Add fd-forwarding transport lib

dt_fd_forward is a jdwpTransport that takes as an address a local
socket file-descriptor and uses it to allow some other piece of code
or program to control the actual attach and communication process.

tools/dt_fds_forward.py is a python program that can be used as a
controller for this comms system. This is useful for testing. It
implements the same behavior (excepting DDMS) that the adbconnection
plugin will but is capable of being easily used on normal host
machines. Unlike the plugin, dt_fds_forward.py works by allowing a
socket file-descriptor to be inherited by the forked Java Language
Runtime.

The overall goal of this transport is to use it to allow us to safely
multiplex the out-bound data with DDMS data. This is needed to let us
match current DDMS behavior which transmits packets in the blind on
the same channel JDWP traffic is sent on.

Test: ./tools/dt_fds_forward.py  \
        ./test/run-test --host --dev 001-HelloWorld
Test: jdb -attach localhost:12345
Test: nc localhost 12345, handshake, disconnect
Test: jdb -attach localhost:12345, detach, attach

Test: ./tools/dt_fds_forward.py  \
            --debug-lib $JAVA_HOME/jre/lib/amd64/libjdwp.so \
            -- \
            ./test/run-test --host --jvm --dev 001-HelloWorld
Test: jdb -attach localhost:12345

Bug: 62821960
Bug: 69169846

Change-Id: I654db6c6991c006933e1e1f0a4e41c13f795f9a8
diff --git a/dt_fd_forward/dt_fd_forward.cc b/dt_fd_forward/dt_fd_forward.cc
new file mode 100644
index 0000000..cf3088d
--- /dev/null
+++ b/dt_fd_forward/dt_fd_forward.cc
@@ -0,0 +1,761 @@
+/* Copyright (C) 2017 The Android Open Source Project
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This file implements interfaces from the file jdwpTransport.h. This
+ * implementation is licensed under the same terms as the file
+ * jdwpTransport.h. The copyright and license information for the file
+ * jdwpTransport.h follows.
+ *
+ * Copyright (c) 2003, 2016, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+#include "dt_fd_forward.h"
+
+#include <string>
+#include <vector>
+
+#include <android-base/endian.h>
+#include <android-base/logging.h>
+#include <android-base/parseint.h>
+#include <android-base/stringprintf.h>
+
+#include <sys/ioctl.h>
+#include <sys/eventfd.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <poll.h>
+
+#include <jni.h>
+#include <jdwpTransport.h>
+
+namespace dt_fd_forward {
+
+// Helper that puts line-number in error message.
+#define DT_IO_ERROR(f) \
+    SetLastError(::android::base::StringPrintf("%s:%d - %s: %s", \
+                                               __FILE__, __LINE__, f, strerror(errno)))
+
+extern const jdwpTransportNativeInterface_ gTransportInterface;
+
+template <typename T> static T HostToNetwork(T in);
+template <typename T> static T NetworkToHost(T in);
+
+template<> int8_t HostToNetwork(int8_t in) { return in; }
+template<> int8_t NetworkToHost(int8_t in) { return in; }
+template<> int16_t HostToNetwork(int16_t in) { return htons(in); }
+template<> int16_t NetworkToHost(int16_t in) { return ntohs(in); }
+template<> int32_t HostToNetwork(int32_t in) { return htonl(in); }
+template<> int32_t NetworkToHost(int32_t in) { return ntohl(in); }
+
+FdForwardTransport::FdForwardTransport(jdwpTransportCallback* cb)
+    : mem_(*cb),
+      read_fd_(-1),
+      write_fd_(-1),
+      wakeup_fd_(eventfd(0, EFD_NONBLOCK)),
+      listen_fd_(-1),
+      close_notify_fd_(-1),
+      state_(TransportState::kClosed),
+      current_seq_num_(0) {}
+
+FdForwardTransport::~FdForwardTransport() { }
+
+bool FdForwardTransport::ChangeState(TransportState old_state, TransportState new_state) {
+  if (old_state == state_) {
+    state_ = new_state;
+    state_cv_.notify_all();
+    return true;
+  } else {
+    return false;
+  }
+}
+
+jdwpTransportError FdForwardTransport::PerformAttach(int listen_fd) {
+  jdwpTransportError err = SetupListen(listen_fd);
+  if (err != OK) {
+    return OK;
+  }
+  err = Accept();
+  StopListening();
+  return err;
+}
+
+static void SendListenMessage(const android::base::unique_fd& fd) {
+  TEMP_FAILURE_RETRY(send(fd, kListenStartMessage, sizeof(kListenStartMessage), MSG_EOR));
+}
+
+jdwpTransportError FdForwardTransport::SetupListen(int listen_fd) {
+  std::lock_guard<std::mutex> lk(state_mutex_);
+  if (!ChangeState(TransportState::kClosed, TransportState::kListenSetup)) {
+    return ERR(ILLEGAL_STATE);
+  } else {
+    listen_fd_.reset(dup(listen_fd));
+    SendListenMessage(listen_fd_);
+    CHECK(ChangeState(TransportState::kListenSetup, TransportState::kListening));
+    return OK;
+  }
+}
+
+static void SendListenEndMessage(const android::base::unique_fd& fd) {
+  TEMP_FAILURE_RETRY(send(fd, kListenEndMessage, sizeof(kListenEndMessage), MSG_EOR));
+}
+
+jdwpTransportError FdForwardTransport::StopListening() {
+  std::lock_guard<std::mutex> lk(state_mutex_);
+  if (listen_fd_ != -1) {
+    SendListenEndMessage(listen_fd_);
+  }
+  // Don't close the listen_fd_ since we might need it for later calls to listen.
+  if (ChangeState(TransportState::kListening, TransportState::kClosed) ||
+      state_ == TransportState::kOpen) {
+    listen_fd_.reset();
+  }
+  return OK;
+}
+
+// Last error message.
+thread_local std::string global_last_error_;
+
+void FdForwardTransport::SetLastError(const std::string& desc) {
+  LOG(ERROR) << desc;
+  global_last_error_ = desc;
+}
+
+IOResult FdForwardTransport::ReadFullyWithoutChecks(void* data, size_t ndata) {
+  uint8_t* bdata = reinterpret_cast<uint8_t*>(data);
+  size_t nbytes = 0;
+  while (nbytes < ndata) {
+    int res = TEMP_FAILURE_RETRY(read(read_fd_, bdata + nbytes, ndata - nbytes));
+    if (res < 0) {
+      DT_IO_ERROR("Failed read()");
+      return IOResult::kError;
+    } else if (res == 0) {
+      return IOResult::kEOF;
+    } else {
+      nbytes += res;
+    }
+  }
+  return IOResult::kOk;
+}
+
+IOResult FdForwardTransport::ReadUpToMax(void* data, size_t ndata, /*out*/size_t* read_amount) {
+  CHECK_GE(read_fd_.get(), 0);
+  int avail;
+  int res = ioctl(read_fd_, FIONREAD, &avail);
+  if (res < 0) {
+    DT_IO_ERROR("Failed ioctl(read_fd_, FIONREAD, &avail)");
+    return IOResult::kError;
+  }
+  size_t to_read = std::min(static_cast<size_t>(avail), ndata);
+  *read_amount = to_read;
+  if (*read_amount == 0) {
+    // Check if the read would cause an EOF.
+    struct pollfd pollfd = { read_fd_, POLLRDHUP, 0 };
+    res = poll(&pollfd, /*nfds*/1, /*timeout*/0);
+    if (res < 0 || (pollfd.revents & POLLERR) == POLLERR) {
+      DT_IO_ERROR("Failed poll on read fd.");
+      return IOResult::kError;
+    }
+    return ((pollfd.revents & (POLLRDHUP | POLLHUP)) == 0) ? IOResult::kOk : IOResult::kEOF;
+  }
+
+  return ReadFullyWithoutChecks(data, to_read);
+}
+
+IOResult FdForwardTransport::ReadFully(void* data, size_t ndata) {
+  uint64_t seq_num = current_seq_num_;
+  size_t nbytes = 0;
+  while (nbytes < ndata) {
+    size_t read_len;
+    struct pollfd pollfds[2];
+    {
+      std::lock_guard<std::mutex> lk(state_mutex_);
+      // Operations in this block must not cause an unbounded pause.
+      if (state_ != TransportState::kOpen || seq_num != current_seq_num_) {
+        // Async-close occurred!
+        return IOResult::kInterrupt;
+      } else {
+        CHECK_GE(read_fd_.get(), 0);
+      }
+      IOResult res = ReadUpToMax(reinterpret_cast<uint8_t*>(data) + nbytes,
+                                 ndata - nbytes,
+                                 /*out*/&read_len);
+      if (res != IOResult::kOk) {
+        return res;
+      } else {
+        nbytes += read_len;
+      }
+
+      pollfds[0] = { read_fd_, POLLRDHUP | POLLIN, 0 };
+      pollfds[1] = { wakeup_fd_, POLLIN, 0 };
+    }
+    if (read_len == 0) {
+      // No more data. Sleep without locks until more is available. We don't actually check for any
+      // errors since possible ones are (1) the read_fd_ is closed or wakeup happens which are both
+      // fine since the wakeup_fd_ or the poll failing will wake us up.
+      int poll_res = poll(pollfds, 2, -1);
+      if (poll_res < 0) {
+        DT_IO_ERROR("Failed to poll!");
+      }
+      // Clear the wakeup_fd regardless.
+      uint64_t val;
+      int unused = read(wakeup_fd_, &val, sizeof(val));
+      DCHECK(unused == sizeof(val) || errno == EAGAIN);
+      if (poll_res < 0) {
+        return IOResult::kError;
+      }
+    }
+  }
+  return IOResult::kOk;
+}
+
+// A helper that allows us to lock the eventfd 'fd'.
+class ScopedEventFdLock {
+ public:
+  explicit ScopedEventFdLock(const android::base::unique_fd& fd) : fd_(fd), data_(0) {
+    TEMP_FAILURE_RETRY(read(fd_, &data_, sizeof(data_)));
+  }
+
+  ~ScopedEventFdLock() {
+    TEMP_FAILURE_RETRY(write(fd_, &data_, sizeof(data_)));
+  }
+
+ private:
+  const android::base::unique_fd& fd_;
+  uint64_t data_;
+};
+
+IOResult FdForwardTransport::WriteFullyWithoutChecks(const void* data, size_t ndata) {
+  ScopedEventFdLock sefdl(write_lock_fd_);
+  const uint8_t* bdata = static_cast<const uint8_t*>(data);
+  size_t nbytes = 0;
+  while (nbytes < ndata) {
+    int res = TEMP_FAILURE_RETRY(write(write_fd_, bdata + nbytes, ndata - nbytes));
+    if (res < 0) {
+      DT_IO_ERROR("Failed write()");
+      return IOResult::kError;
+    } else if (res == 0) {
+      return IOResult::kEOF;
+    } else {
+      nbytes += res;
+    }
+  }
+  return IOResult::kOk;
+}
+
+IOResult FdForwardTransport::WriteFully(const void* data, size_t ndata) {
+  std::lock_guard<std::mutex> lk(state_mutex_);
+  if (state_ != TransportState::kOpen) {
+    return IOResult::kInterrupt;
+  }
+  return WriteFullyWithoutChecks(data, ndata);
+}
+
+static void SendAcceptMessage(int fd) {
+  TEMP_FAILURE_RETRY(send(fd, kAcceptMessage, sizeof(kAcceptMessage), MSG_EOR));
+}
+
+IOResult FdForwardTransport::ReceiveFdsFromSocket() {
+  union {
+    cmsghdr cm;
+    uint8_t buffer[CMSG_SPACE(sizeof(FdSet))];
+  } msg_union;
+  // We don't actually care about the data. Only FDs. We need to have an iovec anyway to tell if we
+  // got the values or not though.
+  char dummy = '\0';
+  iovec iov;
+  iov.iov_base = &dummy;
+  iov.iov_len  = sizeof(dummy);
+
+  msghdr msg;
+  memset(&msg, 0, sizeof(msg));
+  msg.msg_iov = &iov;
+  msg.msg_iovlen = 1;
+  msg.msg_control = msg_union.buffer;
+  msg.msg_controllen = sizeof(msg_union.buffer);
+
+  cmsghdr* cmsg = CMSG_FIRSTHDR(&msg);
+  cmsg->cmsg_len   = msg.msg_controllen;
+  cmsg->cmsg_level = SOL_SOCKET;
+  cmsg->cmsg_type  = SCM_RIGHTS;
+  memset(reinterpret_cast<int*>(CMSG_DATA(cmsg)), -1, FdSet::kDataLength);
+
+  int res = TEMP_FAILURE_RETRY(recvmsg(listen_fd_, &msg, 0));
+  if (res <= 0) {
+    DT_IO_ERROR("Failed to receive fds!");
+    return IOResult::kError;
+  }
+  FdSet out_fds = FdSet::ReadData(CMSG_DATA(cmsg));
+  if (out_fds.read_fd_ < 0 || out_fds.write_fd_ < 0 || out_fds.write_lock_fd_ < 0) {
+    DT_IO_ERROR("Received fds were invalid!");
+    if (out_fds.read_fd_ >= 0) {
+      close(out_fds.read_fd_);
+    }
+    if (out_fds.write_fd_ >= 0) {
+      close(out_fds.write_fd_);
+    }
+    if (out_fds.write_lock_fd_ >= 0) {
+      close(out_fds.write_lock_fd_);
+    }
+    return IOResult::kError;
+  }
+
+  read_fd_.reset(out_fds.read_fd_);
+  write_fd_.reset(out_fds.write_fd_);
+  write_lock_fd_.reset(out_fds.write_lock_fd_);
+
+  // We got the fds. Send ack.
+  close_notify_fd_.reset(dup(listen_fd_));
+  SendAcceptMessage(close_notify_fd_);
+
+  return IOResult::kOk;
+}
+
+// Accept the connection. Note that we match the behavior of other transports which is to just close
+// the connection and try again if we get a bad handshake.
+jdwpTransportError FdForwardTransport::Accept() {
+  // TODO Work with timeouts.
+  while (true) {
+    std::unique_lock<std::mutex> lk(state_mutex_);
+    while (!ChangeState(TransportState::kListening, TransportState::kOpening)) {
+      if (state_ == TransportState::kClosed ||
+          state_ == TransportState::kOpen) {
+        return ERR(ILLEGAL_STATE);
+      }
+      state_cv_.wait(lk);
+    }
+
+    DCHECK_NE(listen_fd_.get(), -1);
+    if (ReceiveFdsFromSocket() != IOResult::kOk) {
+      CHECK(ChangeState(TransportState::kOpening, TransportState::kListening));
+      return ERR(IO_ERROR);
+    }
+
+    current_seq_num_++;
+
+    // Moved to the opening state.
+    char handshake_recv[sizeof(kJdwpHandshake)];
+    memset(handshake_recv, 0, sizeof(handshake_recv));
+    IOResult res = ReadFullyWithoutChecks(handshake_recv, sizeof(handshake_recv));
+    if (res != IOResult::kOk ||
+        strncmp(handshake_recv, kJdwpHandshake, sizeof(kJdwpHandshake)) != 0) {
+      DT_IO_ERROR("Failed to read handshake");
+      CHECK(ChangeState(TransportState::kOpening, TransportState::kListening));
+      CloseFdsLocked();
+      // Retry.
+      continue;
+    }
+    res = WriteFullyWithoutChecks(kJdwpHandshake, sizeof(kJdwpHandshake));
+    if (res != IOResult::kOk) {
+      DT_IO_ERROR("Failed to write handshake");
+      CHECK(ChangeState(TransportState::kOpening, TransportState::kListening));
+      CloseFdsLocked();
+      // Retry.
+      continue;
+    }
+    break;
+  }
+  CHECK(ChangeState(TransportState::kOpening, TransportState::kOpen));
+  return OK;
+}
+
+void SendClosingMessage(int fd) {
+  if (fd >= 0) {
+    TEMP_FAILURE_RETRY(send(fd, kCloseMessage, sizeof(kCloseMessage), MSG_EOR));
+  }
+}
+
+// Actually close the fds associated with this transport.
+void FdForwardTransport::CloseFdsLocked() {
+  // We have a different set of fd's now. Increase the seq number.
+  current_seq_num_++;
+
+  // All access to these is locked under the state_mutex_ so we are safe to close these.
+  {
+    ScopedEventFdLock sefdl(write_lock_fd_);
+    if (close_notify_fd_ >= 0) {
+      SendClosingMessage(close_notify_fd_);
+    }
+    close_notify_fd_.reset();
+    read_fd_.reset();
+    write_fd_.reset();
+    close_notify_fd_.reset();
+  }
+  write_lock_fd_.reset();
+
+  // Send a wakeup in case we have any in-progress reads/writes.
+  uint64_t data = 1;
+  TEMP_FAILURE_RETRY(write(wakeup_fd_, &data, sizeof(data)));
+}
+
+jdwpTransportError FdForwardTransport::Close() {
+  std::lock_guard<std::mutex> lk(state_mutex_);
+  jdwpTransportError res =
+      ChangeState(TransportState::kOpen, TransportState::kClosed) ? OK : ERR(ILLEGAL_STATE);
+  // Send a wakeup after changing the state even if nothing actually happened.
+  uint64_t data = 1;
+  TEMP_FAILURE_RETRY(write(wakeup_fd_, &data, sizeof(data)));
+  if (res == OK) {
+    CloseFdsLocked();
+  }
+  return res;
+}
+
+// A helper class to read and parse the JDWP packet.
+class PacketReader {
+ public:
+  PacketReader(FdForwardTransport* transport, jdwpPacket* pkt)
+      : transport_(transport),
+        pkt_(pkt),
+        is_eof_(false),
+        is_err_(false) {}
+  bool ReadFully() {
+    // Zero out.
+    memset(pkt_, 0, sizeof(jdwpPacket));
+    int32_t len = ReadInt32();         // read len
+    if (is_err_) {
+      return false;
+    } else if (is_eof_) {
+      return true;
+    } else if (len < 11) {
+      transport_->DT_IO_ERROR("Packet with len < 11 received!");
+      return false;
+    }
+    pkt_->type.cmd.len = len;
+    pkt_->type.cmd.id = ReadInt32();
+    pkt_->type.cmd.flags = ReadByte();
+    if (is_err_) {
+      return false;
+    } else if (is_eof_) {
+      return true;
+    } else if ((pkt_->type.reply.flags & JDWPTRANSPORT_FLAGS_REPLY) == JDWPTRANSPORT_FLAGS_REPLY) {
+      ReadReplyPacket();
+    } else {
+      ReadCmdPacket();
+    }
+    return !is_err_;
+  }
+
+ private:
+  void ReadReplyPacket() {
+    pkt_->type.reply.errorCode = ReadInt16();
+    pkt_->type.reply.data = ReadRemaining();
+  }
+
+  void ReadCmdPacket() {
+    pkt_->type.cmd.cmdSet = ReadByte();
+    pkt_->type.cmd.cmd = ReadByte();
+    pkt_->type.cmd.data = ReadRemaining();
+  }
+
+  template <typename T>
+  T HandleResult(IOResult res, T val, T fail) {
+    switch (res) {
+      case IOResult::kError:
+        is_err_ = true;
+        return fail;
+      case IOResult::kOk:
+        return val;
+      case IOResult::kEOF:
+        is_eof_ = true;
+        pkt_->type.cmd.len = 0;
+        return fail;
+      case IOResult::kInterrupt:
+        transport_->DT_IO_ERROR("Failed to read, concurrent close!");
+        is_err_ = true;
+        return fail;
+    }
+  }
+
+  jbyte* ReadRemaining() {
+    if (is_eof_ || is_err_) {
+      return nullptr;
+    }
+    jbyte* out = nullptr;
+    jint rem = pkt_->type.cmd.len - 11;
+    CHECK_GE(rem, 0);
+    if (rem == 0) {
+      return nullptr;
+    } else {
+      out = reinterpret_cast<jbyte*>(transport_->Alloc(rem));
+      IOResult res = transport_->ReadFully(out, rem);
+      jbyte* ret = HandleResult(res, out, static_cast<jbyte*>(nullptr));
+      if (ret != out) {
+        transport_->Free(out);
+      }
+      return ret;
+    }
+  }
+
+  jbyte ReadByte() {
+    if (is_eof_ || is_err_) {
+      return -1;
+    }
+    jbyte out;
+    IOResult res = transport_->ReadFully(&out, sizeof(out));
+    return HandleResult(res, NetworkToHost(out), static_cast<jbyte>(-1));
+  }
+
+  jshort ReadInt16() {
+    if (is_eof_ || is_err_) {
+      return -1;
+    }
+    jshort out;
+    IOResult res = transport_->ReadFully(&out, sizeof(out));
+    return HandleResult(res, NetworkToHost(out), static_cast<jshort>(-1));
+  }
+
+  jint ReadInt32() {
+    if (is_eof_ || is_err_) {
+      return -1;
+    }
+    jint out;
+    IOResult res = transport_->ReadFully(&out, sizeof(out));
+    return HandleResult(res, NetworkToHost(out), -1);
+  }
+
+  FdForwardTransport* transport_;
+  jdwpPacket* pkt_;
+  bool is_eof_;
+  bool is_err_;
+};
+
+jdwpTransportError FdForwardTransport::ReadPacket(jdwpPacket* pkt) {
+  if (pkt == nullptr) {
+    return ERR(ILLEGAL_ARGUMENT);
+  }
+  PacketReader reader(this, pkt);
+  if (reader.ReadFully()) {
+    return OK;
+  } else {
+    return ERR(IO_ERROR);
+  }
+}
+
+// A class that writes a packet to the transport.
+class PacketWriter {
+ public:
+  PacketWriter(FdForwardTransport* transport, const jdwpPacket* pkt)
+      : transport_(transport), pkt_(pkt), data_() {}
+
+  bool WriteFully() {
+    PushInt32(pkt_->type.cmd.len);
+    PushInt32(pkt_->type.cmd.id);
+    PushByte(pkt_->type.cmd.flags);
+    if ((pkt_->type.reply.flags & JDWPTRANSPORT_FLAGS_REPLY) == JDWPTRANSPORT_FLAGS_REPLY) {
+      PushInt16(pkt_->type.reply.errorCode);
+      PushData(pkt_->type.reply.data, pkt_->type.reply.len - 11);
+    } else {
+      PushByte(pkt_->type.cmd.cmdSet);
+      PushByte(pkt_->type.cmd.cmd);
+      PushData(pkt_->type.cmd.data, pkt_->type.cmd.len - 11);
+    }
+    IOResult res = transport_->WriteFully(data_.data(), data_.size());
+    return res == IOResult::kOk;
+  }
+
+ private:
+  void PushInt32(int32_t data) {
+    data = HostToNetwork(data);
+    PushData(&data, sizeof(data));
+  }
+  void PushInt16(int16_t data) {
+    data = HostToNetwork(data);
+    PushData(&data, sizeof(data));
+  }
+  void PushByte(jbyte data) {
+    data_.push_back(HostToNetwork(data));
+  }
+
+  void PushData(void* d, size_t size) {
+    uint8_t* bytes = reinterpret_cast<uint8_t*>(d);
+    data_.insert(data_.end(), bytes, bytes + size);
+  }
+
+  FdForwardTransport* transport_;
+  const jdwpPacket* pkt_;
+  std::vector<uint8_t> data_;
+};
+
+jdwpTransportError FdForwardTransport::WritePacket(const jdwpPacket* pkt) {
+  if (pkt == nullptr) {
+    return ERR(ILLEGAL_ARGUMENT);
+  }
+  PacketWriter writer(this, pkt);
+  if (writer.WriteFully()) {
+    return OK;
+  } else {
+    return ERR(IO_ERROR);
+  }
+}
+
+jboolean FdForwardTransport::IsOpen() {
+  return state_ == TransportState::kOpen;
+}
+
+void* FdForwardTransport::Alloc(size_t s) {
+  return mem_.alloc(s);
+}
+
+void FdForwardTransport::Free(void* data) {
+  mem_.free(data);
+}
+
+jdwpTransportError FdForwardTransport::GetLastError(/*out*/char** err) {
+  std::string data = global_last_error_;
+  *err = reinterpret_cast<char*>(Alloc(data.size() + 1));
+  strcpy(*err, data.c_str());
+  return OK;
+}
+
+static FdForwardTransport* AsFdForward(jdwpTransportEnv* env) {
+  return reinterpret_cast<FdForwardTransport*>(env);
+}
+
+static jdwpTransportError ParseAddress(const std::string& addr,
+                                       /*out*/int* listen_sock) {
+  if (!android::base::ParseInt(addr.c_str(), listen_sock) || *listen_sock < 0) {
+    LOG(ERROR) << "address format is <fd_num> not " << addr;
+    return ERR(ILLEGAL_ARGUMENT);
+  }
+  return OK;
+}
+
+class JdwpTransportFunctions {
+ public:
+  static jdwpTransportError GetCapabilities(jdwpTransportEnv* env ATTRIBUTE_UNUSED,
+                                            /*out*/ JDWPTransportCapabilities* capabilities_ptr) {
+    // We don't support any of the optional capabilities (can_timeout_attach, can_timeout_accept,
+    // can_timeout_handshake) so just return a zeroed capabilities ptr.
+    // TODO We should maybe support these timeout options.
+    memset(capabilities_ptr, 0, sizeof(JDWPTransportCapabilities));
+    return OK;
+  }
+
+  // Address is <sock_fd>
+  static jdwpTransportError Attach(jdwpTransportEnv* env,
+                                   const char* address,
+                                   jlong attach_timeout ATTRIBUTE_UNUSED,
+                                   jlong handshake_timeout ATTRIBUTE_UNUSED) {
+    if (address == nullptr || *address == '\0') {
+      return ERR(ILLEGAL_ARGUMENT);
+    }
+    int listen_fd;
+    jdwpTransportError err = ParseAddress(address, &listen_fd);
+    if (err != OK) {
+      return err;
+    }
+    return AsFdForward(env)->PerformAttach(listen_fd);
+  }
+
+  static jdwpTransportError StartListening(jdwpTransportEnv* env,
+                                           const char* address,
+                                           /*out*/ char** actual_address) {
+    if (address == nullptr || *address == '\0') {
+      return ERR(ILLEGAL_ARGUMENT);
+    }
+    int listen_fd;
+    jdwpTransportError err = ParseAddress(address, &listen_fd);
+    if (err != OK) {
+      return err;
+    }
+    err = AsFdForward(env)->SetupListen(listen_fd);
+    if (err != OK) {
+      return err;
+    }
+    if (actual_address != nullptr) {
+      *actual_address = reinterpret_cast<char*>(AsFdForward(env)->Alloc(strlen(address) + 1));
+      memcpy(*actual_address, address, strlen(address) + 1);
+    }
+    return OK;
+  }
+
+  static jdwpTransportError StopListening(jdwpTransportEnv* env) {
+    return AsFdForward(env)->StopListening();
+  }
+
+  static jdwpTransportError Accept(jdwpTransportEnv* env,
+                                   jlong accept_timeout ATTRIBUTE_UNUSED,
+                                   jlong handshake_timeout ATTRIBUTE_UNUSED) {
+    return AsFdForward(env)->Accept();
+  }
+
+  static jboolean IsOpen(jdwpTransportEnv* env) {
+    return AsFdForward(env)->IsOpen();
+  }
+
+  static jdwpTransportError Close(jdwpTransportEnv* env) {
+    return AsFdForward(env)->Close();
+  }
+
+  static jdwpTransportError ReadPacket(jdwpTransportEnv* env, jdwpPacket *pkt) {
+    return AsFdForward(env)->ReadPacket(pkt);
+  }
+
+  static jdwpTransportError WritePacket(jdwpTransportEnv* env, const jdwpPacket* pkt) {
+    return AsFdForward(env)->WritePacket(pkt);
+  }
+
+  static jdwpTransportError GetLastError(jdwpTransportEnv* env, char** error) {
+    return AsFdForward(env)->GetLastError(error);
+  }
+};
+
+// The actual struct holding all the entrypoints into the jdwpTransport interface.
+const jdwpTransportNativeInterface_ gTransportInterface = {
+  nullptr,  // reserved1
+  JdwpTransportFunctions::GetCapabilities,
+  JdwpTransportFunctions::Attach,
+  JdwpTransportFunctions::StartListening,
+  JdwpTransportFunctions::StopListening,
+  JdwpTransportFunctions::Accept,
+  JdwpTransportFunctions::IsOpen,
+  JdwpTransportFunctions::Close,
+  JdwpTransportFunctions::ReadPacket,
+  JdwpTransportFunctions::WritePacket,
+  JdwpTransportFunctions::GetLastError,
+};
+
+extern "C"
+JNIEXPORT jint JNICALL jdwpTransport_OnLoad(JavaVM* vm ATTRIBUTE_UNUSED,
+                                            jdwpTransportCallback* cb,
+                                            jint version,
+                                            jdwpTransportEnv** /*out*/env) {
+  if (version != JDWPTRANSPORT_VERSION_1_0) {
+    LOG(ERROR) << "unknown version " << version;
+    return JNI_EVERSION;
+  }
+  void* data = cb->alloc(sizeof(FdForwardTransport));
+  if (data == nullptr) {
+    LOG(ERROR) << "Failed to allocate data for transport!";
+    return JNI_ENOMEM;
+  }
+  FdForwardTransport* transport =
+      new (data) FdForwardTransport(cb);
+  transport->functions = &gTransportInterface;
+  *env = transport;
+  return JNI_OK;
+}
+
+}  // namespace dt_fd_forward