AsyncCapabilityStream> stream) = 0;
+  // Transfer a stream.
+
+  Promise<AutoCloseFd> receiveFd();
+  virtual Promise<Maybe<AutoCloseFd>> tryReceiveFd();
+  virtual Promise<void> sendFd(int fd);
+  // Transfer a raw file descriptor. Default implementation throws UNIMPLEMENTED.
+};
+
 struct OneWayPipe {
   // A data pipe with an input end and an output end.  (Typically backed by pipe() system call.)
 
@@ -144,6 +183,12 @@ struct TwoWayPipe {
   Own<AsyncIoStream> ends[2];
 };
 
+struct CapabilityPipe {
+  // Like TwoWayPipe but allowing capability-passing.
+
+  Own<AsyncCapabilityStream> ends[2];
+};
+
 class ConnectionReceiver {
   // Represents a server socket listening on a port.
 
@@ -401,6 +446,13 @@ public:
   // Creates two AsyncIoStreams representing the two ends of a two-way pipe (e.g. created with
   // socketpair(2) system call).  Data written to one end can be read from the other.
 
+  virtual CapabilityPipe newCapabilityPipe();
+  // Creates two AsyncCapabilityStreams representing the two ends of a two-way capability pipe.
+  //
+  // The default implementation throws an unimplemented exception. In particular this is not
+  // implemented by the default AsyncIoProvider on Windows, since Windows lacks any sane way to
+  // pass handles over a stream.
+
   virtual Network& getNetwork() = 0;
   // Creates a new `Network` instance representing the networks exposed by the operating system.
   //
@@ -461,16 +513,11 @@ class LowLevelAsyncIoProvider {
   // Different implementations of this interface might work on top of different event handling
   // primitives, such as poll vs. epoll vs. kqueue vs. some higher-level event library.
   //
-  // On Windows, this interface can be used to import native HANDLEs into the async framework.
+  // On Windows, this interface can be used to import native SOCKETs into the async framework.
   // Different implementations of this interface might work on top of different event handling
   // primitives, such as I/O completion ports vs. completion routines.
-  //
-  // TODO(port):  Actually implement Windows support.
 
 public:
-  // ---------------------------------------------------------------------------
-  // Unix-specific stuff
-
   enum Flags {
     // Flags controlling how to wrap a file descriptor.
 
@@ -501,11 +548,13 @@ public:
 
 #if _WIN32
   typedef uintptr_t Fd;
+  typedef AutoCloseHandle OwnFd;
   // On Windows, the `fd` parameter to each of these methods must be a SOCKET, and must have the
   // flag WSA_FLAG_OVERLAPPED (which socket() uses by default, but WSASocket() wants you to specify
   // explicitly).
 #else
   typedef int Fd;
+  typedef AutoCloseFd OwnFd;
   // On Unix, any arbitrary file descriptor is supported.
 #endif
 
@@ -524,6 +573,12 @@ public:
   //
   // `flags` is a bitwise-OR of the values of the `Flags` enum.
 
+#if !_WIN32
+  virtual Own<AsyncCapabilityStream> wrapUnixSocketFd(Fd fd, uint flags = 0) = 0;
+  // Like wrapSocketFd() but also support capability passing via SCM_RIGHTS. The socket must be
+  // a Unix domain socket.
+#endif
+
   virtual Promise<Own<AsyncIoStream>> wrapConnectingSocketFd(
       Fd fd, const struct sockaddr* addr, uint addrlen, uint flags = 0) = 0;
   // Create an AsyncIoStream wrapping a socket and initiate a connection to the given address.
@@ -562,6 +617,22 @@ public:
   //
   // This timer is not affected by changes to the system date.  It is unspecified whether the timer
   // continues to count while the system is suspended.
+
+  Own<AsyncInputStream> wrapInputFd(OwnFd fd, uint flags = 0);
+  Own<AsyncOutputStream> wrapOutputFd(OwnFd fd, uint flags = 0);
+  Own<AsyncIoStream> wrapSocketFd(OwnFd fd, uint flags = 0);
+#if !_WIN32
+  Own<AsyncCapabilityStream> wrapUnixSocketFd(OwnFd fd, uint flags = 0);
+#endif
+  Promise<Own<AsyncIoStream>> wrapConnectingSocketFd(
+      OwnFd fd, const struct sockaddr* addr, uint addrlen, uint flags = 0);
+  Own<ConnectionReceiver> wrapListenSocketFd(
+      OwnFd fd, NetworkFilter& filter, uint flags = 0);
+  Own<ConnectionReceiver> wrapListenSocketFd(OwnFd fd, uint flags = 0);
+  Own<DatagramPort> wrapDatagramSocketFd(OwnFd fd, NetworkFilter& filter, uint flags = 0);
+  Own<DatagramPort> wrapDatagramSocketFd(OwnFd fd, uint flags = 0);
+  // Convenience wrappers which transfer ownership via AutoCloseFd (Unix) or AutoCloseHandle
+  // (Windows). TAKE_OWNERSHIP will be implicitly added to `flags`.
 };
 
 Own<AsyncIoProvider> newAsyncIoProvider(LowLevelAsyncIoProvider& lowLevel);
@@ -609,6 +680,50 @@ AsyncIoContext setupAsyncIo();
 //   note that this means that server processes which daemonize themselves at startup must wait
 //   until after daemonization to create an AsyncIoContext.
 
+// =======================================================================================
+// Convenience adapters.
+
+class CapabilityStreamConnectionReceiver final: public ConnectionReceiver {
+  // Trivial wrapper which allows an AsyncCapabilityStream to act as a ConnectionReceiver. accept()
+  // calls receiveStream().
+
+public:
+  CapabilityStreamConnectionReceiver(AsyncCapabilityStream& inner)
+      : inner(inner) {}
+
+  Promise<Own<AsyncIoStream>> accept() override;
+  uint getPort() override;
+
+private:
+  AsyncCapabilityStream& inner;
+};
+
+class CapabilityStreamNetworkAddress final: public NetworkAddress {
+  // Trivial wrapper which allows an AsyncCapabilityStream to act as a NetworkAddress.
+  //
+  // connect() is implemented by calling provider.newCapabilityPipe(), sending one end over the
+  // original capability stream, and returning the other end.
+  //
+  // listen().accept() is implemented by receiving new streams over the original stream.
+  //
+  // Note that clone() dosen't work (due to ownership issues) and toString() returns a static
+  // string.
+
+public:
+  CapabilityStreamNetworkAddress(AsyncIoProvider& provider, AsyncCapabilityStream& inner)
+      : provider(provider), inner(inner) {}
+
+  Promise<Own<AsyncIoStream>> connect() override;
+  Own<ConnectionReceiver> listen() override;
+
+  Own<NetworkAddress> clone() override;
+  String toString() override;
+
+private:
+  AsyncIoProvider& provider;
+  AsyncCapabilityStream& inner;
+};
+
 // =======================================================================================
 // inline implementation details
 

c++/src/kj/io.h

@@ -283,6 +283,13 @@ public:
   inline bool operator==(decltype(nullptr)) { return fd < 0; }
   inline bool operator!=(decltype(nullptr)) { return fd >= 0; }
 
+  inline int release() {
+    // Release ownership of an FD. Not recommended.
+    int result = fd;
+    fd = -1;
+    return result;
+  }
+
 private:
   int fd;
   UnwindDetector unwindDetector;
@@ -376,6 +383,13 @@ public:
   inline bool operator==(decltype(nullptr)) { return handle != (void*)-1; }
   inline bool operator!=(decltype(nullptr)) { return handle == (void*)-1; }
 
+  inline void* release() {
+    // Release ownership of an FD. Not recommended.
+    void* result = handle;
+    handle = (void*)-1;
+    return result;
+  }
+
 private:
   void* handle;  // -1 (aka INVALID_HANDLE_VALUE) if not valid.
 };