static function + dispatch_once in header file will cause the once logic executable more than once

[Kyle-Ye]

Background

https://github.com/ChengzhiHuang/__inline__Demo
Credits: @ChengzhiHuang

1. static variables in the static function are shared by the same compile Unit, but not by different compile Units.
2. 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.

swift-corelibs-foundation/Darwin/shims/NSCalendarShims.h

Lines 17 to 26 in 778dde9

	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]);
	}

swift-corelibs-foundation/Darwin/shims/NSLocaleShims.h

Lines 17 to 24 in 778dde9

	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];
	}

swift-corelibs-foundation/Darwin/shims/NSTimeZoneShims.h

Lines 25 to 32 in 778dde9

	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

CDemoKit.zip

Solution

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

Other info

https://forums.swift.org/t/potential-issue-for-ns-inline-dispatch-once-on-swift-corelibs-foundation/70439