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/**                                                                                                                                                                                                                  
                      
 * Return a free transmit buffer, wrapped by its corresponding                
                                                                                                                                                             
 * BufferDescriptor. If there are none, block until one is available.                                                     
                                                                                                                 
 *                                                                                                                                                                                                                                    
     
 */
template<typename Infiniband>
typename Infiniband::BufferDescriptor*
InfRcTransport<Infiniband>::getTransmitBuffer()
{
    CycleCounter<uint64_t> timeThis2;
    // if we've drained our free tx buffer pool, we must wait.                    
                                                                                                                                                         
    while (txBuffers.empty()) {

        ibv_wc retArray[MAX_TX_QUEUE_DEPTH];
        CycleCounter<uint64_t> timeThis;
        int n = infiniband->pollCompletionQueue(commonTxCq,
                                                MAX_TX_QUEUE_DEPTH,
                                                retArray);
        uint64_t gtbPollNanos = cyclesToNanoseconds(timeThis.stop());
        serverStats.gtbPollNanos += gtbPollNanos;
        serverStats.gtbPollCount++;

        if (0 >= n) {
             serverStats.gtbPollZeroNCount++;
             serverStats.gtbPollZeroNanos += gtbPollNanos;
        } else {
             serverStats.gtbPollNonZeroNAvg += n;
             serverStats.gtbPollNonZeroNanos += gtbPollNanos;
        }

        for (int i = 0; i < n; i++) {
            BufferDescriptor* bd =
                reinterpret_cast<BufferDescriptor*>(retArray[i].wr_id);
            txBuffers.push_back(bd);

            if (retArray[i].status != IBV_WC_SUCCESS) {
                LOG(ERROR, "Transmit failed: %s",
                    infiniband->wcStatusToString(retArray[i].status));
            }
        }

    }


    BufferDescriptor* bd = txBuffers.back();
    txBuffers.pop_back();

    serverStats.infrcGetTxBufferNanos += timeThis2.stop();
    return bd;
}