Fallback not used when clock_gettime is unavailable (crash on OSX < 10.12)

[EvanBalster]

Describe the bug
The POSIX implementation of NNG uses clock_gettime which is not available on old versions of Mac OS X. The Mac OS X Deployment Target has no bearing on this behavior.

Expected behavior
Mac OS X Deployment Targets less than 10.12 should enable the NNG_USE_GETTIMEOFDAY macro which enables a less performant fallback implementation.

Actual Behavior
NNG's CMake files make no reference to NNG_USE_GETTIMEOFDAY. POSIX builds will always use clock_gettime regardless of its availability.

To Reproduce
Configure and build NNG with a Mac OS X deployment target of 10.11 or lower. Alternatively, configure NNG to build on a POSIX platform lacking clock_gettime.

Run a test application in a debugger, with a breakpoint on nni_clock. Take note of the code that is executed there.

** Environment Details **

• NNG 1.5.2
• Mac OS X 10.14 (development machine)
• Mac OS X 10.7 (deployment target)
• Apple Clang 11.0.0 (XCode 11.3.1)
• Static library built via makefile

[gdamore]

So the workaround is to set NNG_USE_GETTIMEOFDAY. I need to see if I can convince CMake of this. I expect what is happening is that we are detecting a clock_gettime() symbol on macOS versions that are old, but that the implementation is defective. (I.e. compile time checks find it, but at run time it doesn't work. :(

[gdamore]

well maybe... I do see that we aren't using the check for NNG_HAVE_CLOCKGETTIME.

[gdamore]

Btw, we are likely to retire support for such old versions of macOS.

Pretty much everyone dropping support for macOS versions more than 5 years old. Go dropped support for 10.15 almost a year ago. (Meaning you need macOS 11 -- Big Sur -- or better.)

On the one hand, I'm unlikely to go far out of my way to intentionally break compatibility with an older OS, but I am also unlikely to go far out of my way supporting an old OS. Right now Apple is only providing support for macOS 12 and newer.

A similar situation exists with Windows and to a lesser extent some Linux distributions.