Do not block in RLC TM read queue

lib/src/upper/rlc_tm.cc

@@ -60,9 +60,10 @@ void rlc_tm::empty_queue()
   // Drop all messages in TX queue
   byte_buffer_t *buf;
   while(ul_queue.size() > 0) {
-    ul_queue.read(&buf);
+    if (ul_queue.try_read(&buf)) {
       pool->deallocate(buf);
     }
+  }
 }
 
 void rlc_tm::reset()
@@ -112,7 +113,7 @@ int rlc_tm::read_pdu(uint8_t *payload, uint32_t nof_bytes)
     return 0;
   }
   byte_buffer_t *buf;
-  ul_queue.read(&buf);
+  if (ul_queue.try_read(&buf)) {
     pdu_size = buf->N_bytes;
     memcpy(payload, buf->msg, buf->N_bytes);
     log->debug("%s Complete SDU scheduled for tx. Stack latency: %ld us\n",
@@ -120,6 +121,10 @@ int rlc_tm::read_pdu(uint8_t *payload, uint32_t nof_bytes)
     pool->deallocate(buf);
     log->info_hex(payload, pdu_size, "TX %s, %s PDU", rrc->get_rb_name(lcid).c_str(), rlc_mode_text[RLC_MODE_TM]);
     return pdu_size;
+  } else {
+    log->warning("Queue empty while trying to read\n");
+    return 0;
+  }
 }
 
 void rlc_tm::write_pdu(uint8_t *payload, uint32_t nof_bytes)