adbd: implement a nonblocking USB Connection.

Implement a Connection that implements a nonblocking interface to
functionfs, to replace the existing implementation that uses two
threads that loop and call read and write respectively. The existing
implementation is vulnerable to a race condition that can occur when a
connection is terminated, where one thread can notice failure and
complete reinitialization of the USB endpoints before the other thread
noticed anything went wrong, resulting in either the first packet
coming from the other end disappearing in to the void, or the other end
getting a packet of garbage.

As a side benefit, this improves performance on walleye from:

    push 100MiB: 10 runs: median 49.48 MiB/s, mean 50.00 MiB/s, stddev: 2.77 MiB/s
    pull 100MiB: 10 runs: median 75.82 MiB/s, mean 76.18 MiB/s, stddev: 6.60 MiB/s

to:

    push 100MiB: 10 runs: median 73.90 MiB/s, mean 73.51 MiB/s, stddev: 5.26 MiB/s
    pull 100MiB: 10 runs: median 105.90 MiB/s, mean 107.19 MiB/s, stddev: 6.10 MiB/s

Test: python test_device.py
Change-Id: I9b77c1057965edfef739ed9736e5d76613adf60a

adb/Android.bp

@@ -25,6 +25,7 @@ cc_defaults {
         "-Wvla",
     ],
     rtti: true,
+    cpp_std: "gnu++17",
 
     use_version_lib: true,
 
@@ -313,6 +314,7 @@ cc_library_static {
     srcs: libadb_srcs + libadb_posix_srcs + [
         "daemon/auth.cpp",
         "daemon/jdwp_service.cpp",
+        "daemon/usb.cpp",
         "daemon/usb_ffs.cpp",
         "daemon/usb_legacy.cpp",
     ],

adb/daemon/usb.cpp

@@ -0,0 +1,585 @@
+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#define TRACE_TAG USB
+
+#include "sysdeps.h"
+
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include <linux/usb/functionfs.h>
+#include <sys/eventfd.h>
+
+#include <array>
+#include <future>
+#include <memory>
+#include <mutex>
+#include <optional>
+#include <vector>
+
+#include <asyncio/AsyncIO.h>
+
+#include <android-base/logging.h>
+#include <android-base/macros.h>
+#include <android-base/properties.h>
+#include <android-base/thread_annotations.h>
+
+#include <adbd/usb.h>
+
+#include "adb_unique_fd.h"
+#include "adb_utils.h"
+#include "sysdeps/chrono.h"
+#include "transport.h"
+#include "types.h"
+
+using android::base::StringPrintf;
+
+static constexpr size_t kUsbReadQueueDepth = 16;
+static constexpr size_t kUsbReadSize = 16384;
+
+static constexpr size_t kUsbWriteQueueDepth = 16;
+
+static const char* to_string(enum usb_functionfs_event_type type) {
+    switch (type) {
+        case FUNCTIONFS_BIND:
+            return "FUNCTIONFS_BIND";
+        case FUNCTIONFS_UNBIND:
+            return "FUNCTIONFS_UNBIND";
+        case FUNCTIONFS_ENABLE:
+            return "FUNCTIONFS_ENABLE";
+        case FUNCTIONFS_DISABLE:
+            return "FUNCTIONFS_DISABLE";
+        case FUNCTIONFS_SETUP:
+            return "FUNCTIONFS_SETUP";
+        case FUNCTIONFS_SUSPEND:
+            return "FUNCTIONFS_SUSPEND";
+        case FUNCTIONFS_RESUME:
+            return "FUNCTIONFS_RESUME";
+    }
+}
+
+enum class TransferDirection : uint64_t {
+    READ = 0,
+    WRITE = 1,
+};
+
+struct TransferId {
+    TransferDirection direction : 1;
+    uint64_t id : 63;
+
+    TransferId() : TransferId(TransferDirection::READ, 0) {}
+
+  private:
+    TransferId(TransferDirection direction, uint64_t id) : direction(direction), id(id) {}
+
+  public:
+    explicit operator uint64_t() const {
+        uint64_t result;
+        static_assert(sizeof(*this) == sizeof(result));
+        memcpy(&result, this, sizeof(*this));
+        return result;
+    }
+
+    static TransferId read(uint64_t id) { return TransferId(TransferDirection::READ, id); }
+    static TransferId write(uint64_t id) { return TransferId(TransferDirection::WRITE, id); }
+
+    static TransferId from_value(uint64_t value) {
+        TransferId result;
+        memcpy(&result, &value, sizeof(value));
+        return result;
+    }
+};
+
+struct IoBlock {
+    bool pending;
+    struct iocb control;
+    Block payload;
+
+    TransferId id() const { return TransferId::from_value(control.aio_data); }
+};
+
+struct ScopedAioContext {
+    ScopedAioContext() = default;
+    ~ScopedAioContext() { reset(); }
+
+    ScopedAioContext(ScopedAioContext&& move) { reset(move.release()); }
+    ScopedAioContext(const ScopedAioContext& copy) = delete;
+
+    ScopedAioContext& operator=(ScopedAioContext&& move) {
+        reset(move.release());
+        return *this;
+    }
+    ScopedAioContext& operator=(const ScopedAioContext& copy) = delete;
+
+    static ScopedAioContext Create(size_t max_events) {
+        aio_context_t ctx = 0;
+        if (io_setup(max_events, &ctx) != 0) {
+            PLOG(FATAL) << "failed to create aio_context_t";
+        }
+        ScopedAioContext result;
+        result.reset(ctx);
+        return result;
+    }
+
+    aio_context_t release() {
+        aio_context_t result = context_;
+        context_ = 0;
+        return result;
+    }
+
+    void reset(aio_context_t new_context = 0) {
+        if (context_ != 0) {
+            io_destroy(context_);
+        }
+
+        context_ = new_context;
+    }
+
+    aio_context_t get() { return context_; }
+
+  private:
+    aio_context_t context_ = 0;
+};
+
+struct UsbFfsConnection : public Connection {
+    UsbFfsConnection(unique_fd control, unique_fd read, unique_fd write,
+                     std::promise<void> destruction_notifier)
+        : stopped_(false),
+          destruction_notifier_(std::move(destruction_notifier)),
+          control_fd_(std::move(control)),
+          read_fd_(std::move(read)),
+          write_fd_(std::move(write)) {
+        LOG(INFO) << "UsbFfsConnection constructed";
+        event_fd_.reset(eventfd(0, EFD_CLOEXEC));
+        if (event_fd_ == -1) {
+            PLOG(FATAL) << "failed to create eventfd";
+        }
+
+        aio_context_ = ScopedAioContext::Create(kUsbReadQueueDepth + kUsbWriteQueueDepth);
+    }
+
+    ~UsbFfsConnection() {
+        LOG(INFO) << "UsbFfsConnection being destroyed";
+        Stop();
+        monitor_thread_.join();
+        destruction_notifier_.set_value();
+    }
+
+    virtual bool Write(std::unique_ptr<apacket> packet) override final {
+        LOG(DEBUG) << "USB write: " << dump_header(&packet->msg);
+        Block header(sizeof(packet->msg));
+        memcpy(header.data(), &packet->msg, sizeof(packet->msg));
+
+        std::lock_guard<std::mutex> lock(write_mutex_);
+        write_requests_.push_back(CreateWriteBlock(std::move(header), next_write_id_++));
+        if (!packet->payload.empty()) {
+            write_requests_.push_back(
+                    CreateWriteBlock(std::move(packet->payload), next_write_id_++));
+        }
+        SubmitWrites();
+        return true;
+    }
+
+    virtual void Start() override final { StartMonitor(); }
+
+    virtual void Stop() override final {
+        if (stopped_.exchange(true)) {
+            return;
+        }
+        stopped_ = true;
+        uint64_t notify = 1;
+        ssize_t rc = adb_write(event_fd_.get(), &notify, sizeof(notify));
+        if (rc < 0) {
+            PLOG(FATAL) << "failed to notify eventfd to stop UsbFfsConnection";
+        }
+        CHECK_EQ(static_cast<size_t>(rc), sizeof(notify));
+    }
+
+  private:
+    void StartMonitor() {
+        // This is a bit of a mess.
+        // It's possible for io_submit to end up blocking, if we call it as the endpoint
+        // becomes disabled. Work around this by having a monitor thread to listen for functionfs
+        // lifecycle events. If we notice an error condition (either we've become disabled, or we
+        // were never enabled in the first place), we send interruption signals to the worker thread
+        // until it dies, and then report failure to the transport via HandleError, which will
+        // eventually result in the transport being destroyed, which will result in UsbFfsConnection
+        // being destroyed, which unblocks the open thread and restarts this entire process.
+        static constexpr int kInterruptionSignal = SIGUSR1;
+        static std::once_flag handler_once;
+        std::call_once(handler_once, []() { signal(kInterruptionSignal, [](int) {}); });
+
+        monitor_thread_ = std::thread([this]() {
+            adb_thread_setname("UsbFfs-monitor");
+
+            bool bound = false;
+            bool started = false;
+            bool running = true;
+            while (running) {
+                if (!bound || !started) {
+                    adb_pollfd pfd = {.fd = control_fd_.get(), .events = POLLIN, .revents = 0};
+                    int rc = TEMP_FAILURE_RETRY(adb_poll(&pfd, 1, 5000 /*ms*/));
+                    if (rc == -1) {
+                        PLOG(FATAL) << "poll on USB control fd failed";
+                    } else if (rc == 0) {
+                        // Something in the kernel presumably went wrong.
+                        // Close our endpoints, wait for a bit, and then try again.
+                        aio_context_.reset();
+                        read_fd_.reset();
+                        write_fd_.reset();
+                        control_fd_.reset();
+                        std::this_thread::sleep_for(5s);
+                        HandleError("didn't receive FUNCTIONFS_ENABLE, retrying");
+                        return;
+                    }
+                }
+
+                struct usb_functionfs_event event;
+                if (TEMP_FAILURE_RETRY(adb_read(control_fd_.get(), &event, sizeof(event))) !=
+                    sizeof(event)) {
+                    PLOG(FATAL) << "failed to read functionfs event";
+                }
+
+                LOG(INFO) << "USB event: "
+                          << to_string(static_cast<usb_functionfs_event_type>(event.type));
+
+                switch (event.type) {
+                    case FUNCTIONFS_BIND:
+                        CHECK(!started) << "received FUNCTIONFS_ENABLE while already bound?";
+                        bound = true;
+                        break;
+
+                    case FUNCTIONFS_ENABLE:
+                        CHECK(!started) << "received FUNCTIONFS_ENABLE while already running?";
+                        started = true;
+                        StartWorker();
+                        break;
+
+                    case FUNCTIONFS_DISABLE:
+                        running = false;
+                        break;
+                }
+            }
+
+            pthread_t worker_thread_handle = worker_thread_.native_handle();
+            while (true) {
+                int rc = pthread_kill(worker_thread_handle, kInterruptionSignal);
+                if (rc != 0) {
+                    LOG(ERROR) << "failed to send interruption signal to worker: " << strerror(rc);
+                    break;
+                }
+
+                std::this_thread::sleep_for(100ms);
+
+                rc = pthread_kill(worker_thread_handle, 0);
+                if (rc == 0) {
+                    continue;
+                } else if (rc == ESRCH) {
+                    break;
+                } else {
+                    LOG(ERROR) << "failed to send interruption signal to worker: " << strerror(rc);
+                }
+            }
+
+            worker_thread_.join();
+
+            aio_context_.reset();
+            read_fd_.reset();
+            write_fd_.reset();
+        });
+    }
+
+    void StartWorker() {
+        worker_thread_ = std::thread([this]() {
+            adb_thread_setname("UsbFfs-worker");
+            for (size_t i = 0; i < kUsbReadQueueDepth; ++i) {
+                read_requests_[i] = CreateReadBlock(next_read_id_++);
+                SubmitRead(&read_requests_[i]);
+            }
+
+            while (!stopped_) {
+                uint64_t dummy;
+                ssize_t rc = adb_read(event_fd_.get(), &dummy, sizeof(dummy));
+                if (rc == -1) {
+                    PLOG(FATAL) << "failed to read from eventfd";
+                } else if (rc == 0) {
+                    LOG(FATAL) << "hit EOF on eventfd";
+                }
+
+                WaitForEvents();
+            }
+        });
+    }
+
+    void PrepareReadBlock(IoBlock* block, uint64_t id) {
+        block->pending = false;
+        block->payload.resize(kUsbReadSize);
+        block->control.aio_data = static_cast<uint64_t>(TransferId::read(id));
+        block->control.aio_buf = reinterpret_cast<uintptr_t>(block->payload.data());
+        block->control.aio_nbytes = block->payload.size();
+    }
+
+    IoBlock CreateReadBlock(uint64_t id) {
+        IoBlock block;
+        PrepareReadBlock(&block, id);
+        block.control.aio_rw_flags = 0;
+        block.control.aio_lio_opcode = IOCB_CMD_PREAD;
+        block.control.aio_reqprio = 0;
+        block.control.aio_fildes = read_fd_.get();
+        block.control.aio_offset = 0;
+        block.control.aio_flags = IOCB_FLAG_RESFD;
+        block.control.aio_resfd = event_fd_.get();
+        return block;
+    }
+
+    void WaitForEvents() {
+        static constexpr size_t kMaxEvents = kUsbReadQueueDepth + kUsbWriteQueueDepth;
+        struct io_event events[kMaxEvents];
+        struct timespec timeout = {.tv_sec = 0, .tv_nsec = 0};
+        int rc = io_getevents(aio_context_.get(), 0, kMaxEvents, events, &timeout);
+        if (rc == -1) {
+            HandleError(StringPrintf("io_getevents failed while reading: %s", strerror(errno)));
+            return;
+        }
+
+        for (int event_idx = 0; event_idx < rc; ++event_idx) {
+            auto& event = events[event_idx];
+            TransferId id = TransferId::from_value(event.data);
+
+            if (event.res < 0) {
+                std::string error =
+                        StringPrintf("%s %" PRIu64 " failed with error %s",
+                                     id.direction == TransferDirection::READ ? "read" : "write",
+                                     id.id, strerror(-event.res));
+                HandleError(error);
+                return;
+            }
+
+            if (id.direction == TransferDirection::READ) {
+                HandleRead(id, event.res);
+            } else {
+                HandleWrite(id);
+            }
+        }
+    }
+
+    void HandleRead(TransferId id, int64_t size) {
+        uint64_t read_idx = id.id % kUsbReadQueueDepth;
+        IoBlock* block = &read_requests_[read_idx];
+        block->pending = false;
+        block->payload.resize(size);
+
+        // Notification for completed reads can be received out of order.
+        if (block->id().id != needed_read_id_) {
+            LOG(VERBOSE) << "read " << block->id().id << " completed while waiting for "
+                         << needed_read_id_;
+            return;
+        }
+
+        for (uint64_t id = needed_read_id_;; ++id) {
+            size_t read_idx = id % kUsbReadQueueDepth;
+            IoBlock* current_block = &read_requests_[read_idx];
+            if (current_block->pending) {
+                break;
+            }
+            ProcessRead(current_block);
+            ++needed_read_id_;
+        }
+    }
+
+    void ProcessRead(IoBlock* block) {
+        if (!block->payload.empty()) {
+            if (!incoming_header_.has_value()) {
+                CHECK_EQ(sizeof(amessage), block->payload.size());
+                amessage msg;
+                memcpy(&msg, block->payload.data(), sizeof(amessage));
+                LOG(DEBUG) << "USB read:" << dump_header(&msg);
+                incoming_header_ = msg;
+            } else {
+                size_t bytes_left = incoming_header_->data_length - incoming_payload_.size();
+                Block payload = std::move(block->payload);
+                CHECK_LE(payload.size(), bytes_left);
+                incoming_payload_.append(std::make_unique<Block>(std::move(payload)));
+            }
+
+            if (incoming_header_->data_length == incoming_payload_.size()) {
+                auto packet = std::make_unique<apacket>();
+                packet->msg = *incoming_header_;
+
+                // TODO: Make apacket contain an IOVector so we don't have to coalesce.
+                packet->payload = incoming_payload_.coalesce();
+                read_callback_(this, std::move(packet));
+
+                incoming_header_.reset();
+                incoming_payload_.clear();
+            }
+        }
+
+        PrepareReadBlock(block, block->id().id + kUsbReadQueueDepth);
+        SubmitRead(block);
+    }
+
+    void SubmitRead(IoBlock* block) {
+        block->pending = true;
+        struct iocb* iocb = &block->control;
+        if (io_submit(aio_context_.get(), 1, &iocb) != 1) {
+            HandleError(StringPrintf("failed to submit read: %s", strerror(errno)));
+            return;
+        }
+    }
+
+    void HandleWrite(TransferId id) {
+        std::lock_guard<std::mutex> lock(write_mutex_);
+        auto it =
+                std::find_if(write_requests_.begin(), write_requests_.end(), [id](const auto& req) {
+                    return static_cast<uint64_t>(req->id()) == static_cast<uint64_t>(id);
+                });
+        CHECK(it != write_requests_.end());
+
+        write_requests_.erase(it);
+        size_t outstanding_writes = --writes_submitted_;
+        LOG(DEBUG) << "USB write: reaped, down to " << outstanding_writes;
+
+        SubmitWrites();
+    }
+
+    std::unique_ptr<IoBlock> CreateWriteBlock(Block payload, uint64_t id) {
+        auto block = std::make_unique<IoBlock>();
+        block->payload = std::move(payload);
+        block->control.aio_data = static_cast<uint64_t>(TransferId::write(id));
+        block->control.aio_rw_flags = 0;
+        block->control.aio_lio_opcode = IOCB_CMD_PWRITE;
+        block->control.aio_reqprio = 0;
+        block->control.aio_fildes = write_fd_.get();
+        block->control.aio_buf = reinterpret_cast<uintptr_t>(block->payload.data());
+        block->control.aio_nbytes = block->payload.size();
+        block->control.aio_offset = 0;
+        block->control.aio_flags = IOCB_FLAG_RESFD;
+        block->control.aio_resfd = event_fd_.get();
+        return block;
+    }
+
+    void SubmitWrites() REQUIRES(write_mutex_) {
+        if (writes_submitted_ == kUsbWriteQueueDepth) {
+            return;
+        }
+
+        ssize_t writes_to_submit = std::min(kUsbWriteQueueDepth - writes_submitted_,
+                                            write_requests_.size() - writes_submitted_);
+        CHECK_GE(writes_to_submit, 0);
+        if (writes_to_submit == 0) {
+            return;
+        }
+
+        struct iocb* iocbs[kUsbWriteQueueDepth];
+        for (int i = 0; i < writes_to_submit; ++i) {
+            CHECK(!write_requests_[writes_submitted_ + i]->pending);
+            write_requests_[writes_submitted_ + i]->pending = true;
+            iocbs[i] = &write_requests_[writes_submitted_ + i]->control;
+            LOG(VERBOSE) << "submitting write_request " << static_cast<void*>(iocbs[i]);
+        }
+
+        int rc = io_submit(aio_context_.get(), writes_to_submit, iocbs);
+        if (rc == -1) {
+            HandleError(StringPrintf("failed to submit write requests: %s", strerror(errno)));
+            return;
+        } else if (rc != writes_to_submit) {
+            LOG(FATAL) << "failed to submit all writes: wanted to submit " << writes_to_submit
+                       << ", actually submitted " << rc;
+        }
+
+        writes_submitted_ += rc;
+    }
+
+    void HandleError(const std::string& error) {
+        std::call_once(error_flag_, [&]() {
+            error_callback_(this, error);
+            if (!stopped_) {
+                Stop();
+            }
+        });
+    }
+
+    std::thread monitor_thread_;
+    std::thread worker_thread_;
+
+    std::atomic<bool> stopped_;
+    std::promise<void> destruction_notifier_;
+    std::once_flag error_flag_;
+
+    unique_fd event_fd_;
+
+    ScopedAioContext aio_context_;
+    unique_fd control_fd_;
+    unique_fd read_fd_;
+    unique_fd write_fd_;
+
+    std::optional<amessage> incoming_header_;
+    IOVector incoming_payload_;
+
+    std::array<IoBlock, kUsbReadQueueDepth> read_requests_;
+    IOVector read_data_;
+
+    // ID of the next request that we're going to send out.
+    size_t next_read_id_ = 0;
+
+    // ID of the next packet we're waiting for.
+    size_t needed_read_id_ = 0;
+
+    std::mutex write_mutex_;
+    std::deque<std::unique_ptr<IoBlock>> write_requests_ GUARDED_BY(write_mutex_);
+    size_t next_write_id_ GUARDED_BY(write_mutex_) = 0;
+    size_t writes_submitted_ GUARDED_BY(write_mutex_) = 0;
+};
+
+static void usb_ffs_open_thread() {
+    adb_thread_setname("usb ffs open");
+
+    while (true) {
+        unique_fd control;
+        unique_fd bulk_out;
+        unique_fd bulk_in;
+        if (!open_functionfs(&control, &bulk_out, &bulk_in)) {
+            std::this_thread::sleep_for(1s);
+            continue;
+        }
+
+        atransport* transport = new atransport();
+        transport->serial = "UsbFfs";
+        std::promise<void> destruction_notifier;
+        std::future<void> future = destruction_notifier.get_future();
+        transport->SetConnection(std::make_unique<UsbFfsConnection>(
+                std::move(control), std::move(bulk_out), std::move(bulk_in),
+                std::move(destruction_notifier)));
+        register_transport(transport);
+        future.wait();
+    }
+}
+
+void usb_init_legacy();
+void usb_init() {
+    if (!android::base::GetBoolProperty("persist.adb.nonblocking_ffs", false)) {
+        usb_init_legacy();
+    } else {
+        std::thread(usb_ffs_open_thread).detach();
+    }
+}

adb/daemon/usb_legacy.cpp

@@ -94,8 +94,8 @@ static bool init_functionfs(struct usb_handle* h) {
     return true;
 }
 
-static void usb_ffs_open_thread(usb_handle* usb) {
-    adb_thread_setname("usb ffs open");
+static void usb_legacy_ffs_open_thread(usb_handle* usb) {
+    adb_thread_setname("usb legacy ffs open");
 
     while (true) {
         // wait until the USB device needs opening
@@ -285,12 +285,12 @@ usb_handle* create_usb_handle(unsigned num_bufs, unsigned io_size) {
     return h;
 }
 
-void usb_init() {
-    D("[ usb_init - using FunctionFS ]");
+void usb_init_legacy() {
+    D("[ usb_init - using legacy FunctionFS ]");
     dummy_fd.reset(adb_open("/dev/null", O_WRONLY | O_CLOEXEC));
     CHECK_NE(-1, dummy_fd.get());
 
-    std::thread(usb_ffs_open_thread, create_usb_handle(USB_FFS_NUM_BUFS, USB_FFS_BULK_SIZE))
+    std::thread(usb_legacy_ffs_open_thread, create_usb_handle(USB_FFS_NUM_BUFS, USB_FFS_BULK_SIZE))
             .detach();
 }
 

adb/transport.cpp

@@ -52,7 +52,6 @@
 #include "fdevent.h"
 #include "sysdeps/chrono.h"
 
-static void register_transport(atransport* transport);
 static void remove_transport(atransport* transport);
 static void transport_unref(atransport* transport);
 
@@ -671,7 +670,7 @@ static void transport_registration_func(int _fd, unsigned ev, void*) {
             return true;
         });
         t->connection()->SetErrorCallback([t](Connection*, const std::string& error) {
-            D("%s: connection terminated: %s", t->serial.c_str(), error.c_str());
+            LOG(INFO) << t->serial_name() << ": connection terminated: " << error;
             fdevent_run_on_main_thread([t]() {
                 handle_offline(t);
                 transport_unref(t);
@@ -730,7 +729,7 @@ void kick_all_transports() {
 }
 
 /* the fdevent select pump is single threaded */
-static void register_transport(atransport* transport) {
+void register_transport(atransport* transport) {
     tmsg m;
     m.transport = transport;
     m.action = 1;
@@ -758,6 +757,7 @@ static void transport_unref(atransport* t) {
     CHECK_GT(t->ref_count, 0u);
     t->ref_count--;
     if (t->ref_count == 0) {
+        LOG(INFO) << "destroying transport " << t->serial_name();
         t->connection()->Stop();
 #if ADB_HOST
         if (t->IsTcpDevice() && !t->kicked()) {
@@ -1293,6 +1293,7 @@ void register_usb_transport(usb_handle* usb, const char* serial, const char* dev
     register_transport(t);
 }
 
+#if ADB_HOST
 // This should only be used for transports with connection_state == kCsNoPerm.
 void unregister_usb_transport(usb_handle* usb) {
     std::lock_guard<std::recursive_mutex> lock(transport_lock);
@@ -1304,6 +1305,7 @@ void unregister_usb_transport(usb_handle* usb) {
         return false;
     });
 }
+#endif
 
 bool check_header(apacket* p, atransport* t) {
     if (p->msg.magic != (p->msg.command ^ 0xffffffff)) {

adb/transport.h

@@ -362,6 +362,7 @@ atransport* find_transport(const char* serial);
 void kick_all_tcp_devices();
 void kick_all_transports();
 
+void register_transport(atransport* transport);
 void register_usb_transport(usb_handle* h, const char* serial,
                             const char* devpath, unsigned writeable);