Netplay keyboard hack for keydown/keyup support

An unfortunate hack for cores that translate the keyboard input device
into keydown/keyup events without saving that in the savestate. We
simply replay the previous frame's input before loading the rewound
frame's state, to assure that both the state and the keyup/down state
are correct.

Ideally, cores would save this as part of the state, but it's a bit
proximal and a fairly significant change for those that use it, so it's
easier to fix in netplay.

network/netplay/netplay_handshake.c

@@ -1023,7 +1023,10 @@ bool netplay_handshake_pre_sync(netplay_t *netplay,
       pad.device = ntohl(device);
       netplay->config_devices[i] = pad.device;
       if ((pad.device&RETRO_DEVICE_MASK) == RETRO_DEVICE_KEYBOARD)
+      {
+         netplay->have_updown_device = true;
          netplay_key_hton_init();
+      }
 
       core_set_controller_port_device(&pad);
    }

network/netplay/netplay_init.c

@@ -476,7 +476,10 @@ netplay_t *netplay_new(void *direct_host, const char *server, uint16_t port,
          uint32_t dtype = input_config_get_device(i);
          netplay->config_devices[i] = dtype;
          if ((dtype&RETRO_DEVICE_MASK) == RETRO_DEVICE_KEYBOARD)
+         {
+            netplay->have_updown_device = true;
             netplay_key_hton_init();
+         }
          if (dtype != RETRO_DEVICE_NONE && !netplay_expected_input_size(netplay, 1<<i))
             RARCH_WARN("Netplay does not support input device %u\n", i+1);
       }

network/netplay/netplay_private.h

@@ -439,6 +439,13 @@ struct netplay
     * menu changes, as netplay needs fixed devices. */
    uint32_t config_devices[MAX_INPUT_DEVICES];
 
+   /* Set to true if we have a device that most cores translate to "up/down"
+    * actions, typically a keyboard. We need to keep track of this because with
+    * such a device, we need to "fix" the input state to the frame BEFORE a
+    * state load, then perform the state load, and the up/down states will
+    * proceed as expected */
+   bool have_updown_device;
+
    struct retro_callbacks cbs;
 
    /* TCP port (only set if serving) */

network/netplay/netplay_sync.c

@@ -787,6 +787,20 @@ void netplay_sync_post_frame(netplay_t *netplay, bool stalled)
       /* Replay frames. */
       netplay->is_replay = true;
 
+      /* If we have a keyboard device, we replay the previous frame's input
+       * just to assert that the keydown/keyup events work if the core
+       * translates them in that way */
+      if (netplay->have_updown_device)
+      {
+         netplay->replay_ptr = PREV_PTR(netplay->replay_ptr);
+         netplay->replay_frame_count--;
+         autosave_lock();
+         core_run();
+         autosave_unlock();
+         netplay->replay_ptr = NEXT_PTR(netplay->replay_ptr);
+         netplay->replay_frame_count++;
+      }
+
       if (netplay->quirks & NETPLAY_QUIRK_INITIALIZATION)
          /* Make sure we're initialized before we start loading things */
          netplay_wait_and_init_serialization(netplay);