macOS: Fix a deadlock with CVDisplayLink

I had added an optimization to not pass messages to
main thread every time the CVDisplayLink timer fired, unless
a render frame for that monitor was actually requested.

However, this optimization is impossible to implement wihtout a deadlock
since CVDisplayLink has its own internal lock that it does not expose.

So I guess macOS users with multiple monitors will simply have to take
the performance hit of useless wakeups sixty times a second for every
extra monitor.

Fixes #1779

docs/changelog.rst

@@ -41,6 +41,8 @@ To update |kitty|, :doc:`follow the instructions <binary>`.
 - Linux: Use the system "bell" sound for the terminal bell. Adds libcanberra
   as a new dependency to play the system sound.
 
+- macOS: Fix a rare deadlock causing kitty to hang (:iss:`1779`)
+
 
 0.14.2 [2019-06-09]
 ---------------------

glfw/cocoa_init.m

@@ -399,7 +399,6 @@ int _glfwPlatformInit(void)
     if (!initializeTIS())
         return false;
 
-    _glfw.ns.displayLinks.lock = [NSLock new];
     _glfwInitTimerNS();
     _glfwInitJoysticksNS();
 
@@ -413,11 +412,7 @@ void _glfwPlatformTerminate(void)
 {
     @autoreleasepool {
 
-    if (_glfw.ns.displayLinks.lock) {
     _glfwClearDisplayLinks();
-        [_glfw.ns.displayLinks.lock release];
-        _glfw.ns.displayLinks.lock = nil;
-    }
 
     if (_glfw.ns.inputSource)
     {

glfw/cocoa_monitor.m

@@ -243,16 +243,15 @@ bool refreshMonitorScreen(_GLFWmonitor* monitor)
 //////////////////////////////////////////////////////////////////////////
 
 void _glfwClearDisplayLinks() {
-    [_glfw.ns.displayLinks.lock lock];
     for (size_t i = 0; i < _glfw.ns.displayLinks.count; i++) {
         if (_glfw.ns.displayLinks.entries[i].displayLink) {
             CVDisplayLinkStop(_glfw.ns.displayLinks.entries[i].displayLink);
             CVDisplayLinkRelease(_glfw.ns.displayLinks.entries[i].displayLink);
             _glfw.ns.displayLinks.entries[i].displayLink = nil;
+            _glfw.ns.displayLinks.entries[i].lastRenderFrameRequestedAt = 0;
         }
     }
     _glfw.ns.displayLinks.count = 0;
-    [_glfw.ns.displayLinks.lock unlock];
 }
 
 static CVReturn displayLinkCallback(
@@ -261,28 +260,13 @@ static CVReturn displayLinkCallback(
         CVOptionFlags flagsIn UNUSED, CVOptionFlags* flagsOut UNUSED, void* userInfo)
 {
     CGDirectDisplayID displayID = (CGDirectDisplayID)userInfo;
-    [_glfw.ns.displayLinks.lock lock];
-    bool notify = false;
-    for (size_t i = 0; i < _glfw.ns.displayLinks.count; i++) {
-        if (_glfw.ns.displayLinks.entries[i].displayID == displayID) {
-            if (_glfw.ns.displayLinks.entries[i].renderFrameRequested) {
-                notify = true;
-                _glfw.ns.displayLinks.entries[i].renderFrameRequested = false;
-            }
-            break;
-        }
-    }
-    [_glfw.ns.displayLinks.lock unlock];
-    if (notify) {
     NSNumber *arg = [NSNumber numberWithUnsignedInt:displayID];
     [NSApp performSelectorOnMainThread:@selector(render_frame_received:) withObject:arg waitUntilDone:NO];
     [arg release];
-    }
     return kCVReturnSuccess;
 }
 
 static inline void createDisplayLink(CGDirectDisplayID displayID) {
-    [_glfw.ns.displayLinks.lock lock];
     if (_glfw.ns.displayLinks.count >= sizeof(_glfw.ns.displayLinks.entries)/sizeof(_glfw.ns.displayLinks.entries[0]) - 1) return;
     for (size_t i = 0; i < _glfw.ns.displayLinks.count; i++) {
         if (_glfw.ns.displayLinks.entries[i].displayID == displayID) return;
@@ -292,7 +276,6 @@ static inline void createDisplayLink(CGDirectDisplayID displayID) {
     entry->displayID = displayID;
     CVDisplayLinkCreateWithCGDisplay(displayID, &entry->displayLink);
     CVDisplayLinkSetOutputCallback(entry->displayLink, &displayLinkCallback, (void*)(uintptr_t)displayID);
-    [_glfw.ns.displayLinks.lock unlock];
 }
 
 // Poll for changes in the set of connected monitors

glfw/cocoa_platform.h

@@ -144,7 +144,7 @@ typedef struct _GLFWDisplayLinkNS
 {
     CVDisplayLinkRef displayLink;
     CGDirectDisplayID displayID;
-    bool renderFrameRequested;
+    double lastRenderFrameRequestedAt;
 } _GLFWDisplayLinkNS;
 
 // Cocoa-specific global data
@@ -183,7 +183,6 @@ typedef struct _GLFWlibraryNS
     struct {
         _GLFWDisplayLinkNS entries[256];
         size_t count;
-        id lock;
     } displayLinks;
 
 } _GLFWlibraryNS;

glfw/cocoa_window.m

@@ -67,13 +67,12 @@ static unsigned long long display_link_shutdown_timer = 0;
 
 void
 _glfwShutdownCVDisplayLink(unsigned long long timer_id UNUSED, void *user_data UNUSED) {
-    [_glfw.ns.displayLinks.lock lock];
     display_link_shutdown_timer = 0;
     for (size_t i = 0; i < _glfw.ns.displayLinks.count; i++) {
         _GLFWDisplayLinkNS *dl = &_glfw.ns.displayLinks.entries[i];
         if (dl->displayLink) CVDisplayLinkStop(dl->displayLink);
+        dl->lastRenderFrameRequestedAt = 0;
     }
-    [_glfw.ns.displayLinks.lock unlock];
 }
 
 static inline void
@@ -86,23 +85,26 @@ requestRenderFrame(_GLFWwindow *w, GLFWcocoarenderframefun callback) {
     w->ns.renderFrameCallback = callback;
     w->ns.renderFrameRequested = true;
     CGDirectDisplayID displayID = displayIDForWindow(w);
-    [_glfw.ns.displayLinks.lock lock];
     if (display_link_shutdown_timer) {
         _glfwPlatformUpdateTimer(display_link_shutdown_timer, DISPLAY_LINK_SHUTDOWN_CHECK_INTERVAL, true);
     } else {
         display_link_shutdown_timer = _glfwPlatformAddTimer(DISPLAY_LINK_SHUTDOWN_CHECK_INTERVAL, false, _glfwShutdownCVDisplayLink, NULL, NULL);
     }
+    double now = glfwGetTime();
     for (size_t i = 0; i < _glfw.ns.displayLinks.count; i++) {
         _GLFWDisplayLinkNS *dl = &_glfw.ns.displayLinks.entries[i];
         if (dl->displayID == displayID) {
-            dl->renderFrameRequested = true;
+            dl->lastRenderFrameRequestedAt = now;
             if (!CVDisplayLinkIsRunning(dl->displayLink)) {
                 CVDisplayLinkStart(dl->displayLink);
-            }
+            } else {
-            break;
+                if (dl->lastRenderFrameRequestedAt && now - dl->lastRenderFrameRequestedAt) {
+                    if (dl->displayLink) CVDisplayLinkStop(dl->displayLink);
+                    dl->lastRenderFrameRequestedAt = 0;
+                }
+            }
         }
     }
-    [_glfw.ns.displayLinks.lock unlock];
 }
 
 void