cocoa: Implement OpenGL swap interval support with CVDisplayLink.

Not only does this fix macOS 10.14 ("Mojave")'s broken NSOpenGLCPSwapInterval
support, it also lets us implement "adaptive vsync" on macOS!

CVDisplayLink is supported back to macOS 10.4 ("Tiger"), so we just use it
universally without version checks and dump NSOpenGLCPSwapInterval, Mojave or
not.

src/video/cocoa/SDL_cocoaopengl.h

@@ -36,6 +36,11 @@ struct SDL_GLDriverData
 @interface SDLOpenGLContext : NSOpenGLContext {
     SDL_atomic_t dirty;
     SDL_Window *window;
+    CVDisplayLinkRef displayLink;
+    @public SDL_mutex *swapIntervalMutex;
+    @public SDL_cond *swapIntervalCond;
+    @public SDL_atomic_t swapIntervalSetting;
+    @public SDL_atomic_t swapIntervalsPassed;
 }
 
 - (id)initWithFormat:(NSOpenGLPixelFormat *)format
@@ -43,7 +48,7 @@ struct SDL_GLDriverData
 - (void)scheduleUpdate;
 - (void)updateIfNeeded;
 - (void)setWindow:(SDL_Window *)window;
-
+- (void)dealloc;
 @end
 
 

src/video/cocoa/SDL_cocoaopengl.m

@@ -36,6 +36,23 @@
 
 #define DEFAULT_OPENGL  "/System/Library/Frameworks/OpenGL.framework/Libraries/libGL.dylib"
 
+static CVReturn
+DisplayLinkCallback(CVDisplayLinkRef displayLink, const CVTimeStamp* now, const CVTimeStamp* outputTime, CVOptionFlags flagsIn, CVOptionFlags* flagsOut, void* displayLinkContext)
+{
+    SDLOpenGLContext *nscontext = (SDLOpenGLContext *) displayLinkContext;
+    
+    /*printf("DISPLAY LINK! %u\n", (unsigned int) SDL_GetTicks()); */
+    const int setting = SDL_AtomicGet(&nscontext->swapIntervalSetting);
+    if (setting != 0) { /* nothing to do if vsync is disabled, don't even lock */
+        SDL_LockMutex(nscontext->swapIntervalMutex);
+        SDL_AtomicAdd(&nscontext->swapIntervalsPassed, 1);
+        SDL_CondSignal(nscontext->swapIntervalCond);
+        SDL_UnlockMutex(nscontext->swapIntervalMutex);
+    }
+
+    return kCVReturnSuccess;
+}
+
 @implementation SDLOpenGLContext : NSOpenGLContext
 
 - (id)initWithFormat:(NSOpenGLPixelFormat *)format
@@ -45,6 +62,20 @@
     if (self) {
         SDL_AtomicSet(&self->dirty, 0);
         self->window = NULL;
+        SDL_AtomicSet(&self->swapIntervalSetting, 0);
+        SDL_AtomicSet(&self->swapIntervalsPassed, 0);
+        self->swapIntervalCond = SDL_CreateCond();
+        self->swapIntervalMutex = SDL_CreateMutex();
+        if (!self->swapIntervalCond || !self->swapIntervalMutex) {
+            [self release];
+            return nil;
+        }
+
+        /* !!! FIXME: check return values. */
+        CVDisplayLinkCreateWithActiveCGDisplays(&self->displayLink);
+        CVDisplayLinkSetOutputCallback(self->displayLink, &DisplayLinkCallback, self);
+        CVDisplayLinkSetCurrentCGDisplayFromOpenGLContext(self->displayLink, [self CGLContextObj], [format CGLPixelFormatObj]);
+        CVDisplayLinkStart(displayLink);
     }
     return self;
 }
@@ -114,6 +145,19 @@
     }
 }
 
+- (void)dealloc
+{
+    if (self->displayLink) {
+        CVDisplayLinkRelease(self->displayLink);
+    }
+    if (self->swapIntervalCond) {
+        SDL_DestroyCond(self->swapIntervalCond);
+    }
+    if (self->swapIntervalMutex) {
+        SDL_DestroyMutex(self->swapIntervalMutex);
+    }
+    [super dealloc];
+}
 @end
 
 
@@ -368,21 +412,17 @@ int
 Cocoa_GL_SetSwapInterval(_THIS, int interval)
 { @autoreleasepool
 {
-    NSOpenGLContext *nscontext;
+    SDLOpenGLContext *nscontext = (SDLOpenGLContext *) SDL_GL_GetCurrentContext();
-    GLint value;
     int status;
 
-    if (interval < 0) {  /* no extension for this on Mac OS X at the moment. */
+    if (nscontext == nil) {
-        return SDL_SetError("Late swap tearing currently unsupported");
-    }
-
-    nscontext = (NSOpenGLContext*)SDL_GL_GetCurrentContext();
-    if (nscontext != nil) {
-        value = interval;
-        [nscontext setValues:&value forParameter:NSOpenGLCPSwapInterval];
-        status = 0;
-    } else {
         status = SDL_SetError("No current OpenGL context");
+    } else {
+        SDL_LockMutex(nscontext->swapIntervalMutex);
+        SDL_AtomicSet(&nscontext->swapIntervalsPassed, 0);
+        SDL_AtomicSet(&nscontext->swapIntervalSetting, interval);
+        SDL_UnlockMutex(nscontext->swapIntervalMutex);
+        status = 0;
     }
 
     return status;
@@ -392,17 +432,8 @@ int
 Cocoa_GL_GetSwapInterval(_THIS)
 { @autoreleasepool
 {
-    NSOpenGLContext *nscontext;
+    SDLOpenGLContext *nscontext = (SDLOpenGLContext *) SDL_GL_GetCurrentContext();
-    GLint value;
+    return nscontext ? SDL_AtomicGet(&nscontext->swapIntervalSetting) : 0;
-    int status = 0;
-
-    nscontext = (NSOpenGLContext*)SDL_GL_GetCurrentContext();
-    if (nscontext != nil) {
-        [nscontext getValues:&value forParameter:NSOpenGLCPSwapInterval];
-        status = (int)value;
-    }
-
-    return status;
 }}
 
 int
@@ -411,6 +442,25 @@ Cocoa_GL_SwapWindow(_THIS, SDL_Window * window)
 {
     SDLOpenGLContext* nscontext = (SDLOpenGLContext*)SDL_GL_GetCurrentContext();
     SDL_VideoData *videodata = (SDL_VideoData *) _this->driverdata;
+    const int setting = SDL_AtomicGet(&nscontext->swapIntervalSetting);
+
+    if (setting == 0) {
+        /* nothing to do if vsync is disabled, don't even lock */
+    } else if (setting < 0) {  /* late swap tearing */
+        SDL_LockMutex(nscontext->swapIntervalMutex);
+        while (SDL_AtomicGet(&nscontext->swapIntervalsPassed) == 0) {
+            SDL_CondWait(nscontext->swapIntervalCond, nscontext->swapIntervalMutex);
+        }
+        SDL_AtomicSet(&nscontext->swapIntervalsPassed, 0);
+        SDL_UnlockMutex(nscontext->swapIntervalMutex);
+    } else {
+        SDL_LockMutex(nscontext->swapIntervalMutex);
+        do {  /* always wait here so we know we just hit a swap interval. */
+            SDL_CondWait(nscontext->swapIntervalCond, nscontext->swapIntervalMutex);
+        } while ((SDL_AtomicGet(&nscontext->swapIntervalsPassed) % setting) != 0);
+        SDL_AtomicSet(&nscontext->swapIntervalsPassed, 0);
+        SDL_UnlockMutex(nscontext->swapIntervalMutex);
+    }
 
     /* on 10.14 ("Mojave") and later, this deadlocks if two contexts in two
        threads try to swap at the same time, so put a mutex around it. */