rlc_um: fix reestablishment for sending UM entity

we've reset the rx state variables during reestablishment but not
the tx state variable.

lib/include/srslte/upper/rlc_um_base.h

@@ -105,6 +105,7 @@ protected:
 
     // helper functions
     virtual void debug_state() = 0;
+    virtual void reset()       = 0;
   };
 
   // Receiver sub-class base

lib/include/srslte/upper/rlc_um_lte.h

@@ -63,6 +63,8 @@ private:
     uint32_t get_buffer_state();
 
   private:
+    void reset();
+
     /****************************************************************************
      * State variables and counters
      * Ref: 3GPP TS 36.322 v10.0.0 Section 7

lib/include/srslte/upper/rlc_um_nr.h

@@ -63,6 +63,8 @@ private:
     uint32_t get_buffer_state();
 
   private:
+    void reset();
+
     uint32_t TX_Next = 0; // send state as defined in TS 38.322 v15.3 Section 7
                           // It holds the value of the SN to be assigned for the next newly generated UMD PDU with
                           // segment. It is initially set to 0, and is updated after the UM RLC entity submits a UMD PDU

lib/src/upper/rlc_um_base.cc

@@ -215,6 +215,7 @@ rlc_um_base::rlc_um_base_tx::~rlc_um_base_tx() {}
 void rlc_um_base::rlc_um_base_tx::stop()
 {
   empty_queue();
+  reset();
 }
 
 void rlc_um_base::rlc_um_base_tx::reestablish()

lib/src/upper/rlc_um_lte.cc

@@ -219,6 +219,11 @@ void rlc_um_lte::rlc_um_lte_tx::debug_state()
   log->debug("%s vt_us = %d\n", rb_name.c_str(), vt_us);
 }
 
+void rlc_um_lte::rlc_um_lte_tx::reset()
+{
+  vt_us = 0;
+}
+
 /****************************************************************************
  * Rx subclass implementation
  ***************************************************************************/

lib/src/upper/rlc_um_nr.cc

@@ -205,7 +205,13 @@ int rlc_um_nr::rlc_um_nr_tx::build_data_pdu(unique_byte_buffer_t pdu, uint8_t* p
 
 void rlc_um_nr::rlc_um_nr_tx::debug_state()
 {
-  log->debug("%s TX_Next = %d\n", rb_name.c_str(), TX_Next);
+  log->debug("%s TX_Next=%d, next_so=%d\n", rb_name.c_str(), TX_Next, next_so);
+}
+
+void rlc_um_nr::rlc_um_nr_tx::reset()
+{
+  TX_Next = 0;
+  next_so = 0;
 }
 
 /****************************************************************************