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

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

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

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