module-rtp: Clear ringbuffer in constant delay mode

Clearing the ring buffer is important not only in the direct timestamp
mode, but also in the constant delay mode, since missed packets can lead
to gaps in the ring buffer. These gaps may have stale data inside if the
ringbuffer is not cleared after reading from it.

src/modules/module-rtp/audio.c

@@ -249,6 +249,25 @@ static void rtp_audio_process_playback(void *data)
 					((uint64_t)timestamp * stride) % impl->actual_max_buffer_size,
 					d[0].data, wanted * stride);
 
+			/* Clear the bytes that were just retrieved. Unlike in the
+			 * direct timestamp mode, here, bytes are always read out
+			 * of the ring buffer in sequence - the read pointer does
+			 * not "jump around" (which can happen in direct timestamp
+			 * mode if the last iteration has been a while ago and the
+			 * driver clock time advanced significantly, or if the driver
+			 * time experienced a discontinuity). However, should there
+			 * be packet loss, it could lead to segments in the ring
+			 * buffer that should have been written to but weren't written
+			 * to. These segments would then contain old stale data. By
+			 * clearing data out of the ring buffer after reading it, it
+			 * is ensured that no stale data can exist - in the packet loss
+			 * case, the outcome would be a gap made of nullsamples instead. */
+			ringbuffer_clear(&impl->ring,
+					impl->buffer,
+					impl->actual_max_buffer_size,
+					((uint64_t)timestamp * stride) % impl->actual_max_buffer_size,
+					wanted * stride);
+
 			timestamp += wanted;
 			spa_ringbuffer_read_update(&impl->ring, timestamp);
 		}