Add mechanism to execute code on the main thread

This allows to schedule a runnable to be executed on the main thread,
until the event loop is explicitly terminated.

It is guaranteed that all accepted runnables will be executed (this
avoids possible memory leaks if a runnable owns resources).

PR #5270 <https://github.com/Genymobile/scrcpy/pull/5270>

app/src/events.c

@@ -1,6 +1,7 @@
 #include "events.h"
 
 #include "util/log.h"
+#include "util/thread.h"
 
 bool
 sc_push_event_impl(uint32_t type, const char *name) {
@@ -17,3 +18,49 @@ sc_push_event_impl(uint32_t type, const char *name) {
 
     return true;
 }
+
+bool
+sc_post_to_main_thread(sc_runnable_fn run, void *userdata) {
+    SDL_Event event = {
+        .user = {
+            .type = SC_EVENT_RUN_ON_MAIN_THREAD,
+            .data1 = run,
+            .data2 = userdata,
+        },
+    };
+    int ret = SDL_PushEvent(&event);
+    // ret < 0: error (queue full)
+    // ret == 0: event was filtered
+    // ret == 1: success
+    if (ret != 1) {
+        if (ret == 0) {
+            // if ret == 0, this is expected on exit, log in debug mode
+            LOGD("Could not post runnable to main thread (filtered)");
+        } else {
+            assert(ret < 0);
+            LOGW("Could not post runnable to main thread: %s", SDL_GetError());
+        }
+        return false;
+    }
+
+    return true;
+}
+
+static int SDLCALL
+task_event_filter(void *userdata, SDL_Event *event) {
+    (void) userdata;
+
+    if (event->type == SC_EVENT_RUN_ON_MAIN_THREAD) {
+        // Reject this event type from now on
+        return 0;
+    }
+
+    return 1;
+}
+
+void
+sc_reject_new_runnables(void) {
+    assert(sc_thread_get_id() == SC_MAIN_THREAD_ID);
+
+    SDL_SetEventFilter(task_event_filter, NULL);
+}

app/src/events.h

@@ -9,6 +9,7 @@
 
 enum {
     SC_EVENT_NEW_FRAME = SDL_USEREVENT,
+    SC_EVENT_RUN_ON_MAIN_THREAD,
     SC_EVENT_DEVICE_DISCONNECTED,
     SC_EVENT_SERVER_CONNECTION_FAILED,
     SC_EVENT_SERVER_CONNECTED,
@@ -25,4 +26,12 @@ sc_push_event_impl(uint32_t type, const char *name);
 
 #define sc_push_event(TYPE) sc_push_event_impl(TYPE, # TYPE)
 
+typedef void (*sc_runnable_fn)(void *userdata);
+
+bool
+sc_post_to_main_thread(sc_runnable_fn run, void *userdata);
+
+void
+sc_reject_new_runnables(void);
+
 #endif

app/src/scrcpy.c

@@ -174,6 +174,12 @@ event_loop(struct scrcpy *s) {
             case SDL_QUIT:
                 LOGD("User requested to quit");
                 return SCRCPY_EXIT_SUCCESS;
+            case SC_EVENT_RUN_ON_MAIN_THREAD: {
+                sc_runnable_fn run = event.user.data1;
+                void *userdata = event.user.data2;
+                run(userdata);
+                break;
+            }
             default:
                 if (!sc_screen_handle_event(&s->screen, &event)) {
                     return SCRCPY_EXIT_FAILURE;
@@ -184,6 +190,21 @@ event_loop(struct scrcpy *s) {
     return SCRCPY_EXIT_FAILURE;
 }
 
+static void
+terminate_event_loop(void) {
+    sc_reject_new_runnables();
+
+    SDL_Event event;
+    while (SDL_PollEvent(&event)) {
+        if (event.type == SC_EVENT_RUN_ON_MAIN_THREAD) {
+            // Make sure all posted runnables are run, to avoid memory leaks
+            sc_runnable_fn run = event.user.data1;
+            void *userdata = event.user.data2;
+            run(userdata);
+        }
+    }
+}
+
 // Return true on success, false on error
 static bool
 await_for_server(bool *connected) {
@@ -819,6 +840,7 @@ scrcpy(struct scrcpy_options *options) {
     }
 
     ret = event_loop(s);
+    terminate_event_loop();
     LOGD("quit...");
 
     if (options->video_playback) {