Method KeepWrite

src/brpc/socket.cpp:1782–1866

static const size_t DATA_LIST_MAX = 256;

void* Socket::KeepWrite(void* void_arg) {
    g_vars->nkeepwrite << 1;
    WriteRequest* req = static_cast<WriteRequest*>(void_arg);
    SocketUniquePtr s(req->get_socket());

    // When error occurs, spin until there's no more requests instead of
    // returning directly otherwise _write_head is permantly non-NULL which
    // makes later Write() abnormal.
    WriteRequest* cur_tail = NULL;
    do {
        // req was written, skip it.
        bool need_shutdown = false;
        if (req->next != NULL && req->data.empty()) {
            WriteRequest* const saved_req = req;
            need_shutdown = req->need_shutdown_write();
            req = req->next;
            s->ReturnSuccessfulWriteRequest(saved_req);
        }
        if (need_shutdown) {
            LOG(WARNING) << "Shutdown write of " << *s;
            break;
        }

        const ssize_t nw = s->DoWrite(req);
        if (nw < 0) {
            if (errno != EAGAIN && errno != EOVERCROWDED) {
                const int saved_errno = errno;
                PLOG(WARNING) << "Fail to keep-write into " << *s;
                s->SetFailed(saved_errno, "Fail to keep-write into %s: %s",
                             s->description().c_str(), berror(saved_errno));
                break;
            }
        } else {
            s->AddOutputBytes(nw);
        }
        // Release WriteRequest until non-empty data, last request or shutdown write.
        while (req->next != NULL && req->data.empty()) {
            WriteRequest* const saved_req = req;
            need_shutdown = req->need_shutdown_write();
            req = req->next;
            s->ReturnSuccessfulWriteRequest(saved_req);
            if (need_shutdown) {
                break;
            }
        }
        if (need_shutdown) {
            LOG(WARNING) << "Shutdown write of " << *s;
            break;
        }
        // TODO(gejun): wait for epollout when we actually have written
        // all the data. This weird heuristic reduces 30us delay...
        // Update(12/22/2015): seem not working. better switch to correct code.
        // Update(1/8/2016, r31823): Still working.
        // Update(8/15/2017): Not working, performance downgraded.
        //if (nw <= 0 || req->data.empty()/*note*/) {
        if (nw <= 0) {
            // NOTE: Waiting epollout within timeout is a must to force
            // KeepWrite to check and setup pending WriteRequests periodically,