Potential issue for NS_INLINE+ dispatch_once on swift-corelibs-foundation

Kyle-Ye · 2024-03-05T13:13:13.544Z

Background

static variables in the static function are shared by the same compile Unit, but not by different compile Units.
static variables in normal functions share one copy.

The NS_INLINE is defined as follows

#if !defined(NS_INLINE)
    #if defined(__GNUC__)
        #define NS_INLINE static __inline__ __attribute__((always_inline))
    #elif defined(__MWERKS__) || defined(__cplusplus)
        #define NS_INLINE static inline
    #elif defined(_MSC_VER)
        #define NS_INLINE static __inline
    #endif
#endif

Issue

So for the following code snippet in this repo, if we include and use them in diff compile units(eg. different .m files) the block in dispatch_once will be executed once for each compile units.

eg.

github.com

apple/swift-corelibs-foundation/blob/778dde90ff7cf63f05634c3df6b0788c89249770/Darwin/shims/NSCalendarShims.h#L17-L26


      
          NS_INLINE BOOL __NSCalendarIsAutoupdating(NS_NON_BRIDGED(NSCalendar *) calendar) {
              static dispatch_once_t onceToken;
              static Class autoCalendarClass;
              static Class olderAutoCalendarClass; // Pre 10.12/10.0
              dispatch_once(&onceToken, ^{
                  autoCalendarClass = (Class)objc_lookUpClass("_NSAutoCalendar");
                  olderAutoCalendarClass = (Class)objc_lookUpClass("NSAutoCalendar");
              });
              return (autoCalendarClass && [calendar isKindOfClass:autoCalendarClass]) || (olderAutoCalendarClass && [calendar isKindOfClass:olderAutoCalendarClass]);
          }

github.com

apple/swift-corelibs-foundation/blob/778dde90ff7cf63f05634c3df6b0788c89249770/Darwin/shims/NSLocaleShims.h#L17-L24


      
          NS_INLINE BOOL __NSLocaleIsAutoupdating(NS_NON_BRIDGED(NSLocale *)locale) {
              static dispatch_once_t onceToken;
              static Class autoLocaleClass;
              dispatch_once(&onceToken, ^{
                  autoLocaleClass = (Class)objc_lookUpClass("NSAutoLocale");
              });
              return [locale isKindOfClass:autoLocaleClass];
          }

github.com

apple/swift-corelibs-foundation/blob/778dde90ff7cf63f05634c3df6b0788c89249770/Darwin/shims/NSTimeZoneShims.h#L25-L32


      
          NS_INLINE BOOL __NSTimeZoneIsAutoupdating(NS_NON_BRIDGED(NSTimeZone *)timeZone) {
              static dispatch_once_t onceToken;
              static Class autoTimeZoneClass;
              dispatch_once(&onceToken, ^{
                  autoTimeZoneClass = (Class)objc_lookUpClass("__NSLocalTimeZone");
              });
              return [timeZone isKindOfClass:autoTimeZoneClass];
          }

Example project

image4064×2334 742 KB

CDemoKit.zip

Solution

Use __inline__ __attribute__((always_inline)) directly instead of using NS_INLINE
To be discussed ...

See GitHub issue here static function + dispatch_once in header file will cause the once logic executable more than once · Issue #4904 · apple/swift-corelibs-foundation · GitHub

Tony_Parker · 2024-03-05T15:51:13.771Z

I think the best solution is simply to not inline these at all but instead be regular functions.

Tony_Parker · 2024-03-05T15:52:56.293Z

I’m working on a patch in this area anyway - we can take care of it then, unless this it really urgent.