Make RLC retransmissions deterministic. It was creating unpredictablity

in the RLC AM tests. See issue #2228

lib/include/srslte/upper/rlc_am_lte.h

@@ -126,7 +126,7 @@ private:
 
     bool retx_queue_has_sn(uint32_t sn);
     int  required_buffer_size(rlc_amd_retx_t retx);
-    void retransmit_random_pdu();
+    void retransmit_pdu();
 
     // Helpers
     bool poll_required();

lib/src/upper/rlc_am_lte.cc

@@ -409,9 +409,9 @@ int rlc_am_lte::rlc_am_lte_tx::read_pdu(uint8_t* payload, uint32_t nof_bytes)
     goto unlock_and_exit;
   }
 
-  // Section 5.2.2.3 in TS 36.311, if tx_window is full and retx_queue empty, retransmit random PDU
+  // Section 5.2.2.3 in TS 36.311, if tx_window is full and retx_queue empty, retransmit PDU
   if (tx_window.size() >= RLC_AM_WINDOW_SIZE && retx_queue.empty()) {
-    retransmit_random_pdu();
+    retransmit_pdu();
   }
 
   // RETX if required
@@ -435,11 +435,11 @@ void rlc_am_lte::rlc_am_lte_tx::timer_expired(uint32_t timeout_id)
   pthread_mutex_lock(&mutex);
   if (poll_retx_timer.is_valid() && poll_retx_timer.id() == timeout_id) {
     log->debug("%s Poll reTx timer expired after %dms\n", RB_NAME, poll_retx_timer.duration());
-    // Section 5.2.2.3 in TS 36.311, schedule random PDU for retransmission if
+    // Section 5.2.2.3 in TS 36.311, schedule PDU for retransmission if
     // (a) both tx and retx buffer are empty, or
     // (b) no new data PDU can be transmitted (tx window is full)
     if ((retx_queue.empty() && tx_sdu_queue.size() == 0) || tx_window.size() >= RLC_AM_WINDOW_SIZE) {
-      retransmit_random_pdu();
+      retransmit_pdu();
     }
   }
   pthread_mutex_unlock(&mutex);
@@ -449,12 +449,11 @@ void rlc_am_lte::rlc_am_lte_tx::timer_expired(uint32_t timeout_id)
   }
 }
 
-void rlc_am_lte::rlc_am_lte_tx::retransmit_random_pdu()
+void rlc_am_lte::rlc_am_lte_tx::retransmit_pdu()
 {
   if (not tx_window.empty()) {
-    // randomly select PDU in tx window for retransmission
+    // select PDU in tx window for retransmission
     std::map<uint32_t, rlc_amd_tx_pdu_t>::iterator it = tx_window.begin();
-    std::advance(it, rand() % tx_window.size());
     log->info("%s Schedule SN=%d for reTx.\n", RB_NAME, it->first);
     rlc_amd_retx_t retx = {};
     retx.is_segment     = false;

lib/test/upper/rlc_am_test.cc

@@ -394,8 +394,7 @@ bool retx_test()
   assert(metrics.rx_buffered_bytes == 3);
 
   // Step timers until reordering timeout expires
-  int cnt = 5;
-  while (cnt--) {
+  for (int cnt = 0; cnt < 5; cnt++) {
     timers.step_all();
   }