libs/binder/RpcSession.cpp

/*
 * Copyright (C) 2020 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 LOG_TAG "RpcSession"

#include <binder/RpcSession.h>

#include <dlfcn.h>
#include <inttypes.h>
#include <netinet/tcp.h>
#include <poll.h>
#include <unistd.h>

#include <string_view>

#include <binder/BpBinder.h>
#include <binder/Functional.h>
#include <binder/Parcel.h>
#include <binder/RpcServer.h>
#include <binder/RpcTransportRaw.h>
#include <binder/Stability.h>
#include <utils/String8.h>

#include "BuildFlags.h"
#include "FdTrigger.h"
#include "OS.h"
#include "RpcSocketAddress.h"
#include "RpcState.h"
#include "RpcTransportUtils.h"
#include "RpcWireFormat.h"
#include "Utils.h"

#if defined(__ANDROID__) && !defined(__ANDROID_RECOVERY__)
#include <jni.h>
extern "C" JavaVM* AndroidRuntimeGetJavaVM();
#endif

namespace android {

using namespace android::binder::impl;
using android::binder::borrowed_fd;
using android::binder::unique_fd;

RpcSession::RpcSession(std::unique_ptr<RpcTransportCtx> ctx) : mCtx(std::move(ctx)) {
    LOG_RPC_DETAIL("RpcSession created %p", this);

    mRpcBinderState = std::make_unique<RpcState>();
}
RpcSession::~RpcSession() {
    LOG_RPC_DETAIL("RpcSession destroyed %p", this);

    RpcMutexLockGuard _l(mMutex);
    LOG_ALWAYS_FATAL_IF(mConnections.mIncoming.size() != 0,
                        "Should not be able to destroy a session with servers in use.");
}

sp<RpcSession> RpcSession::make() {
    // Default is without TLS.
    return make(binder::os::makeDefaultRpcTransportCtxFactory());
}

sp<RpcSession> RpcSession::make(std::unique_ptr<RpcTransportCtxFactory> rpcTransportCtxFactory) {
    auto ctx = rpcTransportCtxFactory->newClientCtx();
    if (ctx == nullptr) return nullptr;
    return sp<RpcSession>::make(std::move(ctx));
}

void RpcSession::setMaxIncomingThreads(size_t threads) {
    RpcMutexLockGuard _l(mMutex);
    LOG_ALWAYS_FATAL_IF(mStartedSetup,
                        "Must set max incoming threads before setting up connections");
    mMaxIncomingThreads = threads;
}

size_t RpcSession::getMaxIncomingThreads() {
    RpcMutexLockGuard _l(mMutex);
    return mMaxIncomingThreads;
}

void RpcSession::setMaxOutgoingConnections(size_t connections) {
    RpcMutexLockGuard _l(mMutex);
    LOG_ALWAYS_FATAL_IF(mStartedSetup,
                        "Must set max outgoing threads before setting up connections");
    mMaxOutgoingConnections = connections;
}

size_t RpcSession::getMaxOutgoingThreads() {
    RpcMutexLockGuard _l(mMutex);
    return mMaxOutgoingConnections;
}

bool RpcSession::setProtocolVersionInternal(uint32_t version, bool checkStarted) {
    if (!RpcState::validateProtocolVersion(version)) {
        return false;
    }

    RpcMutexLockGuard _l(mMutex);
    LOG_ALWAYS_FATAL_IF(checkStarted && mStartedSetup,
                        "Must set protocol version before setting up connections");
    if (mProtocolVersion && version > *mProtocolVersion) {
        ALOGE("Cannot upgrade explicitly capped protocol version %u to newer version %u",
              *mProtocolVersion, version);
        return false;
    }

    mProtocolVersion = version;
    return true;
}

bool RpcSession::setProtocolVersion(uint32_t version) {
    return setProtocolVersionInternal(version, true);
}

std::optional<uint32_t> RpcSession::getProtocolVersion() {
    RpcMutexLockGuard _l(mMutex);
    return mProtocolVersion;
}

void RpcSession::setFileDescriptorTransportMode(FileDescriptorTransportMode mode) {
    RpcMutexLockGuard _l(mMutex);
    LOG_ALWAYS_FATAL_IF(mStartedSetup,
                        "Must set file descriptor transport mode before setting up connections");
    mFileDescriptorTransportMode = mode;
}

RpcSession::FileDescriptorTransportMode RpcSession::getFileDescriptorTransportMode() {
    return mFileDescriptorTransportMode;
}

status_t RpcSession::setupUnixDomainClient(const char* path) {
    return setupSocketClient(UnixSocketAddress(path));
}

status_t RpcSession::setupUnixDomainSocketBootstrapClient(unique_fd bootstrapFd) {
    mBootstrapTransport =
            mCtx->newTransport(RpcTransportFd(std::move(bootstrapFd)), mShutdownTrigger.get());
    return setupClient([&](const std::vector<uint8_t>& sessionId, bool incoming) {
        int socks[2];
        if (socketpair(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC | SOCK_NONBLOCK, 0, socks) < 0) {
            int savedErrno = errno;
            ALOGE("Failed socketpair: %s", strerror(savedErrno));
            return -savedErrno;
        }
        unique_fd clientFd(socks[0]), serverFd(socks[1]);

        int zero = 0;
        iovec iov{&zero, sizeof(zero)};
        std::vector<std::variant<unique_fd, borrowed_fd>> fds;
        fds.push_back(std::move(serverFd));

        status_t status = mBootstrapTransport->interruptableWriteFully(mShutdownTrigger.get(), &iov,
                                                                       1, std::nullopt, &fds);
        if (status != OK) {
            ALOGE("Failed to send fd over bootstrap transport: %s", strerror(-status));
            return status;
        }

        return initAndAddConnection(RpcTransportFd(std::move(clientFd)), sessionId, incoming);
    });
}

status_t RpcSession::setupVsockClient(unsigned int cid, unsigned int port) {
    return setupSocketClient(VsockSocketAddress(cid, port));
}

status_t RpcSession::setupInetClient(const char* addr, unsigned int port) {
    auto aiStart = InetSocketAddress::getAddrInfo(addr, port);
    if (aiStart == nullptr) return UNKNOWN_ERROR;
    for (auto ai = aiStart.get(); ai != nullptr; ai = ai->ai_next) {
        InetSocketAddress socketAddress(ai->ai_addr, ai->ai_addrlen, addr, port);
        if (status_t status = setupSocketClient(socketAddress); status == OK) return OK;
    }
    ALOGE("None of the socket address resolved for %s:%u can be added as inet client.", addr, port);
    return NAME_NOT_FOUND;
}

status_t RpcSession::setupPreconnectedClient(unique_fd fd, std::function<unique_fd()>&& request) {
    return setupClient([&](const std::vector<uint8_t>& sessionId, bool incoming) -> status_t {
        if (!fd.ok()) {
            fd = request();
            if (!fd.ok()) return BAD_VALUE;
        }
        if (status_t res = binder::os::setNonBlocking(fd); res != OK) return res;

        RpcTransportFd transportFd(std::move(fd));
        status_t status = initAndAddConnection(std::move(transportFd), sessionId, incoming);
        fd = unique_fd(); // Explicitly reset after move to avoid analyzer warning.
        return status;
    });
}

status_t RpcSession::addNullDebuggingClient() {
    // Note: only works on raw sockets.
    if (auto status = initShutdownTrigger(); status != OK) return status;

    unique_fd serverFd(TEMP_FAILURE_RETRY(open("/dev/null", O_WRONLY | O_CLOEXEC)));

    if (!serverFd.ok()) {
        int savedErrno = errno;
        ALOGE("Could not connect to /dev/null: %s", strerror(savedErrno));
        return -savedErrno;
    }

    RpcTransportFd transportFd(std::move(serverFd));
    auto server = mCtx->newTransport(std::move(transportFd), mShutdownTrigger.get());
    if (server == nullptr) {
        ALOGE("Unable to set up RpcTransport");
        return UNKNOWN_ERROR;
    }
    return addOutgoingConnection(std::move(server), false);
}

sp<IBinder> RpcSession::getRootObject() {
    ExclusiveConnection connection;
    status_t status = ExclusiveConnection::find(sp<RpcSession>::fromExisting(this),
                                                ConnectionUse::CLIENT, &connection);
    if (status != OK) return nullptr;
    return state()->getRootObject(connection.get(), sp<RpcSession>::fromExisting(this));
}

status_t RpcSession::getRemoteMaxThreads(size_t* maxThreads) {
    ExclusiveConnection connection;
    status_t status = ExclusiveConnection::find(sp<RpcSession>::fromExisting(this),
                                                ConnectionUse::CLIENT, &connection);
    if (status != OK) return status;
    return state()->getMaxThreads(connection.get(), sp<RpcSession>::fromExisting(this), maxThreads);
}

bool RpcSession::shutdownAndWait(bool wait) {
    RpcMutexUniqueLock _l(mMutex);
    LOG_ALWAYS_FATAL_IF(mShutdownTrigger == nullptr, "Shutdown trigger not installed");

    mShutdownTrigger->trigger();

    if (wait) {
        LOG_ALWAYS_FATAL_IF(mShutdownListener == nullptr, "Shutdown listener not installed");
        mShutdownListener->waitForShutdown(_l, sp<RpcSession>::fromExisting(this));

        LOG_ALWAYS_FATAL_IF(!mConnections.mThreads.empty(), "Shutdown failed");
    }

    _l.unlock();

    if (status_t res = state()->sendObituaries(sp<RpcSession>::fromExisting(this)); res != OK) {
        ALOGE("Failed to send obituaries as the RpcSession is shutting down: %s",
              statusToString(res).c_str());
    }

    mRpcBinderState->clear();

    return true;
}

status_t RpcSession::transact(const sp<IBinder>& binder, uint32_t code, const Parcel& data,
                              Parcel* reply, uint32_t flags) {
    ExclusiveConnection connection;
    status_t status =
            ExclusiveConnection::find(sp<RpcSession>::fromExisting(this),
                                      (flags & IBinder::FLAG_ONEWAY) ? ConnectionUse::CLIENT_ASYNC
                                                                     : ConnectionUse::CLIENT,
                                      &connection);
    if (status != OK) return status;
    return state()->transact(connection.get(), binder, code, data,
                             sp<RpcSession>::fromExisting(this), reply, flags);
}

status_t RpcSession::sendDecStrong(const BpBinder* binder) {
    // target is 0 because this is used to free BpBinder objects
    return sendDecStrongToTarget(binder->getPrivateAccessor().rpcAddress(), 0 /*target*/);
}

status_t RpcSession::sendDecStrongToTarget(uint64_t address, size_t target) {
    ExclusiveConnection connection;
    status_t status = ExclusiveConnection::find(sp<RpcSession>::fromExisting(this),
                                                ConnectionUse::CLIENT_REFCOUNT, &connection);
    if (status != OK) return status;
    return state()->sendDecStrongToTarget(connection.get(), sp<RpcSession>::fromExisting(this),
                                          address, target);
}

status_t RpcSession::readId() {
    {
        RpcMutexLockGuard _l(mMutex);
        LOG_ALWAYS_FATAL_IF(mForServer != nullptr, "Can only update ID for client.");
    }

    ExclusiveConnection connection;
    status_t status = ExclusiveConnection::find(sp<RpcSession>::fromExisting(this),
                                                ConnectionUse::CLIENT, &connection);
    if (status != OK) return status;

    status = state()->getSessionId(connection.get(), sp<RpcSession>::fromExisting(this), &mId);
    if (status != OK) return status;

    LOG_RPC_DETAIL("RpcSession %p has id %s", this, HexString(mId.data(), mId.size()).c_str());
    return OK;
}

void RpcSession::WaitForShutdownListener::onSessionAllIncomingThreadsEnded(
        const sp<RpcSession>& session) {
    (void)session;
}

void RpcSession::WaitForShutdownListener::onSessionIncomingThreadEnded() {
    mShutdownCount += 1;
    mCv.notify_all();
}

void RpcSession::WaitForShutdownListener::waitForShutdown(RpcMutexUniqueLock& lock,
                                                          const sp<RpcSession>& session) {
    while (mShutdownCount < session->mConnections.mMaxIncoming) {
        if (std::cv_status::timeout == mCv.wait_for(lock, std::chrono::seconds(1))) {
            ALOGE("Waiting for RpcSession to shut down (1s w/o progress): %zu incoming connections "
                  "still %zu/%zu fully shutdown.",
                  session->mConnections.mIncoming.size(), mShutdownCount.load(),
                  session->mConnections.mMaxIncoming);
        }
    }
}

void RpcSession::preJoinThreadOwnership(RpcMaybeThread thread) {
    LOG_ALWAYS_FATAL_IF(thread.get_id() != rpc_this_thread::get_id(), "Must own this thread");

    {
        RpcMutexLockGuard _l(mMutex);
        mConnections.mThreads[thread.get_id()] = std::move(thread);
    }
}

RpcSession::PreJoinSetupResult RpcSession::preJoinSetup(
        std::unique_ptr<RpcTransport> rpcTransport) {
    // must be registered to allow arbitrary client code executing commands to
    // be able to do nested calls (we can't only read from it)
    sp<RpcConnection> connection = assignIncomingConnectionToThisThread(std::move(rpcTransport));

    status_t status;

    if (connection == nullptr) {
        status = DEAD_OBJECT;
    } else {
        status =
                mRpcBinderState->readConnectionInit(connection, sp<RpcSession>::fromExisting(this));
    }

    return PreJoinSetupResult{
            .connection = std::move(connection),
            .status = status,
    };
}

namespace {
#if !defined(__ANDROID__) || defined(__ANDROID_RECOVERY__)
class JavaThreadAttacher {};
#else
// RAII object for attaching / detaching current thread to JVM if Android Runtime exists. If
// Android Runtime doesn't exist, no-op.
class JavaThreadAttacher {
public:
    JavaThreadAttacher() {
        // Use dlsym to find androidJavaAttachThread because libandroid_runtime is loaded after
        // libbinder.
        auto vm = getJavaVM();
        if (vm == nullptr) return;

        char threadName[16];
        if (0 != pthread_getname_np(pthread_self(), threadName, sizeof(threadName))) {
            constexpr const char* defaultThreadName = "UnknownRpcSessionThread";
            memcpy(threadName, defaultThreadName,
                   std::min<size_t>(sizeof(threadName), strlen(defaultThreadName) + 1));
        }
        LOG_RPC_DETAIL("Attaching current thread %s to JVM", threadName);
        JavaVMAttachArgs args;
        args.version = JNI_VERSION_1_2;
        args.name = threadName;
        args.group = nullptr;
        JNIEnv* env;

        LOG_ALWAYS_FATAL_IF(vm->AttachCurrentThread(&env, &args) != JNI_OK,
                            "Cannot attach thread %s to JVM", threadName);
        mAttached = true;
    }
    ~JavaThreadAttacher() {
        if (!mAttached) return;
        auto vm = getJavaVM();
        LOG_ALWAYS_FATAL_IF(vm == nullptr,
                            "Unable to detach thread. No JavaVM, but it was present before!");

        LOG_RPC_DETAIL("Detaching current thread from JVM");
        int ret = vm->DetachCurrentThread();
        if (ret == JNI_OK) {
            mAttached = false;
        } else {
            ALOGW("Unable to detach current thread from JVM (%d)", ret);
        }
    }

private:
    JavaThreadAttacher(const JavaThreadAttacher&) = delete;
    void operator=(const JavaThreadAttacher&) = delete;

    bool mAttached = false;

    static JavaVM* getJavaVM() {
        static auto fn = reinterpret_cast<decltype(&AndroidRuntimeGetJavaVM)>(
                dlsym(RTLD_DEFAULT, "AndroidRuntimeGetJavaVM"));
        if (fn == nullptr) return nullptr;
        return fn();
    }
};
#endif
} // namespace

void RpcSession::join(sp<RpcSession>&& session, PreJoinSetupResult&& setupResult) {
    sp<RpcConnection>& connection = setupResult.connection;

    if (setupResult.status == OK) {
        LOG_ALWAYS_FATAL_IF(!connection, "must have connection if setup succeeded");
        [[maybe_unused]] JavaThreadAttacher javaThreadAttacher;
        while (true) {
            status_t status = session->state()->getAndExecuteCommand(connection, session,
                                                                     RpcState::CommandType::ANY);
            if (status != OK) {
                LOG_RPC_DETAIL("Binder connection thread closing w/ status %s",
                               statusToString(status).c_str());
                break;
            }
        }
    } else {
        ALOGE("Connection failed to init, closing with status %s",
              statusToString(setupResult.status).c_str());
    }

    sp<RpcSession::EventListener> listener;
    {
        RpcMutexLockGuard _l(session->mMutex);
        auto it = session->mConnections.mThreads.find(rpc_this_thread::get_id());
        LOG_ALWAYS_FATAL_IF(it == session->mConnections.mThreads.end());
        it->second.detach();
        session->mConnections.mThreads.erase(it);

        listener = session->mEventListener.promote();
    }

    // done after all cleanup, since session shutdown progresses via callbacks here
    if (connection != nullptr) {
        LOG_ALWAYS_FATAL_IF(!session->removeIncomingConnection(connection),
                            "bad state: connection object guaranteed to be in list");
    }

    session = nullptr;

    if (listener != nullptr) {
        listener->onSessionIncomingThreadEnded();
    }
}

sp<RpcServer> RpcSession::server() {
    RpcServer* unsafeServer = mForServer.unsafe_get();
    sp<RpcServer> server = mForServer.promote();

    LOG_ALWAYS_FATAL_IF((unsafeServer == nullptr) != (server == nullptr),
                        "wp<> is to avoid strong cycle only");
    return server;
}

status_t RpcSession::setupClient(const std::function<status_t(const std::vector<uint8_t>& sessionId,
                                                              bool incoming)>& connectAndInit) {
    {
        RpcMutexLockGuard _l(mMutex);
        LOG_ALWAYS_FATAL_IF(mStartedSetup, "Must only setup session once");
        mStartedSetup = true;

        if constexpr (!kEnableRpcThreads) {
            LOG_ALWAYS_FATAL_IF(mMaxIncomingThreads > 0,
                                "Incoming threads are not supported on single-threaded libbinder");
            // mMaxIncomingThreads should not change from here to its use below,
            // since we set mStartedSetup==true and setMaxIncomingThreads checks
            // for that
        }
    }

    if (auto status = initShutdownTrigger(); status != OK) return status;

    auto oldProtocolVersion = mProtocolVersion;
    auto cleanup = make_scope_guard([&] {
        // if any threads are started, shut them down
        (void)shutdownAndWait(true);

        mShutdownListener = nullptr;
        mEventListener.clear();

        mId.clear();

        mShutdownTrigger = nullptr;
        mRpcBinderState = std::make_unique<RpcState>();

        // protocol version may have been downgraded - if we reuse this object
        // to connect to another server, force that server to request a
        // downgrade again
        mProtocolVersion = oldProtocolVersion;

        mConnections = {};

        // clear mStartedSetup so that we can reuse this RpcSession
        mStartedSetup = false;
    });

    if (status_t status = connectAndInit({}, false /*incoming*/); status != OK) return status;

    {
        ExclusiveConnection connection;
        if (status_t status = ExclusiveConnection::find(sp<RpcSession>::fromExisting(this),
                                                        ConnectionUse::CLIENT, &connection);
            status != OK)
            return status;

        uint32_t version;
        if (status_t status =
                    state()->readNewSessionResponse(connection.get(),
                                                    sp<RpcSession>::fromExisting(this), &version);
            status != OK)
            return status;
        if (!setProtocolVersionInternal(version, false)) return BAD_VALUE;
    }

    // TODO(b/189955605): we should add additional sessions dynamically
    // instead of all at once.
    size_t numThreadsAvailable;
    if (status_t status = getRemoteMaxThreads(&numThreadsAvailable); status != OK) {
        ALOGE("Could not get max threads after initial session setup: %s",
              statusToString(status).c_str());
        return status;
    }

    if (status_t status = readId(); status != OK) {
        ALOGE("Could not get session id after initial session setup: %s",
              statusToString(status).c_str());
        return status;
    }

    size_t outgoingConnections = std::min(numThreadsAvailable, mMaxOutgoingConnections);
    ALOGI_IF(outgoingConnections != numThreadsAvailable,
             "Server hints client to start %zu outgoing threads, but client will only start %zu "
             "because it is preconfigured to start at most %zu outgoing threads.",
             numThreadsAvailable, outgoingConnections, mMaxOutgoingConnections);

    // TODO(b/189955605): we should add additional sessions dynamically
    // instead of all at once - the other side should be responsible for setting
    // up additional connections. We need to create at least one (unless 0 are
    // requested to be set) in order to allow the other side to reliably make
    // any requests at all.

    // we've already setup one client
    LOG_RPC_DETAIL("RpcSession::setupClient() instantiating %zu outgoing connections (server max: "
                   "%zu) and %zu incoming threads",
                   outgoingConnections, numThreadsAvailable, mMaxIncomingThreads);
    for (size_t i = 0; i + 1 < outgoingConnections; i++) {
        if (status_t status = connectAndInit(mId, false /*incoming*/); status != OK) return status;
    }

    for (size_t i = 0; i < mMaxIncomingThreads; i++) {
        if (status_t status = connectAndInit(mId, true /*incoming*/); status != OK) return status;
    }

    cleanup.release();

    return OK;
}

status_t RpcSession::setupSocketClient(const RpcSocketAddress& addr) {
    return setupClient([&](const std::vector<uint8_t>& sessionId, bool incoming) {
        return setupOneSocketConnection(addr, sessionId, incoming);
    });
}

status_t RpcSession::setupOneSocketConnection(const RpcSocketAddress& addr,
                                              const std::vector<uint8_t>& sessionId,
                                              bool incoming) {
    for (size_t tries = 0; tries < 5; tries++) {
        if (tries > 0) usleep(10000);

        unique_fd serverFd(TEMP_FAILURE_RETRY(
                socket(addr.addr()->sa_family, SOCK_STREAM | SOCK_CLOEXEC | SOCK_NONBLOCK, 0)));
        if (!serverFd.ok()) {
            int savedErrno = errno;
            ALOGE("Could not create socket at %s: %s", addr.toString().c_str(),
                  strerror(savedErrno));
            return -savedErrno;
        }

        if (addr.addr()->sa_family == AF_INET || addr.addr()->sa_family == AF_INET6) {
            int noDelay = 1;
            int result =
                    setsockopt(serverFd.get(), IPPROTO_TCP, TCP_NODELAY, &noDelay, sizeof(noDelay));
            if (result < 0) {
                int savedErrno = errno;
                ALOGE("Could not set TCP_NODELAY on %s: %s", addr.toString().c_str(),
                      strerror(savedErrno));
                return -savedErrno;
            }
        }

        RpcTransportFd transportFd(std::move(serverFd));

        if (0 != TEMP_FAILURE_RETRY(connect(transportFd.fd.get(), addr.addr(), addr.addrSize()))) {
            int connErrno = errno;
            if (connErrno == EAGAIN || connErrno == EINPROGRESS) {
                // For non-blocking sockets, connect() may return EAGAIN (for unix domain socket) or
                // EINPROGRESS (for others). Call poll() and getsockopt() to get the error.
                status_t pollStatus = mShutdownTrigger->triggerablePoll(transportFd, POLLOUT);
                if (pollStatus != OK) {
                    ALOGE("Could not POLLOUT after connect() on non-blocking socket: %s",
                          statusToString(pollStatus).c_str());
                    return pollStatus;
                }
                // Set connErrno to the errno that connect() would have set if the fd were blocking.
                socklen_t connErrnoLen = sizeof(connErrno);
                int ret = getsockopt(transportFd.fd.get(), SOL_SOCKET, SO_ERROR, &connErrno,
                                     &connErrnoLen);
                if (ret == -1) {
                    int savedErrno = errno;
                    ALOGE("Could not getsockopt() after connect() on non-blocking socket: %s. "
                          "(Original error from connect() is: %s)",
                          strerror(savedErrno), strerror(connErrno));
                    return -savedErrno;
                }
                // Retrieved the real connErrno as if connect() was called with a blocking socket
                // fd. Continue checking connErrno.
            }
            if (connErrno == ECONNRESET) {
                ALOGW("Connection reset on %s", addr.toString().c_str());
                continue;
            }
            // connErrno could be zero if getsockopt determines so. Hence zero-check again.
            if (connErrno != 0) {
                ALOGE("Could not connect socket at %s: %s", addr.toString().c_str(),
                      strerror(connErrno));
                return -connErrno;
            }
        }
        LOG_RPC_DETAIL("Socket at %s client with fd %d", addr.toString().c_str(),
                       transportFd.fd.get());

        return initAndAddConnection(std::move(transportFd), sessionId, incoming);
    }

    ALOGE("Ran out of retries to connect to %s", addr.toString().c_str());
    return UNKNOWN_ERROR;
}

status_t RpcSession::initAndAddConnection(RpcTransportFd fd, const std::vector<uint8_t>& sessionId,
                                          bool incoming) {
    LOG_ALWAYS_FATAL_IF(mShutdownTrigger == nullptr);
    auto server = mCtx->newTransport(std::move(fd), mShutdownTrigger.get());
    if (server == nullptr) {
        ALOGE("%s: Unable to set up RpcTransport", __PRETTY_FUNCTION__);
        return UNKNOWN_ERROR;
    }

    LOG_RPC_DETAIL("Socket at client with RpcTransport %p", server.get());

    if (sessionId.size() > std::numeric_limits<uint16_t>::max()) {
        ALOGE("Session ID too big %zu", sessionId.size());
        return BAD_VALUE;
    }

    RpcConnectionHeader header{
            .version = mProtocolVersion.value_or(RPC_WIRE_PROTOCOL_VERSION),
            .options = 0,
            .fileDescriptorTransportMode = static_cast<uint8_t>(mFileDescriptorTransportMode),
            .sessionIdSize = static_cast<uint16_t>(sessionId.size()),
    };

    if (incoming) {
        header.options |= RPC_CONNECTION_OPTION_INCOMING;
    }

    iovec headerIov{&header, sizeof(header)};
    auto sendHeaderStatus = server->interruptableWriteFully(mShutdownTrigger.get(), &headerIov, 1,
                                                            std::nullopt, nullptr);
    if (sendHeaderStatus != OK) {
        ALOGE("Could not write connection header to socket: %s",
              statusToString(sendHeaderStatus).c_str());
        return sendHeaderStatus;
    }

    if (sessionId.size() > 0) {
        iovec sessionIov{const_cast<void*>(static_cast<const void*>(sessionId.data())),
                         sessionId.size()};
        auto sendSessionIdStatus =
                server->interruptableWriteFully(mShutdownTrigger.get(), &sessionIov, 1,
                                                std::nullopt, nullptr);
        if (sendSessionIdStatus != OK) {
            ALOGE("Could not write session ID ('%s') to socket: %s",
                  HexString(sessionId.data(), sessionId.size()).c_str(),
                  statusToString(sendSessionIdStatus).c_str());
            return sendSessionIdStatus;
        }
    }

    LOG_RPC_DETAIL("Socket at client: header sent");

    if (incoming) {
        return addIncomingConnection(std::move(server));
    } else {
        return addOutgoingConnection(std::move(server), true /*init*/);
    }
}

status_t RpcSession::addIncomingConnection(std::unique_ptr<RpcTransport> rpcTransport) {
    RpcMutex mutex;
    RpcConditionVariable joinCv;
    RpcMutexUniqueLock lock(mutex);
    RpcMaybeThread thread;
    sp<RpcSession> thiz = sp<RpcSession>::fromExisting(this);
    bool ownershipTransferred = false;
    thread = RpcMaybeThread([&]() {
        RpcMutexUniqueLock threadLock(mutex);
        std::unique_ptr<RpcTransport> movedRpcTransport = std::move(rpcTransport);
        // NOLINTNEXTLINE(performance-unnecessary-copy-initialization)
        sp<RpcSession> session = thiz;
        session->preJoinThreadOwnership(std::move(thread));

        // only continue once we have a response or the connection fails
        auto setupResult = session->preJoinSetup(std::move(movedRpcTransport));

        ownershipTransferred = true;
        threadLock.unlock();
        joinCv.notify_one();
        // do not use & vars below

        RpcSession::join(std::move(session), std::move(setupResult));
    });
    rpcJoinIfSingleThreaded(thread);
    joinCv.wait(lock, [&] { return ownershipTransferred; });
    LOG_ALWAYS_FATAL_IF(!ownershipTransferred);
    return OK;
}

status_t RpcSession::initShutdownTrigger() {
    // first client connection added, but setForServer not called, so
    // initializaing for a client.
    if (mShutdownTrigger == nullptr) {
        mShutdownTrigger = FdTrigger::make();
        mEventListener = mShutdownListener = sp<WaitForShutdownListener>::make();
        if (mShutdownTrigger == nullptr) return INVALID_OPERATION;
    }
    return OK;
}

status_t RpcSession::addOutgoingConnection(std::unique_ptr<RpcTransport> rpcTransport, bool init) {
    sp<RpcConnection> connection = sp<RpcConnection>::make();
    {
        RpcMutexLockGuard _l(mMutex);
        connection->rpcTransport = std::move(rpcTransport);
        connection->exclusiveTid = binder::os::GetThreadId();
        mConnections.mOutgoing.push_back(connection);
    }

    status_t status = OK;
    if (init) {
        status =
                mRpcBinderState->sendConnectionInit(connection, sp<RpcSession>::fromExisting(this));
    }

    clearConnectionTid(connection);

    return status;
}

bool RpcSession::setForServer(const wp<RpcServer>& server, const wp<EventListener>& eventListener,
                              const std::vector<uint8_t>& sessionId,
                              const sp<IBinder>& sessionSpecificRoot) {
    LOG_ALWAYS_FATAL_IF(mForServer != nullptr);
    LOG_ALWAYS_FATAL_IF(server == nullptr);
    LOG_ALWAYS_FATAL_IF(mEventListener != nullptr);
    LOG_ALWAYS_FATAL_IF(eventListener == nullptr);
    LOG_ALWAYS_FATAL_IF(mShutdownTrigger != nullptr);
    LOG_ALWAYS_FATAL_IF(mCtx != nullptr);

    mShutdownTrigger = FdTrigger::make();
    if (mShutdownTrigger == nullptr) return false;

    mId = sessionId;
    mForServer = server;
    mEventListener = eventListener;
    mSessionSpecificRootObject = sessionSpecificRoot;
    return true;
}

sp<RpcSession::RpcConnection> RpcSession::assignIncomingConnectionToThisThread(
        std::unique_ptr<RpcTransport> rpcTransport) {
    RpcMutexLockGuard _l(mMutex);

    if (mConnections.mIncoming.size() >= mMaxIncomingThreads) {
        ALOGE("Cannot add thread to session with %zu threads (max is set to %zu)",
              mConnections.mIncoming.size(), mMaxIncomingThreads);
        return nullptr;
    }

    // Don't accept any more connections, some have shutdown. Usually this
    // happens when new connections are still being established as part of a
    // very short-lived session which shuts down after it already started
    // accepting new connections.
    if (mConnections.mIncoming.size() < mConnections.mMaxIncoming) {
        return nullptr;
    }

    sp<RpcConnection> session = sp<RpcConnection>::make();
    session->rpcTransport = std::move(rpcTransport);
    session->exclusiveTid = binder::os::GetThreadId();

    mConnections.mIncoming.push_back(session);
    mConnections.mMaxIncoming = mConnections.mIncoming.size();

    return session;
}

bool RpcSession::removeIncomingConnection(const sp<RpcConnection>& connection) {
    RpcMutexUniqueLock _l(mMutex);
    if (auto it =
                std::find(mConnections.mIncoming.begin(), mConnections.mIncoming.end(), connection);
        it != mConnections.mIncoming.end()) {
        mConnections.mIncoming.erase(it);
        if (mConnections.mIncoming.size() == 0) {
            sp<EventListener> listener = mEventListener.promote();
            if (listener) {
                _l.unlock();
                listener->onSessionAllIncomingThreadsEnded(sp<RpcSession>::fromExisting(this));
            }
        }
        return true;
    }
    return false;
}

void RpcSession::clearConnectionTid(const sp<RpcConnection>& connection) {
    RpcMutexUniqueLock _l(mMutex);
    connection->exclusiveTid = std::nullopt;
    if (mConnections.mWaitingThreads > 0) {
        _l.unlock();
        mAvailableConnectionCv.notify_one();
    }
}

std::vector<uint8_t> RpcSession::getCertificate(RpcCertificateFormat format) {
    return mCtx->getCertificate(format);
}

status_t RpcSession::ExclusiveConnection::find(const sp<RpcSession>& session, ConnectionUse use,
                                               ExclusiveConnection* connection) {
    connection->mSession = session;
    connection->mConnection = nullptr;
    connection->mReentrant = false;

    uint64_t tid = binder::os::GetThreadId();
    RpcMutexUniqueLock _l(session->mMutex);

    session->mConnections.mWaitingThreads++;
    while (true) {
        sp<RpcConnection> exclusive;
        sp<RpcConnection> available;

        // CHECK FOR DEDICATED CLIENT SOCKET
        //
        // A server/looper should always use a dedicated connection if available
        findConnection(tid, &exclusive, &available, session->mConnections.mOutgoing,
                       session->mConnections.mOutgoingOffset);

        // WARNING: this assumes a server cannot request its client to send
        // a transaction, as mIncoming is excluded below.
        //
        // Imagine we have more than one thread in play, and a single thread
        // sends a synchronous, then an asynchronous command. Imagine the
        // asynchronous command is sent on the first client connection. Then, if
        // we naively send a synchronous command to that same connection, the
        // thread on the far side might be busy processing the asynchronous
        // command. So, we move to considering the second available thread
        // for subsequent calls.
        if (use == ConnectionUse::CLIENT_ASYNC && (exclusive != nullptr || available != nullptr)) {
            session->mConnections.mOutgoingOffset = (session->mConnections.mOutgoingOffset + 1) %
                    session->mConnections.mOutgoing.size();
        }

        // USE SERVING SOCKET (e.g. nested transaction)
        if (use != ConnectionUse::CLIENT_ASYNC) {
            sp<RpcConnection> exclusiveIncoming;
            // server connections are always assigned to a thread
            findConnection(tid, &exclusiveIncoming, nullptr /*available*/,
                           session->mConnections.mIncoming, 0 /* index hint */);

            // asynchronous calls cannot be nested, we currently allow ref count
            // calls to be nested (so that you can use this without having extra
            // threads). Note 'drainCommands' is used so that these ref counts can't
            // build up.
            if (exclusiveIncoming != nullptr) {
                if (exclusiveIncoming->allowNested) {
                    // guaranteed to be processed as nested command
                    exclusive = exclusiveIncoming;
                } else if (use == ConnectionUse::CLIENT_REFCOUNT && available == nullptr) {
                    // prefer available socket, but if we don't have one, don't
                    // wait for one
                    exclusive = exclusiveIncoming;
                }
            }
        }

        // if our thread is already using a connection, prioritize using that
        if (exclusive != nullptr) {
            connection->mConnection = exclusive;
            connection->mReentrant = true;
            break;
        } else if (available != nullptr) {
            connection->mConnection = available;
            connection->mConnection->exclusiveTid = tid;
            break;
        }

        if (session->mConnections.mOutgoing.size() == 0) {
            ALOGE("Session has no outgoing connections. This is required for an RPC server to make "
                  "any non-nested (e.g. oneway or on another thread) calls. Use code request "
                  "reason: %d. Incoming connections: %zu. %s.",
                  static_cast<int>(use), session->mConnections.mIncoming.size(),
                  (session->server()
                           ? "This is a server session, so see RpcSession::setMaxIncomingThreads "
                             "for the corresponding client"
                           : "This is a client session, so see "
                             "RpcSession::setMaxOutgoingConnections "
                             "for this client or RpcServer::setMaxThreads for the corresponding "
                             "server"));
            return WOULD_BLOCK;
        }

        LOG_RPC_DETAIL("No available connections (have %zu clients and %zu servers). Waiting...",
                       session->mConnections.mOutgoing.size(),
                       session->mConnections.mIncoming.size());
        session->mAvailableConnectionCv.wait(_l);
    }
    session->mConnections.mWaitingThreads--;

    return OK;
}

void RpcSession::ExclusiveConnection::findConnection(uint64_t tid, sp<RpcConnection>* exclusive,
                                                     sp<RpcConnection>* available,
                                                     std::vector<sp<RpcConnection>>& sockets,
                                                     size_t socketsIndexHint) {
    LOG_ALWAYS_FATAL_IF(sockets.size() > 0 && socketsIndexHint >= sockets.size(),
                        "Bad index %zu >= %zu", socketsIndexHint, sockets.size());

    if (*exclusive != nullptr) return; // consistent with break below

    for (size_t i = 0; i < sockets.size(); i++) {
        sp<RpcConnection>& socket = sockets[(i + socketsIndexHint) % sockets.size()];

        // take first available connection (intuition = caching)
        if (available && *available == nullptr && socket->exclusiveTid == std::nullopt) {
            *available = socket;
            continue;
        }

        // though, prefer to take connection which is already inuse by this thread
        // (nested transactions)
        if (exclusive && socket->exclusiveTid == tid) {
            *exclusive = socket;
            break; // consistent with return above
        }
    }
}

RpcSession::ExclusiveConnection::~ExclusiveConnection() {
    // reentrant use of a connection means something less deep in the call stack
    // is using this fd, and it retains the right to it. So, we don't give up
    // exclusive ownership, and no thread is freed.
    if (!mReentrant && mConnection != nullptr) {
        mSession->clearConnectionTid(mConnection);
    }
}

bool RpcSession::hasActiveConnection(const std::vector<sp<RpcConnection>>& connections) {
    for (const auto& connection : connections) {
        if (connection->exclusiveTid != std::nullopt && !connection->rpcTransport->isWaiting()) {
            return true;
        }
    }
    return false;
}

bool RpcSession::hasActiveRequests() {
    RpcMutexUniqueLock _l(mMutex);
    if (hasActiveConnection(mConnections.mIncoming)) {
        return true;
    }
    if (hasActiveConnection(mConnections.mOutgoing)) {
        return true;
    }
    return mConnections.mWaitingThreads != 0;
}

} // namespace android