Buffer management bug in radio.cc

[Ksk190809]

Apologies if this is a duplicate -- I didn't see a match in the multiple pages of issues I reviewed.

In the radio::tx() method in lib/src/radio/radio.cc when interpolating (ratio > 1), the buffer argument (passed by reference) has its channel buffer overwritten with the interpolated buffer pointer:

       // Set the buffer pointer
      buffer.set(ch, tx_buffer[ch].data());

In srsUE method phy_common::worker_end() in srsue/src/phy/phy_common.cc there is code to send zeros when the method is_continuous_tx() returns true -- controlled by configuration parameter rf.continuous_tx defaulting to true -- using a statically declared buffer zeroes_multi that gets its channel array pointer overwritten by the above code. Consequently, zeros are transmitted under the continuous_tx mode until after the first time the radio::tx() method is asked to send actual data, placing non-zero content in the buffer now held by both zeroes_multi and the tx_buffer held by the radio object. This results in non-zero data being transmitted when not expected. An easy work-around would be to disable the continuous_tx option, as these transmissions appear to be unnecessary. The following code patch introduces a holding buffer in the radio class to avoid unnecessary copying of data while preserving the integrity of the passed-in buffer argument:

diff --git a/lib/include/srsran/radio/radio.h b/lib/include/srsran/radio/radio.h
index 367ef35d8..c50ecfa65 100644
--- a/lib/include/srsran/radio/radio.h
+++ b/lib/include/srsran/radio/radio.h
@@ -106,6 +106,7 @@ private:
   std::array<std::vector<cf_t>, SRSRAN_MAX_CHANNELS>      dummy_buffers;
   std::mutex                                              tx_mutex;
   std::mutex                                              rx_mutex;
+  rf_buffer_t                                             tx_buffer_holder;
   std::array<std::vector<cf_t>, SRSRAN_MAX_CHANNELS>      tx_buffer;
   std::array<std::vector<cf_t>, SRSRAN_MAX_CHANNELS>      rx_buffer;
   std::array<srsran_resampler_fft_t, SRSRAN_MAX_CHANNELS> interpolators = {};
diff --git a/lib/src/radio/radio.cc b/lib/src/radio/radio.cc
index 8124185e6..700e0d235 100644
--- a/lib/src/radio/radio.cc
+++ b/lib/src/radio/radio.cc
@@ -426,6 +426,12 @@ bool radio::tx(rf_buffer_interface& buffer, const rf_timestamp_interface& tx_tim
   // Get number of samples at the low rate
   uint32_t nof_samples = buffer.get_nof_samples();
 
+  // Make a copy of "buffer" into the holding buffer; use holding buffer thereafter
+  for (uint32_t ch = 0; ch < nof_channels; ch++) {
+    tx_buffer_holder.set(ch, buffer.get(ch));
+  }
+  tx_buffer_holder.set_nof_samples(nof_samples);
+
   // Check that number of the interpolated samples does not exceed the buffer size
   if (ratio > 1 && (size_t)nof_samples * (size_t)ratio > tx_buffer[0].size()) {
     // This is a corner case that could happen during sample rate change transitions, as it does not have a negative
@@ -433,8 +439,8 @@ bool radio::tx(rf_buffer_interface& buffer, const rf_timestamp_interface& tx_tim
     fmt::memory_buffer buff;
     fmt::format_to(buff,
                    "Tx number of samples ({}/{}) exceeds buffer size ({})\n",
-                   buffer.get_nof_samples(),
-                   buffer.get_nof_samples() * ratio,
+                   tx_buffer_holder.get_nof_samples(),
+                   tx_buffer_holder.get_nof_samples() * ratio,
                    tx_buffer[0].size());
     logger.info("%s", to_c_str(buff));
 
@@ -446,18 +452,18 @@ bool radio::tx(rf_buffer_interface& buffer, const rf_timestamp_interface& tx_tim
   if (interpolators[0].ratio > 1) {
     for (uint32_t ch = 0; ch < nof_channels; ch++) {
       // Perform actual interpolation
-      srsran_resampler_fft_run(&interpolators[ch], buffer.get(ch), tx_buffer[ch].data(), nof_samples);
+      srsran_resampler_fft_run(&interpolators[ch], tx_buffer_holder.get(ch), tx_buffer[ch].data(), nof_samples);
 
       // Set the buffer pointer
-      buffer.set(ch, tx_buffer[ch].data());
+      tx_buffer_holder.set(ch, tx_buffer[ch].data());
     }
 
     // Set buffer size after applying the interpolation
-    buffer.set_nof_samples(nof_samples * ratio);
+    tx_buffer_holder.set_nof_samples(nof_samples * ratio);
   }
 
   for (uint32_t device_idx = 0; device_idx < (uint32_t)rf_devices.size(); device_idx++) {
-    ret &= tx_dev(device_idx, buffer, tx_time.get(device_idx));
+    ret &= tx_dev(device_idx, tx_buffer_holder, tx_time.get(device_idx));
   }
 
   is_start_of_burst = false;