rlc_am_lte: use try-lock when building status PDU

the patch uses try-lock whenever a status PDU is tried
to be built. This makes sure that when the lock is currently
hold (e.g. by a thread processing rx PDUs) the generation
of the status PDUs is not taking too long and blocking the calling
thread. Instead the status PDU generation is deferred to the next
Tx opportunity.

It's a probabilistic approach that assumes that at some stage the
lock can in fact be acquired.

lib/src/rlc/rlc_am_lte.cc

@@ -666,8 +666,10 @@ bool rlc_am_lte::rlc_am_lte_tx::poll_required()
 int rlc_am_lte::rlc_am_lte_tx::build_status_pdu(uint8_t* payload, uint32_t nof_bytes)
 {
   int pdu_len = parent->rx.get_status_pdu(&tx_status, nof_bytes);
-  log_rlc_am_status_pdu_to_string(logger.debug, "%s", &tx_status);
-  if (pdu_len > 0 && nof_bytes >= static_cast<uint32_t>(pdu_len)) {
+  if (pdu_len == SRSRAN_ERROR) {
+    logger.debug("%s Deferred Status PDU. Cause: Failed to acquire Rx lock", RB_NAME);
+    pdu_len = 0;
+  } else if (pdu_len > 0 && nof_bytes >= static_cast<uint32_t>(pdu_len)) {
     log_rlc_am_status_pdu_to_string(logger.info, "%s Tx status PDU - %s", &tx_status, RB_NAME);
 
     parent->rx.reset_status();
@@ -1922,9 +1924,14 @@ void rlc_am_lte::rlc_am_lte_rx::timer_expired(uint32_t timeout_id)
 }
 
 // Called from Tx object to pack status PDU that doesn't exceed a given size
+// If lock-acquisition fails, return -1. Otherwise it returns the length of the generated PDU.
 int rlc_am_lte::rlc_am_lte_rx::get_status_pdu(rlc_status_pdu_t* status, const uint32_t max_pdu_size)
 {
-  std::lock_guard<std::mutex> lock(mutex);
+  std::unique_lock<std::mutex> lock(mutex, std::try_to_lock);
+  if (not lock.owns_lock()) {
+    return SRSRAN_ERROR;
+  }
+
   status->N_nack = 0;
   status->ack_sn = vr_r; // start with lower edge of the rx window
 
@@ -1970,7 +1977,10 @@ int rlc_am_lte::rlc_am_lte_rx::get_status_pdu(rlc_status_pdu_t* status, const ui
 // Called from Tx object to obtain length of the full status PDU
 int rlc_am_lte::rlc_am_lte_rx::get_status_pdu_length()
 {
-  std::lock_guard<std::mutex> lock(mutex);
+  std::unique_lock<std::mutex> lock(mutex, std::try_to_lock);
+  if (not lock.owns_lock()) {
+    return 0;
+  }
   rlc_status_pdu_t            status = {};
   status.ack_sn                      = vr_ms;
   uint32_t i                         = vr_r;