Clarify which thread is the main thread

If video is initialized, the main thread is the one that initialized video, otherwise if events are initialized, the main thread is the thread that initialized events, otherwise the main thread is the one that called the main function.

Fixes https://github.com/libsdl-org/SDL/issues/14511

src/SDL.c

@@ -189,6 +189,8 @@ static bool SDL_MainIsReady = false;
 static bool SDL_MainIsReady = true;
 #endif
 static SDL_ThreadID SDL_MainThreadID = 0;
+static SDL_ThreadID SDL_EventsThreadID = 0;
+static SDL_ThreadID SDL_VideoThreadID = 0;
 static bool SDL_bInMainQuit = false;
 static Uint8 SDL_SubsystemRefCount[32];
 
@@ -266,14 +268,16 @@ void SDL_SetMainReady(void)
 
 bool SDL_IsMainThread(void)
 {
-    if (SDL_MainThreadID == 0) {
-        // Not initialized yet?
-        return true;
+    if (SDL_VideoThreadID) {
+        return (SDL_GetCurrentThreadID() == SDL_VideoThreadID);
     }
-    if (SDL_MainThreadID == SDL_GetCurrentThreadID()) {
-        return true;
+    if (SDL_EventsThreadID) {
+        return (SDL_GetCurrentThreadID() == SDL_EventsThreadID);
     }
-    return false;
+    if (SDL_MainThreadID) {
+        return (SDL_GetCurrentThreadID() == SDL_MainThreadID);
+    }
+    return true;
 }
 
 // Initialize all the subsystems that require initialization before threads start
@@ -281,6 +285,11 @@ void SDL_InitMainThread(void)
 {
     static bool done_info = false;
 
+    // If we haven't done it by now, mark this as the main thread
+    if (SDL_MainThreadID == 0) {
+        SDL_MainThreadID = SDL_GetCurrentThreadID();
+    }
+
     SDL_InitTLSData();
     SDL_InitEnvironment();
     SDL_InitTicks();
@@ -335,6 +344,11 @@ bool SDL_InitSubSystem(SDL_InitFlags flags)
     if (flags & SDL_INIT_EVENTS) {
         if (SDL_ShouldInitSubsystem(SDL_INIT_EVENTS)) {
             SDL_IncrementSubsystemRefCount(SDL_INIT_EVENTS);
+
+            // Note which thread initialized events
+            // This is the thread which should be pumping events
+            SDL_EventsThreadID = SDL_GetCurrentThreadID();
+
             if (!SDL_InitEvents()) {
                 SDL_DecrementSubsystemRefCount(SDL_INIT_EVENTS);
                 goto quit_and_error;
@@ -354,12 +368,16 @@ bool SDL_InitSubSystem(SDL_InitFlags flags)
                 goto quit_and_error;
             }
 
+            SDL_IncrementSubsystemRefCount(SDL_INIT_VIDEO);
+
             // We initialize video on the main thread
             // On Apple platforms this is a requirement.
             // On other platforms, this is the definition.
-            SDL_MainThreadID = SDL_GetCurrentThreadID();
+            SDL_VideoThreadID = SDL_GetCurrentThreadID();
+#ifdef SDL_PLATFORM_APPLE
+            SDL_assert(SDL_VideoThreadID == SDL_MainThreadID);
+#endif
 
-            SDL_IncrementSubsystemRefCount(SDL_INIT_VIDEO);
             if (!SDL_VideoInit(NULL)) {
                 SDL_DecrementSubsystemRefCount(SDL_INIT_VIDEO);
                 SDL_PushError();
@@ -609,6 +627,7 @@ void SDL_QuitSubSystem(SDL_InitFlags flags)
         if (SDL_ShouldQuitSubsystem(SDL_INIT_VIDEO)) {
             SDL_QuitRender();
             SDL_VideoQuit();
+            SDL_VideoThreadID = 0;
             // video implies events
             SDL_QuitSubSystem(SDL_INIT_EVENTS);
         }
@@ -619,6 +638,7 @@ void SDL_QuitSubSystem(SDL_InitFlags flags)
     if (flags & SDL_INIT_EVENTS) {
         if (SDL_ShouldQuitSubsystem(SDL_INIT_EVENTS)) {
             SDL_QuitEvents();
+            SDL_EventsThreadID = 0;
         }
         SDL_DecrementSubsystemRefCount(SDL_INIT_EVENTS);
     }

src/SDL_internal.h

@@ -265,6 +265,7 @@ extern "C" {
 #include "SDL_utils_c.h"
 #include "SDL_hashtable.h"
 
+
 /* SDL_ExitProcess is not declared in any public header, although
    it is shared between some parts of SDL, because we don't want
    anything calling it without an extremely good reason. */

src/events/SDL_events.c

@@ -1494,6 +1494,9 @@ void SDL_PumpEventMaintenance(void)
 // Run the system dependent event loops
 static void SDL_PumpEventsInternal(bool push_sentinel)
 {
+    // This should only be called on the main thread, check in debug builds
+    SDL_assert(SDL_IsMainThread());
+
     // Free any temporary memory from old events
     SDL_FreeTemporaryMemory();
 

src/main/SDL_main_callbacks.c

@@ -147,13 +147,19 @@ void SDL_QuitMainCallbacks(SDL_AppResult result)
     SDL_Quit();
 }
 
-void SDL_CheckDefaultArgcArgv(int *argc, char ***argv)
+static void SDL_CheckDefaultArgcArgv(int *argc, char ***argv)
 {
-    if (!argv)
-    {
+    if (!*argv) {
         static char dummyargv0[] = { 'S', 'D', 'L', '_', 'a', 'p', 'p', '\0' };
         static char *argvdummy[2] = { dummyargv0, NULL };
         *argc = 1;
         *argv = argvdummy;
     }
 }
+
+int SDL_CallMainFunction(int argc, char *argv[], SDL_main_func mainFunction)
+{
+    SDL_CheckDefaultArgcArgv(&argc, &argv);
+    SDL_SetMainReady();
+    return mainFunction(argc, argv);
+}

src/main/SDL_main_callbacks.h

@@ -27,10 +27,7 @@ SDL_AppResult SDL_InitMainCallbacks(int argc, char *argv[], SDL_AppInit_func app
 SDL_AppResult SDL_IterateMainCallbacks(bool pump_events);
 void SDL_QuitMainCallbacks(SDL_AppResult result);
 
-// (not a callback thing, but convenient to stick this in here.)
-// If *_argv is NULL, update *_argc and *_argv to point at a static array of { "SDL_app", NULL }.
-void SDL_CheckDefaultArgcArgv(int *_argc, char ***_argv);
+// Check args and call the main function
+extern int SDL_CallMainFunction(int argc, char *argv[], SDL_main_func mainFunction);
 
 #endif // SDL_main_callbacks_h_
-
-

src/main/SDL_runapp.c

@@ -28,8 +28,7 @@
 int SDL_RunApp(int argc, char *argv[], SDL_main_func mainFunction, void * reserved)
 {
     (void)reserved;
-    SDL_CheckDefaultArgcArgv(&argc, &argv);
-    return mainFunction(argc, argv);
+    return SDL_CallMainFunction(argc, argv, mainFunction);
 }
 
 #endif

src/main/emscripten/SDL_sysmain_runapp.c

@@ -37,8 +37,6 @@ int SDL_RunApp(int argc, char *argv[], SDL_main_func mainFunction, void * reserv
 {
     (void)reserved;
 
-    SDL_CheckDefaultArgcArgv(&argc, &argv);
-
     // Move any URL params that start with "SDL_" over to environment
     //  variables, so the hint system can pick them up, etc, much like a user
     //  can set them from a shell prompt on a desktop machine. Ignore all
@@ -59,7 +57,7 @@ int SDL_RunApp(int argc, char *argv[], SDL_main_func mainFunction, void * reserv
         }
     }, SDL_setenv_unsafe);
 
-    return mainFunction(argc, argv);
+    return SDL_CallMainFunction(argc, argv, mainFunction);
 }
 
 #endif

src/main/gdk/SDL_sysmain_runapp.cpp

@@ -24,6 +24,7 @@ extern "C" {
 #include "../../core/gdk/SDL_gdk.h"
 #include "../../core/windows/SDL_windows.h"
 #include "../../events/SDL_events_c.h"
+#include "../SDL_main_callbacks.h"
 }
 #include <XGameRuntime.h>
 #include <xsapi-c/services_c.h>
@@ -65,14 +66,12 @@ int SDL_RunApp(int argc, char **argv, SDL_main_func mainFunction, void *reserved
             SDL_SetError("[GDK] Unable to get titleid. Will not call XblInitialize. Check MicrosoftGame.config!");
         }
 
-        SDL_SetMainReady();
-
         if (!GDK_RegisterChangeNotifications()) {
             return -1;
         }
 
         // Run the application main() code
-        result = mainFunction(argc, argv);
+        result = SDL_CallMainFunction(argc, argv, mainFunction);
 
         GDK_UnregisterChangeNotifications();
 

src/main/n3ds/SDL_sysmain_runapp.c

@@ -31,14 +31,11 @@ int SDL_RunApp(int argc, char *argv[], SDL_main_func mainFunction, void * reserv
 {
     int result;
 
-    SDL_CheckDefaultArgcArgv(&argc, &argv);
-
     // init
     osSetSpeedupEnable(true);
     romfsInit();
 
-    SDL_SetMainReady();
-    result = mainFunction(argc, argv);
+    result = SDL_CallMainFunction(argc, argv, mainFunction);
 
     // quit
     romfsExit();

src/main/ps2/SDL_sysmain_runapp.c

@@ -68,21 +68,17 @@ static void deinit_drivers(void)
 
 int SDL_RunApp(int argc, char *argv[], SDL_main_func mainFunction, void * reserved)
 {
-    int res;
+    int result;
     (void)reserved;
 
-    SDL_CheckDefaultArgcArgv(&argc, &argv);
-
     prepare_IOP();
     init_drivers();
 
-    SDL_SetMainReady();
-
-    res = mainFunction(argc, argv);
+    result = SDL_CallMainFunction(argc, argv, mainFunction);
 
     deinit_drivers();
 
-    return res;
+    return result;
 }
 
 #endif // SDL_PLATFORM_PS2

src/main/psp/SDL_sysmain_runapp.c

@@ -74,13 +74,9 @@ int SDL_RunApp(int argc, char *argv[], SDL_main_func mainFunction, void * reserv
 {
     (void)reserved;
 
-    SDL_CheckDefaultArgcArgv(&argc, &argv);
-
     sdl_psp_setup_callbacks();
 
-    SDL_SetMainReady();
-
-    return mainFunction(argc, argv);
+    return SDL_CallMainFunction(argc, argv, mainFunction);
 }
 
 #endif // SDL_PLATFORM_PSP

src/main/windows/SDL_sysmain_runapp.c

@@ -23,6 +23,7 @@
 #ifdef SDL_PLATFORM_WIN32
 
 #include "../../core/windows/SDL_windows.h"
+#include "../SDL_main_callbacks.h"
 
 /* Win32-specific SDL_RunApp(), which does most of the SDL_main work,
   based on SDL_windows_main.c, placed in the public domain by Sam Lantinga  4/13/98 */
@@ -37,8 +38,7 @@ int MINGW32_FORCEALIGN SDL_RunApp(int argc, char *argv[], SDL_main_func mainFunc
     if (args_error) {
         SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Fatal Error", args_error, NULL);
     } else {
-        SDL_SetMainReady();
-        result = mainFunction(argc, argv);
+        result = SDL_CallMainFunction(argc, argv, mainFunction);
         if (heap_allocated) {
             HeapFree(GetProcessHeap(), 0, heap_allocated);
         }

src/video/uikit/SDL_uikitappdelegate.m

@@ -42,8 +42,6 @@ static int exit_status;
 
 int SDL_RunApp(int argc, char *argv[], SDL_main_func mainFunction, void *reserved)
 {
-    SDL_CheckDefaultArgcArgv(&argc, &argv);
-
     // store arguments
     forward_main = mainFunction;
     forward_argc = argc;
@@ -483,7 +481,7 @@ API_AVAILABLE(ios(13.0))
     [self performSelector:@selector(hideLaunchScreen) withObject:nil afterDelay:0.0];
 
     SDL_SetiOSEventPump(true);
-    exit_status = forward_main(forward_argc, forward_argv);
+    exit_status = SDL_CallMainFunction(forward_argc, forward_argv, forward_main);
     SDL_SetiOSEventPump(false);
 
     if (launchWindow) {
@@ -553,7 +551,7 @@ API_AVAILABLE(ios(13.0))
 
     // run the user's application, passing argc and argv
     SDL_SetiOSEventPump(true);
-    exit_status = forward_main(forward_argc, forward_argv);
+    exit_status = SDL_CallMainFunction(forward_argc, forward_argv, forward_main);
     SDL_SetiOSEventPump(false);
 
     if (launchWindow) {