Rather than stalling by blocking and becoming unresponsive, stall by

replaying the same frame. TODO: Maybe mute the audio?

network/netplay/netplay.c

@@ -149,6 +149,13 @@ static bool get_self_input_state(netplay_t *netplay)
    if (!netplay_delta_frame_ready(netplay, ptr, netplay->self_frame_count))
       return false;
 
+   if (ptr->have_local)
+   {
+      /* We've already read this frame! */
+      netplay->self_ptr = NEXT_PTR(netplay->self_ptr);
+      return true;
+   }
+
    if (!input_driver_is_libretro_input_blocked() && netplay->self_frame_count > 0)
    {
       unsigned i;
@@ -206,6 +213,7 @@ static bool get_self_input_state(netplay_t *netplay)
    }
 
    memcpy(ptr->self_state, state, sizeof(state));
+   ptr->have_local = true;
    netplay->self_ptr = NEXT_PTR(netplay->self_ptr);
    return true;
 }
@@ -296,14 +304,8 @@ static bool netplay_get_cmd(netplay_t *netplay)
 
          if (buffer[0] != netplay->read_frame_count)
          {
-             /* FIXME: JUST drop it? */
-             return netplay_cmd_nak(netplay);
-         }
-
-         if (!netplay_delta_frame_ready(netplay, &netplay->buffer[netplay->read_ptr], netplay->read_frame_count))
-         {
-             /* FIXME: If we're here, we're desyncing. */
-             netplay->must_fast_forward = true;
+             /* FIXME: Except on the first (null) frame, this should be
+              * impossible, so maybe just disconnect? */
              return netplay_cmd_nak(netplay);
          }
 
@@ -313,7 +315,7 @@ static bool netplay_get_cmd(netplay_t *netplay)
          netplay->read_ptr = NEXT_PTR(netplay->read_ptr);
          netplay->read_frame_count++;
          netplay->timeout_cnt = 0;
-         return netplay_cmd_ack(netplay);
+         return true;
       }
 
       case NETPLAY_CMD_FLIP_PLAYERS:
@@ -431,7 +433,7 @@ static int poll_input(netplay_t *netplay, bool block)
 
       RARCH_LOG("Network is stalling, resending packet... Count %u of %d ...\n",
             netplay->timeout_cnt, MAX_RETRIES);
-   } while (had_input || (block && netplay->read_frame_count < netplay->self_frame_count));
+   } while (had_input);
 
    /*if (block)
       return -1;*/
@@ -537,9 +539,8 @@ static bool netplay_poll(netplay_t *netplay)
       return true;
    }
 
-   /* We might have reached the end of the buffer, where we 
-    * simply have to block. */
-   res = poll_input(netplay, netplay->read_frame_count < netplay->self_frame_count - 10); /* FIXME: configure stalling intervals */
+   /* Read Netplay input */
+   res = poll_input(netplay, 0); /* FIXME: configure stalling intervals */
    if (res == -1)
    {
       netplay->has_connection = false;
@@ -582,6 +583,18 @@ static bool netplay_poll(netplay_t *netplay)
    else
       netplay->buffer[PREV_PTR(netplay->self_ptr)].used_real = true;
 
+   /* Consider stalling */
+   switch (netplay->stall) {
+       case RARCH_NETPLAY_STALL_RUNNING_FAST:
+          if (netplay->read_frame_count >= netplay->self_frame_count)
+             netplay->stall = RARCH_NETPLAY_STALL_NONE;
+          break;
+
+       default: /* not stalling */
+          if (netplay->read_frame_count < netplay->self_frame_count - 10)
+             netplay->stall = RARCH_NETPLAY_STALL_RUNNING_FAST;
+   }
+
    return true;
 }
 

network/netplay/netplay_net.c

@@ -166,6 +166,20 @@ static void netplay_net_post_frame(netplay_t *netplay)
    }
 #endif
 
+   /* If we're supposed to stall, rewind */
+   if (netplay->stall)
+   {
+      retro_ctx_serialize_info_t serial_info;
+
+      netplay->self_ptr = PREV_PTR(netplay->self_ptr);
+      netplay->self_frame_count--;
+
+      serial_info.data       = NULL;
+      serial_info.data_const = netplay->buffer[netplay->self_ptr].state;
+      serial_info.size       = netplay->state_size;
+
+      core_unserialize(&serial_info);
+   }
 }
 static bool netplay_net_init_buffers(netplay_t *netplay)
 {

network/netplay/netplay_private.h

@@ -49,8 +49,13 @@ struct delta_frame
    uint32_t simulated_input_state[WORDS_PER_FRAME - 1];
    uint32_t self_state[WORDS_PER_FRAME - 1];
 
+   /* Have we read local input? */
    bool have_local;
+
+   /* Badly named: This is !have_real(_remote) */
    bool is_simulated;
+
+   /* Is the current state as of self_frame_count using the real data? */
    bool used_real;
 };
 
@@ -60,6 +65,12 @@ struct netplay_callbacks {
    bool (*info_cb)   (netplay_t *netplay, unsigned frames);
 };
 
+enum rarch_netplay_stall_reasons
+{
+    RARCH_NETPLAY_STALL_NONE = 0,
+    RARCH_NETPLAY_STALL_RUNNING_FAST
+};
+
 struct netplay
 {
    char nick[32];
@@ -131,6 +142,9 @@ struct netplay
    bool pause;
    uint32_t pause_frame;
 
+   /* And stalling */
+   int stall;
+
    struct netplay_callbacks* net_cbs;
 };