app: [macOS] fix display link callback race

Commit c0c25b777 replaced the synchronizing of the display link callback
from a sync.Map to a cgo.Handle. However, the change didn't take into
account the lifecycle issues: a callback may happen just as the cgo.Handle
is freed, leading to a misuse crash.

This change restores the sync.Map synchronization, which avoids the
lifecycle issue.

Fixes: https://todo.sr.ht/~eliasnaur/gio/526
Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/os_darwin.go

@@ -6,7 +6,7 @@ package app
 #include <Foundation/Foundation.h>
 
 __attribute__ ((visibility ("hidden"))) void gio_wakeupMainThread(void);
-__attribute__ ((visibility ("hidden"))) CFTypeRef gio_createDisplayLink(uintptr_t handle);
+__attribute__ ((visibility ("hidden"))) CFTypeRef gio_createDisplayLink(void);
 __attribute__ ((visibility ("hidden"))) void gio_releaseDisplayLink(CFTypeRef dl);
 __attribute__ ((visibility ("hidden"))) int gio_startDisplayLink(CFTypeRef dl);
 __attribute__ ((visibility ("hidden"))) int gio_stopDisplayLink(CFTypeRef dl);
@@ -42,7 +42,7 @@ static CFTypeRef newNSString(unichar *chars, NSUInteger length) {
 import "C"
 import (
 	"errors"
-	"runtime/cgo"
+	"sync"
 	"sync/atomic"
 	"time"
 	"unicode/utf16"
@@ -70,6 +70,9 @@ type displayLink struct {
 	running uint32
 }
 
+// displayLinks maps CFTypeRefs to *displayLinks.
+var displayLinks sync.Map
+
 var mainFuncs = make(chan func(), 1)
 
 // runOnMain runs the function on the main thread.
@@ -128,18 +131,18 @@ func NewDisplayLink(callback func()) (*displayLink, error) {
 		states:   make(chan bool),
 		dids:     make(chan uint64),
 	}
-	h := cgo.NewHandle(d)
-	dl := C.gio_createDisplayLink(C.uintptr_t(h))
+	dl := C.gio_createDisplayLink()
 	if dl == 0 {
 		return nil, errors.New("app: failed to create display link")
 	}
-	go d.run(dl, h)
+	go d.run(dl)
 	return d, nil
 }
 
-func (d *displayLink) run(dl C.CFTypeRef, h cgo.Handle) {
+func (d *displayLink) run(dl C.CFTypeRef) {
 	defer C.gio_releaseDisplayLink(dl)
-	defer h.Delete()
+	displayLinks.Store(dl, d)
+	defer displayLinks.Delete(dl)
 	var stopTimer *time.Timer
 	var tchan <-chan time.Time
 	started := false
@@ -200,10 +203,14 @@ func (d *displayLink) SetDisplayID(did uint64) {
 }
 
 //export gio_onFrameCallback
-func gio_onFrameCallback(dl C.CFTypeRef, handle C.uintptr_t) {
-	d := cgo.Handle(handle).Value().(*displayLink)
-	if atomic.LoadUint32(&d.running) != 0 {
-		d.callback()
+func gio_onFrameCallback(ref C.CFTypeRef) {
+	d, exists := displayLinks.Load(ref)
+	if !exists {
+		return
+	}
+	dl := d.(*displayLink)
+	if atomic.LoadUint32(&dl.running) != 0 {
+		dl.callback()
 	}
 }
 

app/os_ios.m

@@ -123,6 +123,9 @@ static void handleTouches(int last, UIView *view, NSSet<UITouch *> *touches, UIE
 
 @implementation GioView
 NSArray<UIKeyCommand *> *_keyCommands;
++ (void)onFrameCallback:(CADisplayLink *)link {
+       gio_onFrameCallback((__bridge CFTypeRef)link);
+}
 + (Class)layerClass {
     return gio_layerClass();
 }
@@ -227,23 +230,8 @@ NSArray<UIKeyCommand *> *_keyCommands;
 }
 @end
 
-@interface DisplayLinkHandle : NSObject {
-}
-@property uintptr_t handle;
-@end
-
-@implementation DisplayLinkHandle {
-}
-
-- (void)onFrameCallback:(CADisplayLink *)link {
-	gio_onFrameCallback((__bridge CFTypeRef)link, _handle);
-}
-@end
-
-CFTypeRef gio_createDisplayLink(uintptr_t handle) {
-	DisplayLinkHandle *h = [DisplayLinkHandle alloc];
-	h.handle = handle;
-	CADisplayLink *dl = [CADisplayLink displayLinkWithTarget:h selector:@selector(onFrameCallback:)];
+CFTypeRef gio_createDisplayLink(void) {
+	CADisplayLink *dl = [CADisplayLink displayLinkWithTarget:[GioView class] selector:@selector(onFrameCallback:)];
 	dl.paused = YES;
 	NSRunLoop *runLoop = [NSRunLoop mainRunLoop];
 	[dl addToRunLoop:runLoop forMode:[runLoop currentMode]];

app/os_macos.m

@@ -193,14 +193,14 @@ static void handleMouse(NSView *view, NSEvent *event, int typ, CGFloat dx, CGFlo
 static GioWindowDelegate *globalWindowDel;
 
 static CVReturn displayLinkCallback(CVDisplayLinkRef dl, const CVTimeStamp *inNow, const CVTimeStamp *inOutputTime, CVOptionFlags flagsIn, CVOptionFlags *flagsOut, void *handle) {
-	gio_onFrameCallback(dl, (uintptr_t)handle);
+	gio_onFrameCallback(dl);
 	return kCVReturnSuccess;
 }
 
-CFTypeRef gio_createDisplayLink(uintptr_t handle) {
+CFTypeRef gio_createDisplayLink(void) {
 	CVDisplayLinkRef dl;
 	CVDisplayLinkCreateWithActiveCGDisplays(&dl);
-	CVDisplayLinkSetOutputCallback(dl, displayLinkCallback, (void *)(handle));
+	CVDisplayLinkSetOutputCallback(dl, displayLinkCallback, nil);
 	return dl;
 }