Method IsWriteComplete

src/brpc/socket.cpp:1166–1219

Check if there're new requests appended. If yes, point old_head to reversed new requests and return false; If no: old_head is fully written, set _write_head to NULL and return true; old_head is not written yet, keep _write_head unchanged and return false; `old_head' is last new_head got from this function or (in another word) tail of current writing list. `singular_node' is true iff `old_head' is

// tail of current writing list.
// `singular_node' is true iff `old_head' is the only node in its list.
bool Socket::IsWriteComplete(Socket::WriteRequest* old_head,
                             bool singular_node,
                             Socket::WriteRequest** new_tail) {
    CHECK(NULL == old_head->next);
    // Try to set _write_head to NULL to mark that the write is done.
    WriteRequest* new_head = old_head;
    WriteRequest* desired = NULL;
    bool return_when_no_more = true;
    if (!old_head->data.empty() || !singular_node) {
        desired = old_head;
        // Write is obviously not complete if old_head is not fully written.
        return_when_no_more = false;
    }
    if (_write_head.compare_exchange_strong(
            new_head, desired, butil::memory_order_acquire)) {
        // No one added new requests.
        if (new_tail) {
            *new_tail = old_head;
        }
        return return_when_no_more;
    }
    CHECK_NE(new_head, old_head);
    // Above acquire fence pairs release fence of exchange in Write() to make
    // sure that we see all fields of requests set.

    // Someone added new requests.
    // Reverse the list until old_head.
    WriteRequest* tail = NULL;
    WriteRequest* p = new_head;
    do {
        while (p->next == WriteRequest::UNCONNECTED) {
            // TODO(gejun): elaborate this
            sched_yield();
        }
        WriteRequest* const saved_next = p->next;
        p->next = tail;
        tail = p;
        p = saved_next;
        CHECK(p != NULL);
    } while (p != old_head);

    // Link old list with new list.
    old_head->next = tail;
    // Call Setup() from oldest to newest, notice that the calling sequence
    // matters for protocols using pipelined_count, this is why we don't
    // call Setup in above loop which is from newest to oldest.
    for (WriteRequest* q = tail; q; q = q->next) {
        q->Setup(this);
    }
    if (new_tail) {
        *new_tail = new_head;
    }
    return false;
}

int Socket::WaitEpollOut(int fd, bool pollin, const timespec* abstime) {
    if (!ValidFileDescriptor(fd)) {