trysend.cpp

                bool TrySend(connection *const c)
                {
                        iovec iov[128];
                        int fd = c->fd;

sendv:
                        auto it = c->respTail;
                        int32_t r;
                        bool haveCork;

                        // If we have multiple requests(therefore multiple writev() calls are likely required)
                        // and/or have say data to stream using sendfile(), then we need a cork
                        // We don't use sendfile(), but that's how it'd work. Also, if we did need sendfile()
                        // c could have been == nullptr.

                        if (!it->next) // Fast path: single request(no pipelining)
                        {
                                r = writev(fd, it->iov + it->iovCur, it->iovCnt - it->iovCur);
                                haveCork = false;
                        }
                        else
                        {
                                // Need to reduce writev() calls to absolute minimum
                                uint8_t i{0};
                                auto cur= it->iovCur;


                                if (!SwitchNetwork::SetTCPCork(fd, 1))
                                        haveCork = true;

                                do
                                {
                                        if (cur >= 127)
                                        {
                                                // Patch, starting from it->iovCnt - 127. Reset it afterwards
                                                const auto cnt = it->iovCnt;

                                                for (decltype(it->iovCnt) i = it->iovCur - 127; i != cnt; ++i)
                                                {
                                                        auto &v = it->iov[i];

                                                        if (v.iov_len >= (1U<<30))
                                                        {
                                                                v.iov_len -= 1U<<30;
                                                                v.iov_base = it->content->At((uint32_t)(uintptr_t)v.iov_base);
                                                        }
                                                }
                                                cur = it->iovCur = 0;
                                        }

                                        const auto n = Min<uint8_t>(sizeof_array(iov) - i, it->iovCnt - cur);

                                        (void)memcpy(iov + i, it->iov + it->iovCur, n * sizeof(iovec));
                                        i+=n;
                                        cur = 0;
                                } while (i != sizeof_array(iov) && (it = it->next));


                                // Timings:
                                // 5%: 4
                                // 10%: 5
                                // 25%: 5
                                // 50%: 7
                                // 75%: 8
                                // 90%: 13
                                // 95%: 19
                                // 99%: 31
                                r = writev(c->fd, iov, i);
                        }

                        if (unlikely(r == -1))
                        {
                                const auto err = errno;

                                if (unlikely(haveCork))
                                        SwitchNetwork::SetTCPCork(fd, 0);

                                if (err == EAGAIN)
                                {
                                        if (!c->needIoAvail)
                                        {
                                                c->needIoAvail = true;
                                                (void)poller.SetDataAndEvents(c->fd, c, POLLIN|POLLOUT);
                                        }
                                }
                                else if (err != EINTR)
                                {
                                        Shutdown(c, __LINE__);
                                        return false;
                                }

                                return true;
                        }

                        for (it = c->respTail; r >= it->iov[it->iovCur].iov_len; )
                        {
                                r-=it->iov[it->iovCur].iov_len;
                                if (++(it->iovCur) == it->iovCnt)
                                {
                                        auto *const n = it->next;

                                        PutResponse(it);
                                        it = c->respTail = n;
                                        if (!it)
                                        {
                                                if (unlikely(haveCork))
                                                        SwitchNetwork::SetTCPCork(fd, 0);

                                                if (!c->keepAlive)
                                                {
                                                        Shutdown(c, __LINE__);
                                                        return false;
                                                }
                                                else
                                                {
                                                        c->respHead = nullptr;
                                                        if (c->needIoAvail)
                                                        {
                                                                (void)poller.SetDataAndEvents(c->fd, c, POLLIN);
                                                                c->needIoAvail = false;
                                                        }
                                                        return true;
                                                }
                                        }
                                }
                        }

                        auto &v = it->iov[it->iovCur];

                        v.iov_len-=r;
                        v.iov_base = (char *)v.iov_base + r;

                        // We didn't get to send all the content out - because we are limited in terms of sizeof_array(iov)
                        // so we need another short at it
                        // Ideally, we 'd have a large enough iov[] anyway. 
                        //
                        // However, this could have been because there is no more space left in the socket send buf
                        // so it only accepted as much as it could. In this case, another call to writev() will
                        // return EAGAIN. 
                        // 
                        // We could probably better deal with this (enter needIoAvail mode if not all scheduled
                        // content was dispatched, instead of retrying).
                        goto sendv;
                }