From 1fb20f0ab9f1b1ad51a79dbf59c50db5cc91d6f9 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <icculus@icculus.org>
Date: Thu, 18 Oct 2018 23:38:27 -0400
Subject: [PATCH] cocoa: Put a mutex around GL_SwapBuffers.

Prevents deadlock when swapping two different GL contexts on two different
threads at the same time on macOS 10.14 ("Mojave").

Fixes Bugzilla #4278.
---
 src/video/cocoa/SDL_cocoaopengl.m | 6 ++++++
 src/video/cocoa/SDL_cocoavideo.h  | 2 +-
 src/video/cocoa/SDL_cocoavideo.m  | 8 ++++++++
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/src/video/cocoa/SDL_cocoaopengl.m b/src/video/cocoa/SDL_cocoaopengl.m
index c7d0c60e52c12..9539c17799e68 100644
--- a/src/video/cocoa/SDL_cocoaopengl.m
+++ b/src/video/cocoa/SDL_cocoaopengl.m
@@ -410,8 +410,14 @@ - (void)setWindow:(SDL_Window *)newWindow
 { @autoreleasepool
 {
     SDLOpenGLContext* nscontext = (SDLOpenGLContext*)SDL_GL_GetCurrentContext();
+    SDL_VideoData *videodata = (SDL_VideoData *) _this->driverdata;
+
+    /* 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. */
+    SDL_LockMutex(videodata->swaplock);
     [nscontext flushBuffer];
     [nscontext updateIfNeeded];
+    SDL_UnlockMutex(videodata->swaplock);
     return 0;
 }}
 
diff --git a/src/video/cocoa/SDL_cocoavideo.h b/src/video/cocoa/SDL_cocoavideo.h
index 05bbd34838185..b1c26fa7e8c6e 100644
--- a/src/video/cocoa/SDL_cocoavideo.h
+++ b/src/video/cocoa/SDL_cocoavideo.h
@@ -107,7 +107,7 @@ typedef struct SDL_VideoData
     Uint32 screensaver_activity;
     BOOL screensaver_use_iopm;
     IOPMAssertionID screensaver_assertion;
-
+    SDL_mutex *swaplock;
 } SDL_VideoData;
 
 /* Utility functions */
diff --git a/src/video/cocoa/SDL_cocoavideo.m b/src/video/cocoa/SDL_cocoavideo.m
index 977022494a0ff..20bdfa79165cb 100644
--- a/src/video/cocoa/SDL_cocoavideo.m
+++ b/src/video/cocoa/SDL_cocoavideo.m
@@ -175,15 +175,23 @@
     /* The IOPM assertion API can disable the screensaver as of 10.7. */
     data->screensaver_use_iopm = floor(NSAppKitVersionNumber) > NSAppKitVersionNumber10_6;
 
+    data->swaplock = SDL_CreateMutex();
+    if (!data->swaplock) {
+        return -1;
+    }
+
     return 0;
 }
 
 void
 Cocoa_VideoQuit(_THIS)
 {
+    SDL_VideoData *data = (SDL_VideoData *) _this->driverdata;
     Cocoa_QuitModes(_this);
     Cocoa_QuitKeyboard(_this);
     Cocoa_QuitMouse(_this);
+    SDL_DestroyMutex(data->swaplock);
+    data->swaplock = NULL;
 }
 
 /* This function assumes that it's called from within an autorelease pool */