Improve startup time

On startup, the client has to:
 1. listen on a port
 2. push and start the server to the device
 3. wait for the server to connect (accept)
 4. read device name and size
 5. initialize SDL
 6. initialize the window and renderer
 7. show the window

From the execution of the app_process command to start the server on the
device, to the execution of the java main method, it takes ~800ms. As a
consequence, step 3 also takes ~800ms on the client.

Once complete, the client initializes SDL, which takes ~500ms.

These two expensive actions are executed sequentially:

                     HOST              DEVICE
listen on port        |                  |
push/start the server |----------------->|| app_process loads the jar
accept the connection .   ^              ||
                      .   |              ||
                      .   | WASTE        ||
                      .   |  OF          ||
                      .   | TIME         ||
                      .   |              ||
                      .   |              ||
                      .   v              X execution of our java main
connection accepted   |<-----------------| connect to the host
init SDL             ||                  |
                     || ,----------------| send frames
                     || |,---------------|
                     || ||,--------------|
                     || |||,-------------|
                     || ||||,------------|
init window/renderer  | |||||,-----------|
display frames        |<++++++-----------|
(many frames skipped)

The rationale for step 3 occuring before step 5 is that initializing
SDL replaces the SIGTERM handler to receive the event in the event loop,
so pressing Ctrl+C during step 5 would not work (since it blocks the
event loop).

But this is not so important; let's parallelize the SDL initialization
with the app_process execution (we'll just add a timeout to the
connection):

                     HOST              DEVICE
listen on port        |                  |
push/start the server |----------------->||app_process loads the jar
init SDL             ||                  ||
                     ||                  ||
                     ||                  ||
                     ||                  ||
                     ||                  ||
                     ||                  ||
accept the connection .                  ||
                      .                  X execution of our java main
connection accepted   |<-----------------| connect to the host
init window/renderer  |                  |
display frames        |<-----------------| send frames
                      |<-----------------|

In addition, show the window only once the first frame is available to
avoid flickering (opening a black window for 100~200ms).

Note: the window and renderer are initialized after the connection is
accepted because they use the device information received from the
device.

app/src/scrcpy.c

@@ -57,6 +57,11 @@ static void event_loop(void) {
                 SDL_LogDebug(SDL_LOG_CATEGORY_APPLICATION, "User requested to quit");
                 return;
             case EVENT_NEW_FRAME:
+                if (!screen.has_frame) {
+                    screen.has_frame = SDL_TRUE;
+                    // this is the very first frame, show the window
+                    screen_show_window(&screen);
+                }
                 if (!screen_update_frame(&screen, &frames)) {
                     return;
                 }
@@ -101,6 +106,11 @@ SDL_bool scrcpy(const char *serial, Uint16 local_port, Uint16 max_size, Uint32 b
 
     SDL_bool ret = SDL_TRUE;
 
+    if (!sdl_init_and_configure()) {
+        ret = SDL_FALSE;
+        goto finally_destroy_server;
+    }
+
     // to reduce startup time, we could be tempted to init other stuff before blocking here
     // but we should not block after SDL_Init since it handles the signals (Ctrl+C) in its
     // event loop: blocking could lead to deadlock
@@ -149,11 +159,6 @@ SDL_bool scrcpy(const char *serial, Uint16 local_port, Uint16 max_size, Uint32 b
         goto finally_destroy_controller;
     }
 
-    if (!sdl_init_and_configure()) {
-        ret = SDL_FALSE;
-        goto finally_stop_and_join_controller;
-    }
-
     if (!screen_init_rendering(&screen, device_name, frame_size)) {
         ret = SDL_FALSE;
         goto finally_stop_and_join_controller;

app/src/screen.c

@@ -137,7 +137,7 @@ SDL_bool screen_init_rendering(struct screen *screen, const char *device_name, s
 
     struct size window_size = get_initial_optimal_size(frame_size);
     screen->window = SDL_CreateWindow(device_name, SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
-                                      window_size.width, window_size.height, SDL_WINDOW_SHOWN | SDL_WINDOW_RESIZABLE);
+                                      window_size.width, window_size.height, SDL_WINDOW_HIDDEN | SDL_WINDOW_RESIZABLE);
     if (!screen->window) {
         SDL_LogCritical(SDL_LOG_CATEGORY_SYSTEM, "Could not create window: %s", SDL_GetError());
         return SDL_FALSE;
@@ -178,6 +178,10 @@ SDL_bool screen_init_rendering(struct screen *screen, const char *device_name, s
     return SDL_TRUE;
 }
 
+void screen_show_window(struct screen *screen) {
+    SDL_ShowWindow(screen->window);
+}
+
 void screen_destroy(struct screen *screen) {
     if (screen->texture) {
         SDL_DestroyTexture(screen->texture);
@@ -231,7 +235,6 @@ static SDL_bool prepare_for_frame(struct screen *screen, struct size new_frame_s
 
 // write the frame into the texture
 static void update_texture(struct screen *screen, const AVFrame *frame) {
-    screen->texture_initialized = SDL_TRUE;
     SDL_UpdateYUVTexture(screen->texture, NULL,
             frame->data[0], frame->linesize[0],
             frame->data[1], frame->linesize[1],
@@ -255,9 +258,7 @@ SDL_bool screen_update_frame(struct screen *screen, struct frames *frames) {
 
 void screen_render(struct screen *screen) {
     SDL_RenderClear(screen->renderer);
-    if (screen->texture_initialized) {
     SDL_RenderCopy(screen->renderer, screen->texture, NULL, NULL);
-    }
     SDL_RenderPresent(screen->renderer);
 }
 

app/src/screen.h

@@ -14,7 +14,7 @@ struct screen {
     struct size frame_size;
     //used only in fullscreen mode to know the windowed window size
     struct size windowed_window_size;
-    SDL_bool texture_initialized;
+    SDL_bool has_frame;
     SDL_bool fullscreen;
 };
 
@@ -30,7 +30,7 @@ struct screen {
         .width = 0,           \
         .height = 0,          \
     },                        \
-    .texture_initialized = SDL_FALSE, \
+    .has_frame = SDL_FALSE,   \
     .fullscreen = SDL_FALSE,  \
 }
 
@@ -40,11 +40,14 @@ SDL_bool sdl_init_and_configure(void);
 // initialize default values
 void screen_init(struct screen *screen);
 
-// initialize screen, create window, renderer and texture
+// initialize screen, create window, renderer and texture (window is hidden)
 SDL_bool screen_init_rendering(struct screen *screen,
                                const char *device_name,
                                struct size frame_size);
 
+// show the window
+void screen_show_window(struct screen *screen);
+
 // destroy window, renderer and texture (if any)
 void screen_destroy(struct screen *screen);