[SR-2587] Optimisation for zero length String fails on Android build of Foundation

[johnno1962]

Previous ID	SR-2587
Radar	None
Original Reporter	@johnno1962
Type	Bug
Status	Closed
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Foundation
Labels	Bug
Assignee	@johnno1962
Priority	Medium

md5: 647012f97ad3fe4b3b854ad423c13a8a

Issue Description:

During the port of Foundation to Android it was found that an optimisation for creating zero length Strings from Data objects was causing a SEGFAULT.

#622

The change made to remove this optimisation is here:

https://github.com/SwiftJava/swift-corelibs-foundation/blob/master/CoreFoundation/String.subproj/CFString.c#L1267

[johnno1962]

This was resolved by moving the alias from __CFConstantStringClassReference to _TMC10Foundation19_NSCFConstantString from being applied as a linker option into CFInternal.h:

#ifdef ANDROID
// Avoids crashes on Android
// https://bugs.swift.org/browse/SR-2587
// https://bugs.swift.org/browse/SR-2588
// Seemed to be a linker/relocation? problem.
// CFStrings using CONST_STRING_DECL() were not working
// Applies reference to _NSCFConstantString's isa here
// rather than using a linker option to create an alias.
#define __CFConstantStringClassReference _TMC10Foundation19_NSCFConstantString
#endif